CETME-LC (the short one): Safety PSA

As you may or may not be aware, I recently built a semiauto CETME-LV rifle using a parts kit imported by Apex Gun Parts and an 80% flat from Hill & Mac Gunworks. Now, the LV model is identical to the standard CETME-L except for the rear sight block, which includes a STANAG optics mount not present on the regular CETME-L rifle. That is a totally non-mechanical difference – it has no impact on how the rifle functions. There is, however, one other model of the rifle – the CETME-LC.

cetme_model_l

The LC is the short (corto) carbine version of the gun, with a 12.6″ barrel and a collapsing stock. Because the recoil spring in the normal rifles extends deep into the stock, it must be changed if the rifle is to have a different type of buttstock. So the LC model has its recoil spring assembly moved up to the area of the charging handle tube. The spring tension was also changed, and the bolt carrier weight was reduced by about 3 ounces. What this all leads to is an increase in bolt velocity. No problem, if the other parts of the design are adjusted accordingly. However, in this case we have a problem, thanks to the poor spring quality endemic to the CETME-L. The firing pin spring is not nearly strong enough.

The original production run of CETME-LC carbines was done near the end of the gun’s service period, and the receiver stamping tooling was getting old and worn. The LC receivers generally did not fit all that well, and there was excessive friction dragging against the bolt carrier as it traveled. With a new well-made receiver (like an HMG flat), this added resistance does not exist. As a result, the bolt velocity goes up, and the guns will slamfire. Because the bolt carrier is lighter and moving faster, the firing pin is also moving faster, traveling within the bolt assembly. The bolt will chamber a round and come to a dead stop, and the firing pin will continue moving forward with more energy than the firing pin spring can contain. It will hit the primer of the newly chambered round without needing to be hit with the hammer. The resulting recoil will unlock the gun (and faster than normal, because the hammer was never released, so the bolt doesn’t have to expend any energy recocking it) and the process will repeat, increasing in bolt velocity bit by bit until it finally runs out of ammo or outruns the magazine and jams. Cyclic rate doing this is a rather frightening 1000RPM or so.

Judging from the LC kits imported from Spain, this was definitely a systematic problem. All of the kits that HMG has inspected have shown extensive wear to the auto trips, which would be expected from this type of malfunction.

Now, the solution is a simple and easy one, once the problem is known: increase the strength of the firing pin spring to just under the point where you begin to get light strikes with the hammer.

Some people will read this and get clever ideas about machine guns. Those people are wrong. This is not an easy way to make a machine gun, it is an easy way to make a self-destructing gun. At this rate of fire, the gun is hammering itself to bits and simultaneously creating so enough recoil to be extremely dangerous to the shooter and people around. Yes, some people can control shoulder rifles at that rate of fire – but the kind of people who seriously consider exploiting this type of malfunctioning as an opportunity are not the sort of people who can do it. If you have that much trigger time on machine guns, you know that this condition is DANGEROUS. Best case, you will break your gun. Worst case, recoil will pull the rifle out of your control and kill a bystander.

This is a PSA for the folks who plan to build themselves CETME-LC rifles, so they can do it safely. These kits are still available from Apex Gun Parts as of this post’s writing: CETME-LC Parts Kit.

75 Comments

  1. Fascinating.
    But also quite frightening. I own a registered Ruger KAC556k with a 13″ barrel. It can be a (fun) handful to shoot so I can only imagine what this would be like.

    BTW, I would love to put it in your hands for a video if you are ever in the southeast.

    • Military purposes- Folk never use full auto. Unless they are “spraying” say, a trench… Or such. 1000 Rpm, seems about right to me. Your trying to speed up the time it takes for the enemy to desist to be such. Thus, get some. Nobody “walk fires”. Whats the point of 600 or so Rpm in an assault rifle, any “assaulting” is done at SMG ranges. So, you want semi… Or Ppsh.

      You can’t hit anything at 600 reliably, beyond close up and personal range with a light weapon, no tripod, in a rifle calibre.

      • A 25″ barrel, in 5.56mm would be of more use, particularly now the Russians have discovered optics and can see us “no offence Russia, traditional’ish etc having you as the bogeyman” I question the assault rifle concept- Particularly if the auto rate is slow, given it isn’t used often. 25″ 1000 Rpm. It doesn’t need to be controlled, take the British. Post a grenade, spray. No walking fire, the enemy is well within Ppsh range I.e. Useful full auto fire rate.

