Headspace on a M1 Carbine if you change out bolt


November 11, 2010, 06:12 AM
Just a quick question,
After a search of the forum I have not found a answer.
On the M1 Carbine if you swap out bolts (like if you are trying to switch to a flat bolt) would this effect your head space? Any other issues I should be aware of?
Thank you in advance.

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Lee Roder
November 11, 2010, 08:56 AM
It may or may not. Bolts on all five of my carbines each headspace correctly with any other barrelled assy. Probably just lucky. You really must check it with a field gage yourself and assume nothing.


November 11, 2010, 12:13 PM
On the M1 Carbine if you swap out bolts (like if you are trying to switch to a flat bolt) would this effect your head space?

Yes, it will affect headspace. That's how armorers rebuilt M1 rifles and carbines; they would have a bucket of bolts and try them until the found one that appropriately headspaced. Some bolts are longer than others, and lug wear varies.

Having said that, you may very well be able to swap bolts with no appreciable change in headspace. It's an experiment.

You really must check it with a field gage yourself and assume nothing.

November 11, 2010, 12:43 PM
I would really question how critical headspace is on the .30 carbine, as long as it is close enough for the extractor to snap over the rim when it chambers the round and fires when you pull the trigger.

The straight tapered case is not going to stretch & break like a bottle-neck rifle caliber with excess headspace anyway.

For a quick and dirty test, get yourself a roll of Scotch Magic tape.

Start applying layers of it to a case head & trimming it even with the rim until the bolt won't close on it all the way.

Each layer of tape is about .0025" so two layers = .005", etc.

It should close on one or two layers, and I really doubt four layers, or .010", would hurt anything.


Jim K
November 11, 2010, 04:01 PM
I agree with rcmodel, the combination of low pressure and a straight case greatly reduce headspace concerns with the .30 Carbine. I would just replace the bolt and fire the gun. If there are no signs of trouble, no problem.


November 11, 2010, 08:21 PM
Thank you guys for your help.

Mac's Precision
November 11, 2010, 09:53 PM
Head space on the .30 Carbine should be a non issue. I would bet that you could fire that with .030 and get away with it. I would suspect that misfires due to excess head space would be a bigger issue than pressure problems. Obviously we want to have as little head space as we can but that being said I have seen some rifles with a LOT...and ran fine. Head space issues just aren't as big an issue as some folks assume.


November 4, 2012, 10:38 AM
I have read all the responses but have a question.
I am not an expert but it seems to me one can simply take of the charging handle spring and slip a live factory round or a non primed resized brass (to be safer) under the extractor and close the bolt.

If the bolt closes easily and does not rattle or move when you wiggle it with your finger you are good to go.
I am as always open to other's ideas about this as I recently had a Underwood WW2 bolt crack and bought a new old stock GI bolt for one of my two M1 Carbines and the above is what I did with the trigger assembly off of the gun.
I am considering buying a field gage for it but money is short right now.

November 4, 2012, 11:23 AM
See post #4, and the later posts by others.


November 4, 2012, 11:57 AM
Remember also that the .30 carbine headspaces on the case mouth like most autoloading pistols. If your bolt has slightly excess headspace, perhaps trimming the cases a little less (longer) can compensate for the added room at the back. That is of course if you are wise enough to reload.


November 5, 2012, 03:03 PM
Practiced by very very few, it is possible to determine the effect the replacement bolt will have on the length of the chamber, a more common practice is order a bucket of bolts then try them one at a time. Problem, no one knows the length of the chamber before they start.

then there are reloaders with a press with threads, and dies with threads, my presses and dies have threads, threads make my dies in my presses adjustable, I use the length of the case to off set the length of the chamber.

F. Guffey

November 14, 2012, 11:36 AM
FWIW, when I was in the Air Force I was an armorer and that was way back when they used M2 carbines. We always stored the rifles in the vault minus bolts for security. Bolts were always tagged with string tags with the SN of the rifle, but when reassembling the bolts to the rifles there were always a few cases where tags came up missing. We always checked headspace before firing them, but to my recollection, we never had a problem with a bolt not functioning properly.

I'd say put 1-2 thicknesses of tape on a case head, and see if that still allows the bolt to close. 3 thicknesses or more probably should not work. Or, just buy a field gauge from Brownells if it worries you. After checking your headspace, you can sell it on evilbay for about the new cost. Hey, it is new, right?

