View Full Version : M-1 Carbine Bolt Question

Col. Mosby
February 12, 2011, 12:31 PM
I have a Underwood carbine that has the round M-2 bolt in it. May I replace the round bolt with a complete flat style bolt without affecting the headspace? How is the head space determined on the carbine? Thanks for any and all help.

February 12, 2011, 01:31 PM
Since the M1 is a blow back opperated carbine, you should be able to do this with no problem. FYI - the M2 bolt was supplied as a field drop in replacement part with no gun smithing required. Don't see why you can't go the other way. The M2 bolt will work just as well as the M1 bolt. I don't see much reason to change it.

Col. Mosby
February 12, 2011, 01:58 PM
Thanks for the info. I was entertaining the thought of putting the carbine back to it's original configuration. Do you happen to know when the M-2 bolts first started to be used?

February 12, 2011, 03:49 PM
The bolts can normally be dropped in if the chamber and receiver are within spec, but that should be verified, IMHO. They are not, however, blowback operated, as most .22 rimfire semi-auto systems are, for example. The M1 bolts are gas-operated rotating bolt actions, operated by a short-stroke gas piston system, usually called a tappet and slide system.

February 12, 2011, 03:53 PM
Whoa! The M1 carbine is NOT a blow back design. It is an air cooled, locked breach, magazine fed rifle. Headspace needs to be checked if swapping bolts. This is done with go/no-go gages. This rifle develops 46412 psi chamber pressure. Nothing to fool with and be wrong!

February 12, 2011, 04:11 PM
Sorry, you're right about the breach lock but the pressure is a bit high. More like 36,000. It doesn't have much more power than a 357 magnum.

February 12, 2011, 04:26 PM
That chamber pressure figure I quoted is from my QuickLoad software and is the MAXIMUM for the cartridge. Most loads are 35-40000 psi.

In late 1943, Inland experimented with a fully round bolt for the M1 carbine. The round bolt design proved successful and was adopted for the M1 carbine in March 1944. These are considered Type III bolts. M2 carbines were a development of the Korean war, but the round bolts were used in WWII in M1 carbines.

James K
February 12, 2011, 07:29 PM
Round bolts were adopted as easier to manufacture and also because there had been some cracked bolts of the earlier type reported. The round bolt was standard for the M2, but was used in original production and as a replacement bolt in both M1 and M2 carbines.


February 12, 2011, 07:39 PM
The key point here is that M1 Carbine bolts, like virtually all rifle bolts DO NOT just drop in with no safety check.
They need to be checked for proper head space.

This was true in WWII when they were new, and it even more true today more than 60 years later when the rifles and bolts are all used and have worn.

Col. Mosby
February 12, 2011, 08:16 PM
Thanks to all for their imput. My Underwood has the round bolt and shoots like a champ as it is. Like they say, "if it aint broke don't fix it". I just wanted to have my carbine to be in proper WWII configuration. It appears that it is. Thanks again to all for helping this novice. :)

February 13, 2011, 10:56 AM

Note that because QuickLOAD was developed in Germany, the default maximums are CIP rather than SAAMI specs except where (SAAMI) appears in parentheses or U.S. Military specs. That's how the .30 Carbine has a nice even 3200 bar or 320 MPa maximum pressure in QuickLOAD (depending which units you chose) and has all those odd trailing digits. SAAMI specs for Maximumum Average Pressure (MAP) are always rounded to the nearest 500 psi.

Note that the military and the CIP both reported copper crusher results as PSI up into the mid-90's, rather than segregating them into CUP and PSI as SAAMI did. This causes a lot of confusion. Many people still believe commercial .308 is hotter than military 7.62 because of that, when they are actually the same if measured in the same test apparatus.

In the case of the .30 Carbine, it's pressures run in the range over which copper crushers sometimes report close to or even higher results than Piezo transducers do, give or take 2,000 to 5,000 thousand PSI. This is an artifact of the measuring systems having an irregular relationship such that math conversions from one to the other cannot be relied on not to change the real pressure limits. Fortunately for us, guns are pretty tough and tend to withstand the irregularities.

.30 Carbine Maximum Average Pressure (peak)

CIP MAP = 320 MPa = 46,212 PSI
SAAMI MAPs = 40,000 PSI and 40,000 CUP (old)
Military MAP = 40,000 PSI (actually CUP reported as PSI)
Military Proof Load (Blue Pill) = 47,500 PSI (actually CUP reported as PSI)

The CIP spec used to match the military spec pretty well when the CIP still used copper crushers, too. At that time it was 280 MPa or about 40,600 PSI. If I do my own trend analysis of MAPs, the expected change would have been to about 43,000 PSI or 295 MPa (2950 bar) going from crusher to Piezo transducer, but CIP typically just uses a fixed multiplier in switching from copper crushers to Piezo transducer numbers, and that's not always realistic. There's just too much noise in the relationship. For that reason I tend to trust the SAAMI numbers more. There are other complications in that the CIP Piezo transducer ports aren't located in the same position as SAAMI test devices are. This can fudge the numbers another couple thousand PSI.

Unfortunately, only measuring military reference loads designed to produce the exact correct MAP, and doing it with both SAAMI and CIP Piezo pressure test rigs could resolve this. I wish I had the gear available, but don't.

I expect that makes it all as clear as mud. Here's a place (http://kwk.us/pressures.html) with some SAAMI and CIP pressures listed, though the CIP numbers are rounded PSI numbers in this table.