        • “I question the assault rifle concept”
          But (at least Russian) conception of avtomat is selective-fire intermediate cartridge weapon.
          Sub-machine guns have limited range and due to this were mainly replaced by avtomat. Also notice that in modern conditions you must consider possibility that enemy use bulletproof vest, stopping pistol ammunition, but not avtomat ammo. One of Russian solutions for that problem is 9x39mm cartridge and small dimension avtomat (see СР-3 Вихрь for example)

          • I watched a video of a Ppsh firing, and became sentimental. I’ll be honest I’d had a few drinks, but I was very impressed with it’s fire rate, layout, cartridge. I can see why whole units had them. Yes I like that 9x39mm and the CP-3, most impressive cheers.

      • Well you can, better, hit stuff, at 600, than 1000. But, why… It’s intermittent. Hence, support guns- Pin them down etc. It isn’t you, what does it. Suppose you could… But it isn’t particularly effective, can’t be with a rifle.

      • Bipod.

        Folk call the Mg42 fire rate excessive, it isn’t on a tripod. It’s depleting ze enemy quick- Bonus. If they dare, likely enough more would… Be depleted, than with a gun firing at a lesser rate of fire. Toodle pip.

        • “fire rate excessive”
          MG3 developed from MG42 has reduced rate-of-fire. Other universal machine gun developed after WWII (PK, FN MAG and others) has lower rate-of-fire.
          Higher rate-of-fire mean that you have to supply more ammunition for it.

  2. “The LC receivers generally did not fit all that well, and there was excessive friction dragging against the bolt carrier as it traveled. With a new well-made receiver (like an HMG flat), this added resistance does not exist.”
    Interesting example of It’s not a bug, it’s a feature

  3. and the third round goes up your nose…

    if it doesn’t and you live, you might get what David Olofson got for “transferring” a gun that would slam fire – a 30 Month sentence, and while in there you might even become Bubba’s new bitch.

  4. Out of battery explosion might become a reality in this situation taking your rifle and possibly a piece of you with it. I had a similar incident with a MAS 49/56 when reloading with commercial primers. The free floating firing pin has enough inertia to strike the primer and set off the cartridge. Only use military grade primers in these weapons for reloading purposes. This is not good time full auto fun either. This is an out of control weapon dumping a magazine of 7.5 MAS. Very disconcerting and dangerous.

    • Does the CETME have a firing pin stop in the rear part of the bolt to keep it from firing until the rollers are wedged into place?

      • I believe you are correct in that the stop would limit the firing pin travel until the bolt was fully in battery. However if during the 1000 rpm cycling the firing pin were to separate just ahead of the shoulder and jam into the firing pin hole due to inertia, the protrusion of the pin out of the head could cause premature primer ignition before the rollers were locked fully in the trunnion. According to H&Ks drawings the firing pin is not physically constrained by the Locking Piece, so this scenario might be possible.

    • “This is not good time full auto fun either.”
      Full auto is never good when you except self-loading.
      This remind me:
      2 loudest sounds in world?
      BANG when you except CLICK
      CLICK when you except BANG

  5. My comment is somewhat, off topic. But in a similar vein, in respect of “questionable” functioning, as applicable to U.S/States therein, law. Would a mechanism/layout that facilitates two lever actions say Marlins in 30.30 to operate on the Gast gun principle, with no physical modifications to the rifles be legal? Objective, automatic fire via Gast principle. Fed from twin Lewis gun mags* Think, bump fire stock “loophole” lark.
    Mechanism/layout: Essentially you mount the rifles eitherside of each other on twin racks, said racks are two horizontally placed parallel plates with sliding/pivoting rifle attachments affixed which themselves pivot on a vertically aligned (in relation to the above) being laid flat, base plate. There’s two circular plates which rotate, attached to a vertical “upward” extension to the base plate aforesaid. These plates have pins extending outward from there sides which engage the rifles lever from behind the trigger guard section. When one rifle is forward, the other will be rearward. This means one muzzle will be further back from the other, the forward one will be seated inside a recoil booster at this point.
    Both rifles will be empty, but one lever will be cocked- because the rotation of the “wheel” will enable this via the pins interaction with the lever. One rack will interact with a spring running along the opposite side of the other, and will end in a handle “Think a handle bar” The triggers are two sliding rods inside a mount attached appropriately to the base plate, rods which depress the triggers when either gun is fully forward.
    You pull one gun rearwards, against the spring and let go. It goes forward, the lever closes- Wheel rotation. Hits trigger bar, bang. Recoil booster, rifle moves rearward against spring, opposite rifle moves forward, bang. Repeats.

