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.
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.