Barrel Install on M1 Carbine

Hi I'm new here and this is my first post
To the proficient and knowledgeable, could one of you tell me how to measure the draw on an M1 Carbine barrel installation? I took my carbine with a new barrel to a gunsmith and asked to have it installed. Also installing the front sight and the gas valve. When I got the gun back the barrel was still .025 from tick mark to tick mark. This also made the front sight out of alignment. I was told to just adjust the rear sight and it would be good to go. I was under the impression you lined-up the tick marks so everything was timed and square. I took the barrel back off the receiver and then hand tightened it and measured mark to mark. It measured .230, which is about 29.3 degrees. Can this be reinstalled to the marks. If I reinstall it the headspace will be reduced by .00044 if my calculation are correct.

Thank you,

Hobberdobber

The barrel needs to be indicated with the front sight at top center. This should put the gas block and tapit at bottom center. It should also put the flat on the right hand side 90 degrees. The flat is also important because the operating slide rides in a grove in the flat. I am curious if you can remove the barrel yourself you should be able to install it yourself or did your gunsmith only have the barrel hand tight? I am suspect of your gunsmiths mechanical/technical prowess regarding M1 carbines. Also if memory serves me the thread pitch for the carbine is 16tpi. With that in mind thread crest to crest is .062. So .062 divided by 360 degrees times 29.3 degrees should give a headspace decrease of .005.

Depending on the receiver quality you may also find that the barrel threads contact the inside surface of the receiver prior to the "torque tab" hitting the inner torque surface in the receiver. You may also have to remove a bit of material off that small tab protruding from the chamber end of the barrel to make it mate up just right. I think if you get it to within 1/16th shy when you hand tighten you should be able to torque it on and line up the two indicator marks(one on barrel one on receiver).

Edited to add that a pull through reamer to finish it up will be required most likely.

I would not trust tick marks... On many rifles they are assembly marks for that barrel and receiver only..

The Carbine repair manual, says to use the marks, but that is with GI parts only.. (The flats on bottom of barrel and receiver, can be also used, per manual)

Be sure barrel extractor clearance cut, is is proper location.

Use long straight edges from a flat surface on barrel and receiver. Adjust barrel until they are parallel.

Indicating off of flats, is also doable, just takes more time...

4 tenths headspace on a M1 carbine, is a non issue.. Length of the cartridges vary more than that...

As to .005 go check your math... . 25 thou rotation (at ~`1 in OD) is not 1/12 of 360 degrees or 30 degrees..

Headspace is only properly checked, when the barrel is in its proper position...

And if it is a US military carbine, receiver quality should be OK...

I think the barrel tenon thread pitch on an M1 Carbine is 20TPI.

GGaskill said:

I think the barrel tenon thread pitch on an M1 Carbine is 20TPI.

Click to expand...

That's correct.
I just did a 30 carbine barrel swap today and measured the thread pitch so that I could calculate how much to face off the barrel to get it to clock properly with about 1/10 of a turn of draw.
I ended up having to take off 0.006" as the barrel was clocking 90 degrees off when hand turned into the receiver.
The new barrel is a Criterion and they come 0.010" short on the chamber as well so I expect to have to ream the chamber about 0.012" to 0.016" to get it into headspace spec once the reamer and gauges get here.

Just a caution. Many of the NOS USGI barrels sold in the last twenty years are products of a Vietnam-era program that sought to increase the dispersion by decreasing the rate of twist in the rifling specs. These barrels function well, but may not give the 100 yd. accuracy results you desire. Regards, Clark

So who cares about 1/2 a thousandts less head space.

Carbine receiver threads are 20tpi.

My experience with measuring new factory ammo for .30 Carbine leads me to believe that case length (and necessarily headspace) is not tightly controlled in this caliber. I found cases that were as much as .006" longer than max. Best results will come from trimming cases after each resizing to maximum length so you have minimum headspace.