    * If 30.30 can be fitted into a .303 mag- Modify said mag function to operate via the above. Rounds drop in, onto elevator from above.

      • There’s a problem with coming up with very clever things that use the currently understood definitions of words to get around the words used in laws. The problem is that regulatory agencies and the courts decide what the words mean.

        You build this thing and then kick the buttstock of one of the rifles and then they both fire until they’re out of ammunition, and the ATF is going to invite you to come stay at Club Fed. If your argument against this is “I only touched the buttstock, the triggers are just being manipulated by these rods and springs, and the law says ‘trigger’.” and the ATF is just going to tell you that it turns out the buttstock is the “trigger” now, or something.

        cf. the split ring on a string nestled up against the back of the mini-14 trigger guard, with the string wrapped around the operating rod such that the motion of the rod slackened and tightened the string, pulling the trigger. That’s a “machinegun” according to the tech branch, just one with a trigger made out of a split ring and a sear made of cotton twine.

        see also the akins accelerator – machinegun with the spring, not a machinegun without it

        You can’t win semantics games with the guys who decide what words mean

          • Lever actions, in the Gast gun configuration would fire per trigger pull, but “automatically” as envisaged… Or doesn’t that count legally, does it need to be a finger that pulls it?

          • The primary difference between the cranks and bump fire vs. a runaway is that they still require a single trigger pull per shot.

          • These “bum-fire” stocks make me wonder: what is the purpose? Why people bother to go thru such pain and be on edge of legal while getting involved with such contraption. Rifle is after all designed to fire one pull – one shot.

          • Some people think that BUFAF (Basically Uncontrollable Fully-Automatic Fire) is cool. They don’t understand the reason for the M16A2’s three-shot burst control; I know, I’ve tried to explain it to them.

            Hollywood has taught them that it’s cool and macho to blast off an entire magazine or belt at one go, and nothing you can do will convince them otherwise. That’s not even including the ones who scream like Rambo while doing it. For them, nothing less than MG-42-level mayhem will satisfy.

            I’m not military, but I’ve used my share of selective-fire weapons and a few autofire-only ones (MP38/40, for instance), so I know from experience that a short burst is the second most practical way to both hit what you’re shooting at, conserve ammunition, and avoid excessive wear on the weapon, especially the barrel.

            The most practical way is, of course, single-shot, sight picture, trigger squeeze.

            Full-auto is mainly a great way to expend a lot of ammunition while missing.

            And it isn’t just “amateurs” who make this level of mistake. I’ve seen NG-issue M60 and M60A1 GPMGs in every imaginable stage of being FUBAR’d, due to use and abuse by people who’d had too much “movie” and not enough “manual”, IYKWIMAITYD. I’d rather not get into the abuse I’ve seen inflicted on ARs. (Hint; go easy on the 5.56 tracer, folks, and don’t SLAM down the top cover on the ’60, ‘kay?)

            For some people, the “super-cryo-chilled-mega-cool” of full-auto overrides any objective assessment of its practicality.

            BTW, my affection for the M1921/28 Thompson is mainly due to its single-shot setting, closed-bolt/hammer firing, and that lovely Lyman rear sight. Set on Single and using that sight plus a rest, the “Tommy” is remarkable accurate out to about 250 yards, and I’ve won some bets and offed some woodchucks in proving it.

            Most selective-fire small arms are quite decent and accurate shooters on single-shot or with a three-shot burst control.

            These “bump-fire” gadgets, “trigger rifflers”, and etc. turn reasonably accurate self-loading weapons into clumsy and inaccurate “mechanical machine guns”. Which sort of negates the whole concept of “reliable, accurate, and inexpensive” which is what we really should be “shooting for”, pardon the pun.