November 16, 2012, 10:09 PM
I never had any problems swapping bolts. The only time I had trouble was with worn locking lugs (.020) on an M2. It was kind of neat as it would blow the primers out, simplified one step when I would reload the brass.

November 24, 2012, 09:19 PM
One reason for controlling headspace is to keep the case from moving rearwards under pressure and pounding the bolt face, and causing setback to the bolt lugs. Continued firing with excessive headspace will make the problem grow worse.

30 Carbine is a ~ 40kpsi cartridge. It is medium pressure compared with modern sporting cartridges, but I would not call it low pressure. It is a tapered case, not straight walled, so will have more bolt thrust than a cartridge with some amount of straight wall sticking to the chamber walls.

Be careful when you light the fuse on tens of thousands of psi a few inches from your face.

November 25, 2012, 10:34 AM
I have built, rebuilt and re-barreled several carbines. They normally head spaces just fine. I have experienced short chamber space when replacing new Winchester barrels. I have one now that is short and will require a reamer. The carbine head spaces on the end of the cartridge therefore you should check it with a go gauge and a field gauge.

December 19, 2012, 04:16 PM
Has anyone replaced a barrel due to head space issues or do you only replace them due to muzzle/rifling wear? A friend has a Postal Meter that recently had a cartridge have the case almost separate and had the primer almost push out. Is this a case of the head space being too large? Would you replace the bolt to solve the problem? Just starting to learn about the M1 Carbine. So any thoughts would be appreciated

December 19, 2012, 04:37 PM
I would not replace anything until I measured the headspace and probably a chamber cast as a minimum. Careful inspection and measurement of the gun using appropriate print specs to verify everything is in spec would be best. Ammo would be suspect, too, until I ruled it out.

December 19, 2012, 05:33 PM
How do you do a chamber cast?

December 19, 2012, 05:43 PM
Chamber cast ...


December 19, 2012, 05:46 PM
Get some cerrosafe from Brownells. Follow the instructions to the letter. Get the headspace gages and the Kuhnhausen book on the service rifles, too.

December 19, 2012, 07:28 PM
Thanks for the info!

December 19, 2012, 07:34 PM
The shop manual BBBBill was kind enough to reference can be found here (http://www.midwayusa.com/product/660048/the-us-30-caliber-gas-operated-carbines-a-shop-manual-book-by-jerry-kuhnhausen) from Midway.

Canuck-IL was kind enough to provide a link to chamber casting alloy.

Forrester headspace gauges from Brownells can be found here (http://www.brownells.com/gunsmith-tools-supplies/measuring-tools/headspace-gauges/forster-headspace-gauges-prod19183.aspx).

The problem is that head space gauges run about $35 EA so if you get a set of three (Go - No Go- Field) you have about $105 in gauges. That plus $32 for the shop manual. Toss in the casting alloy and when all is said and done you have about $150 in tools and gauges.

The .30 Carbine cartridge while not real strange is not as common as most in how it is head spaced. You may want to give this link a read (http://www.m1carbinesinc.com/safety.html) and scroll down to Head Space. Something else to consider is when checking the head space on a 30 Carbine the rifle is field stripped and then the bolt needs stripped completely removing the firing pin, extractor and the ejector. If you do this watch yourself as things in there are spring loaded and tend to go flying. The bolt is like a miniature M1 Garand bolt in miniature and re assembly can be a challenge without the proper tool. Not impossible, just not fun. The ejector and ejector spring don't always cooperate.

A Google of "Making a chamber casting" will show you plenty of examples on how it is done. While not a 30 Carbine this video (http://www.youtube.com/watch?v=voMp0IQwDLU) does cover making a chamber casting.

So the question becomes if you want to make the investment(s) to check a single rifle or take it to a competent smith who is familiar with the 30 carbine and has the right tools to work on one?


December 20, 2012, 12:30 AM
In 10 words or less, no,

I'm, if not the only person that knows how to switch out bolts, I am one of a small group that can change a bolt and get it correct every time without a go, no or beyond gage, convincing someone it can be done is another matter. It is helpful when the length of the chamber is to remain the same length, reduced or lengthen.

On the Internet the advise has always been, purchase a bucket of bolts and start replacing and checking, and hope one of them solves/remedies the problem.

F. Guffey

345 DeSoto
January 4, 2014, 01:47 PM

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