Back in the 1960s, the US Navy decided to rechamber a bunch of its M1 Garand rifles to 7.62 NATO. The proper way to do this would have been to simply put new 7.62 barrels into the rifles – but the Navy wanted a cheaper route. So instead, they decided to make chamber inserts that could allow the use of 7.62 NATO in existing .30-06 barrels. The notion was that the chamber insert could be fixed in place simply by slipping it into the chamber of an M1 Garand and firing two rounds of M80 7.62 ball ammo to seat it in place. No threading, no adhesives – just a pressure fit. Well, Springfield Arsenal took several of these rifles and tested out the conversion.

We have a copy of the original test report, dated October 30, 1964, which you can download right here (complete with dimensioned drawings of the chambers and adapters):

US Navy 7.62 NATO M1 Garand Conversion Report - 1964 (English)
US Navy 7.62 NATO M1 Garand Conversion Report - 1964 (English)

Well, the test results were less than spectacular. Springfield determined that the chamber adapter dimensions didn’t quite fit either the .30-06 chambers OR the 7.62×51 cartridges. So the procedure to properly install one was to first clean the chamber, then use a special reamer to open it up slightly. Then clean the chamber again, and load the chamber adapter and a round of 7.62. This could require a couple attempts to fully seat, since the adapters were 0.008 too tight at the neck to fit the 7.62 cartridges. Once the round and adapter were chambered and the bolt fully closed, the round would be fired, and then a second round also fired. This would fully seat the chamber adapter, after which a second special reamer was used to open up the adapter to the correct dimensions.

In addition, the gas port on the barrel needed to be enlarged. The M80 7.62 cartridge developed greater velocity than M2 .30-06, but had less pressure at the gas port, hence the larger port required. Both .0995 and .1065 inch gas ports were tested, with 0.1065 being found best for proper functioning of the rifle. However, when grenade launching was tested the 7.62 conversions were found to have dangerously high bolt opening velocity, and deemed unsafe.

Folks with commercially-made .308/7.62 Garand rifles should take note of this, as it applies to these rifles as well. It may be tempting to get some blanks and dummy grenades to launch for fun (the thought has certainly occurred to me) – but don’t! The M1 gas system when opened up for cycling with .308 produces too much pressure in this situation for safe operating.

Anyway, back to the Navy conversions. The testing evaluation found a slew of objections to the chamber conversions. In addition to the multiple reaming steps required for proper installation, the adapters were still liable to come out unexpectedly. Use of a broken shell extractor could also loosen an adapter. They left about half an inch of freebore for the 7.62 bullet to travel before engaging rifling, which hindered accuracy and increased throat erosion. They tended to have out of spec headspace as a result of the conversion. Needless to say, the test conclusions suggested just replacing the whole barrel to convert an M1 to 7.62 NATO. The Navy eventually chose this route, but many of the adapters were reportedly sold as surplus, and may be encountered today.


  1. I remember seeing these adapters being sold in Shotgun News back in the late 90’s and early 2000’s. However I believe these were commercial/aftermarket inserts.

    • I have a question. Let’s assume the person I’m dealing with is telling the truth and the M1 he has is a navy done new barrel conversion with a #’s matching bayonet. What’s the ball park figure on that rifle in great condition.

  2. Hi:
    Nice article. I recently acquired one of CMP’s navy barreled actions (reamed; not sleeved).
    I originally installed the CMP plastic safety spacer which prevents accidentally chambering
    a 30.06 round. I discovered another steel spacer (Italian) which I like better for durability.
    I am very happy with the 7.62 NATO conversion.
    The CMP stripped barreled action presently costs $350 (non sleeved version). I’ve had it out
    to the range and fired several 7.62 NATO rounds through it without a malfunction.
    It’s a little pricey buying the other needed parts, but I’ve had fun putting it together….
    Don in Salem, VA

    • I was thinking about doing the same thing with one of the CMP Navy 308 barreled actions. Could the conversion be done by simply swapping the 308 barreled action with the original Garand 30.06 action? What else is involved?

  3. i HAVE ONE OF THE “NEW BARREL” Naval Garands” SA SN
    well below 1,000,000. Shoots well and in exc.cond. Hope to leave it to my kids.

  4. Note, there should be NO bolt opening velocity for the use of grenade cartridges. The grenade launcher bleeds off the excess gasses by holding the gas cylinder lock screw relief valve open during firing. All you should get is recoil from the launcher. If this simple fact is in error, how factual is the rest of the article?

  5. Waaaaaaaaay back in the 80s, when I was a wee lad, I participated in WWII reenactments and was in a group that portrayed A/1/501 PIR of the 101st Airborne. I got to borrow a pal’s new Springfield Armory M1 Garand that was chambered for 7.62. I remember not being able to cycle blanks (probably de-linked M60 blank rounds) through that thing. Possibly because the BFA was set up for the higher pressures of the .30 (30-06) ammo? In any case, it was a beautiful rifle and I hope I didn’t screw it up!

  6. I’m a current WWII reenactor, and I used a current manufacture chamber insert converter to convert my M1 to 308. However, the key difference is that I used permanent Loctite (you can use non-permanent, however I replaced the original 30-06 barrel with a junker, as the vast majority of the time I would be shooting blanks). After 8 years and countless thousands of blanks, and some live 308 thrown in for good measure, the insert has not come out (knock on wood). As in the article, I did have to open up the gas port.

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