            Just IMHO.

            cheers

            eon

          • I *have* used full-auto weapons in a military context. Dumping a magazine is undeniably fun – but useless and wasteful in almost every situation. Call ‘full auto’ 99.44% useless. Even belt-fed weapons are used in controlled bursts.

          • Aye, points taken. But the legal machine guns in the U.S won’t last forever, being old’ish. So, in order to preserve… The capacity for civilian “automatic” fire maybe, these types of devices provide an alternative.

            Type thing.

            Boost the economy, etc.

          • to Eon:

            yes, absolutely – it must be a pop-culture more than anything else. It might be of diversion to you when I say that within Rambo-style video reach (which is EU full now) kids want to do it – since as you say it is “cool”.

            No one wants to think that ammo belt ends at one point.

  6. United States v. Olofson (2009) established that a malfunctioning semi-auto rifle that slam-fires is legally equivalent to a machine gun, so this leaves a huge question unanswered — what would be the proper legal course of action when you take your homemade gun out to the range for the first time and you –and everyone else within earshot– get to witness a fully automatic gun firing off?

    Leaving the range and taking the gun back home to try to figure out what went wrong (or worse, giving it to a friend to fix) might not mean much when the ATF comes knocking on your door, and then determines that the sometimes-malfunctioning rifle is an unregistered machine gun.

    To stay completely on the right side of the law, would you need to immediately call the local police or Feds and turn the gun in to them? I’m sure many people would just brave it out –or wrongly assume that they’re legally clean– but 10 years is a long time to spend in prison, so it’s not the kind of thing anyone should ever take lightly.

    • More importantly, one should NEVER test fire a homemade rifle with a full magazine if it has not been determined that it won’t automatically dump the entire magazine’s load into whatever is in the sights. Try chaining the subject rifle to the bench so that if it does slam-fire, it won’t whip around. Next, load a magazine with only two or three rounds in it. Do this with a very good suppressor over the muzzle so as not to attract the ATF. And in any case, never use parts with questionable safety records!!

      Most amateur riflemen will do the wrong thing if they have an AR-15 or SKS suddenly going Rambo: drop the rifle and hope it stops, and usually it DOES NOT STOP. And if you’re evil and don’t want jail time, why not just MASSACRE the ATF agents when they show up and then throw their corpses in front of a speeding freight train? Yes, I am playing the Villain Card.

      • “SKS suddenly going Rambo”
        SKS will probably deplete all magazine (10) before you would able to do anything. I don’t have data of cyclic rate-of-fire for said weapon, but I suspect it might be quite high. Assuming 1000 rpm it will deplete all magazine in 0.6 sec.

        • Watched a modded f/a type 56 SKS fed from AK mags run at the Knob Creek machine gun shoot. My listening guess was that it was running in the 700 to 750rpm range. It sounded about as fast as a M16A1 or a M2 carbine.

      • The safest way to check is as follows;

        1. Load ONE cartridge in the chamber.

        2. Fire. Observe ejection, if any.

        3. If bolt is held open, release it, WITHOUT touching the trigger,allowing bolt/slide/etc. to go into battery.

        3. VISUALLY check hammer, sear, firing pin;

        A. Did the hammer “follow down” as the bolt, etc. closed?

        B. Is the firing pin (on a non-hammer fired system)still held by the sear?

        C. Is the trigger under tension (i.e., engaging the sear correctly) or not?

        If any of these indicators is present-

        A. hammer down

        B. firing pin NOT engaged by sear

        C. Trigger NOT engaging sear

        Then there is a strong indication that the searage is damaged or incorrectly adjusted in a way that WILL cause “runaway fire”.

        If you feel the need, put one more round in the chamber, one in the magazine, and determine if it “doubles” when fired.

        Generally, I don’t bother, I just deadline that weapon until it can be repaired, which generally requires new searage parts and careful adjustment. Which I can do, but I prefer to let a licensed gunsmith do it, so there’s a permanent official record that it was in fact done. This saves problems with ATF.

        Not to mention my nerves.

        cheers

        eon

      • When something unexpected does happen, like an unexpected burst of full auto, or even more recoil than expected

        the instinctive response which we inherited from our tree dwelling monkey ancestors, is to grab tightly so we don’t fall out of our none existant tree.

        There was an abseiling descender called the “Petzl Stop” that on the old versions had a “dead man’s handle” that you had to keep squeezed to release a cam brake,

        The idea was that the handle should be fully squeezed and control of your descent was then supposed to be like any other abseiling device, by tension on the rope below you.

        some idiots never read the instructions and used the handle to control their descent.

        if the descent got too fast or they got frightened, they’d end up gripping tighter, and a few idiots got hurt, other times, the idiots got saved by friends who pulled the rope tight below them.
        more recent stop type descenders have been re designed and lock on the rope if you squeeze them tighter. They’re all horrible bouncy things, a good 6 bar rack is a far more civilised descender.

        the point is,

        if something frightening happens, the frightened person finds it almost impossible to relax their tight grip.

        and the third round goes straight up their nose!

        That tight grip reflex is probably one of the reasons for people firing two shots by accident when firing a really hard kicking double action revolver (think a Cassull or a Linbaugh)

        The clench reflex used to be well recognised in guns, which is why double action trigger pulls used to be at least 15 pounds, and usually more like 17 to 20 pounds – you had to be able to consciously and deliberately feel that pull, it had to be almost, or actually painful, and way too heavy for a small child. The lighter pulls in Glockenspawn, could account for an awful lot of extra hole in things that weren’t intended to get perforated.

      • “carbide will do as well”
        This mean tungsten carbide or something others (exist material having “carbide” in name, but not being tungsten carbide)?

        • By carbide Denny probably means tungsten carbide (WC). I hear that “carbide” is used as an abbreviation for WC in English-speaking metalworking circles. There are of course other carbides besides WC, but most of them are not dense and would not make sense in this context.

          • Yes, that’s what I meant; in NA it is colloquial expression. I recall someone told me they used WC to slow down ROF on AR15 type carbine. It worked, no problem any longer.

    • Hi Denny,

      Tungsten would be the obvious choice to dovetail in or to use as a material for a lightly stressed part.

      There are machineable tungsten alloys available, some are for use as EDM elctrodes. IIRC companies like Johnson Mattey have a range of machineable alloys with a density of around 18g /cm^3 compared to around 12g / cm^3 for lead.

      Even if the tungsten part rattled, it could still perform a useful bufferring as well as slowing role https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2495460A&KC=A&FT=D&ND=3&date=19500124&DB=EPODOC&locale=en_EP

      • Excellent idea Keith

        What I foresee after some initial (shabby) reaction is this:
        – obtain the type of material you talk about
        – machine off top lug which houses spring
        – machine new replacement piece
        – fasten in place reliably (such as dovetail, as you say) and secure by cross pin

        You would have suddenly assembly with substantially larger mass which may be desired fix.

  7. I’ve been on the range when a consumer-built AR-1st an away. Bad stuff. O.o
    Fortunately, in that instance there was “only” property damage, not blood and tears.

    When first testing a consumer-built self-loader, start with only a single round in the magazine to check basic function. The goto TWO round a only in the magazine, to check for proper disconnector function or other ‘run away’ problems. Two rounds autofired is a lot less dangerous than a full magazine dump…

  8. Call it morbid curiosity, but i almost wanna see this happen in a video. In a controlled enviroment without anyone around.
    I guess thats due to Mythbusters being cancelled.

  9. If a military rifle of at least sixty years of past has battery off securty engaged solely to the tension of a spring, there should be a over serious design fault in its construction. The springs will weaken and even break through time in using. No rifle designer can be fool enough to be in such an unbelivable approach. There should be another reason.

    • Chances of out of battery fire are almost none with all conventional designs (including type like Famas). This is built-in into 2-pice bolt ass’y design – secured by fact that firing pin will not reach primer. That simple.

      This having said however, concretely with roller locking, it does not mean that when action is “in battery” that the action is locked. Firing pin may not reach primer, but some other effect may cause initiation – for example if slam is strong enough and/ or gun is real hot.

      I had one time chance to see locking shoulder from heavy barrel select fire FAL which had visible ‘step’ about 1/3 down pushed into it which means that action was not fully closed when fired. I concluded that this was caused by self-ignition when firing with (very) hot gun. It happened very, very fast right at closing; maybe even before.

      • “hot gun”
        To prevent unintentional activating cartridge .30 caliber Browning tank machine gun was equipped with bolt latch, http://browningmgs.com/Info/BoltLatch.htm states that:
        The Army conducted extensive tests to determine how the heat build up would affect the new air cooled Browning.� The tests determined that after firing 400 rounds at a rate of 400 rounds per minute that a chambered cartridge would cook-off in about 16 seconds.� After firing another 500 rounds the cook-off time dropped to 11 seconds. This, rightfully, caused much concern,� on November 2, 1918, 9 days before the Armistice ending WWI was signed, the Army ordered that the bolt latch be fabricated and mounted on the Tank guns.

      • Usually, the cause of slamfiring comes from stuck firing pins either by breaking or by fouling and changing the firing pin spring generally may solve the problem by the reason of both detecting the broken part or simply cleaning the firing pin recess through dismounting. However, in G3 derived rifles, there it be a lever at top side of firing pin housing functioning as the bolt carrier retainer and its rear tip, by a clever forming, can be used as firing pin arrester until the carrier to go its foremost position. IMHO.

        • The part you are describing is in the manual called “palanca de bloqueo”(stop lever). It arrests Bolt assembly with respect to Carrier in one of 2 positions – opened and closed. Basically, it is there to prevent Bolt bounce. It is common on all roller-locked HK firearms starting with MP5. To extend its function to retain FP might be little too much to handle.

          • HK’s new M16 lookalike rifles have such a battery off safety in the location where original gun’s carrying gas intake pipe hovever. Transmiting the idea to old roller delay rifles should not be difficult. The company even got a patent for this valuable innovation.

          • I’ll have to look that patent out

            I bet it’s a re-tread of the blade infront of the mainspring flange on a Mauser 98, that’s purpose was to prevent a broken striker from reaching a primer unless the bolt was locked when the striker broke.

            98 strikers could break due to repeatedly being dry fired.

            H&K seem to have got a lot of mileage out of later patenting stuff that was invented pre and during WWII in the Mauser Werke.

          • Since the new piston stroke M16 lookalikes do not use gas intake pipe, this valuable innovation seems as using that useless space for a usefull purpose. IMHO.

  10. Re the “cumulative” increase in cyclic rate:

    The bolt velocity in recoil after the second shot might be faster due to the absence of resistance from the hammer but it is also slowed down from having to arrest and reverse the momentum of the bolt as the second shot is slam-fired by inertia.
    Which influence is greater can only be measured by empirical measurement.

    However, there is no explanation why there should be any progressive increase in cyclic rate after the second shot.

    M

    • I tend to think that way too: there is a definite rate of elasticity in materials which seemingly increases with growing temperatures, but it does not. If anything, it slightly decreases since material ‘softens’. Springs are the first which show slowdown.

      But the point is: did the assembly achieve its maximum ‘statutory’ rate of elastic response or not? Therefore, initially it may look that increase of ROF is progressively increasing to alarming value. It will and cannot go to infinity, even if gun stays integral.

  11. Some thoughts at following;

    -This is roller delay blowback gun. That is; the faster the bolt carrier, the slower the bolt, resulting, only the initial hammer cocking force should not effect the eventual recoiling speed of combination.

    – In case of well made flat would work in due course, there should be no additional speed for the bolt combination than the normal, it should be another cause for ascending speed.

    – Hammer friction under the bolt should be present all along the travel of bolt combination reciprocal movement. In case of its absence, speed would rise both rearward and forward. If the hammer stops deeper than normal, this would be the cause. This kind of a issue should be looked for.

    – There might be cause to compress the recoil spring than normal. Its back stop might be on a place nearer to the front and this might be another cause for rather than normal speed.

    – And other possible reasons like these… IMHO.
    ,

  12. Just a note on Idiots bump firing

    there’s a truism, that it is impossible to miss fast enough to win either a fight or a match.

    • Next you’ll be saying you disapprove of folk firing soda cans out of AR barrel replacement extensions, you clearly don’t have a population of ravenous 2 litres roaming wild in your patch.

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