M1 Carbine gas piston

[eb in oregon]

I'm well aware this topic has been beat up, tossed about, argued about, and discussed numerous times, but it bears mention again.

I've owned and shot M1 Carbines for over 45 years. My first rifle purchase after finishing my first tour in the Army in 1974 was a Plainfield carbine. It was identical to the military carbine except for a birch stock with a plastic butt plate and it worked flawlessly. Until it was stolen during a home robbery when we were at work over 30 years ago. However before it was stolen I'd experienced gas pistol/cylinder problems from shooting quantities of lead bullets in it. One day it just quit working.

I found the gas piston stuck in the extended position and it wouldn't budge. Disassembling the piston (I've a set of armorer's wrenches and tools) I found the gas cylinder packed with lead. It took some careful work to carve it out without scratching the bore. Once that was done it worked like gang busters again, up until it was stolen.

But then last year I was shooting with my older brother and he was shooting his M1 carbine and it was having functioning problems. I brought the carbine home, disassembled it, and found the gas piston stuck in the extended position. The gas cylinder was full of bits of lead, thus rendering the piston's function inoperative. I cleaned it and reassembled it and test fired it. It worked like gang busters.

So, the other day I was cleaning another Plainfield I'd acquired a few years back after firing it after it had been sitting for a couple of years. The piston was stuck and refused to budge after removing the piston nut. Not wanting to damage anything I cruised the internet for an hour or so looking up the subject. One guys post said "If it works don't mess with it." So I put it back together and fired three rounds. It worked perfectly. So I took it apart again to find the piston still stuck. This really bugged me so I took the nut back out, reassembled it, and fired one round with a magazine with a bolt stop follower. The gas piston was found inside the stock, no damage anywhere.

I found a "coin" of lead at the bottom of the gas cylinder, along with some nasty carbon. So I carefully carved it out and cleaned the gas cylinder removing all lead and carbon. What was happening is that when firing the piston would do it's job but when pushed back in by the slide it would stick to the lead thus capturing the piston. While it worked before the question is "for how long?" Having lead and carbon in the gas assembly is a bad thing. It gums up the works, has nowhere to go, and will stay within the gas cylinder until it is cleaned out as described.

So, I've had three carbines I've worked on for the same issue, which is the heart of the M1 Carbines function. The very first time I encountered the problem I stopped using lead bullets in my carbine. The Plainfield I now have I've shot nothing but jacketed bullets through it, so whoever owned it before me shot a bunch of lead bullets through it. My brother had shot lead bullets in his, bullets I'd given him. While many people shoot lead in their M1 Carbines I'm convinced it's not the best thing to do as it blows small bits of lead into the gas cylinder and eventually they will have this problem. I myself don't want to make disassembly of the gas assembly a part of normal maintenance so I'll stick with jacked bullets.

This is only for informational purposes, I'll not tell anyone what they can or can't shoot in their firearm, but it is information that could save heartache for others in the future.

Information

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[jimb16]

For what its worth....I've had literally dozens of carbines. I shot lead rounds in every one of them including a registered M2. Out of all of them, I had lead in the gas cylinder in exactly one. When the gas port was drilled, they left a tiny burr on the mouth of it. It didn't seem to effect any jacketed rounds, but boy did it shave lead. None of the others ever exhibited any build up of lead in the gas cylinder. That one was a Saginaw SG. I guess it depends on how carefully the gas port was drilled.

[painter777]

Nothing better than a informative post Thank You Eb.

Too bad the Thief got away with your Plainfield 'After' you had it fine tuned.
On your test fire, I'd of had to of spun the nut on a couple of threads, just to knock down the chances of any damage from the flying piston.

I've always wondered about the effect of a heated up barrel and some of the soft cast lead bullets.
Anyone else ever wonder if there is enough heat to partially melt some of that lead and it ending up in the gas cylinder?
Ordnance called for FMJ Ball.

I really don't shoot lead, not even the FMJ's with the small diameter lead tip. Then again I don't hunt with a carbine. But I do keep a few mags loaded with FMJ LHP tipped that cycled dependably if needed for home defense.

I can say I was warned by Bruce McAskill to watch out for shavings from Plated FMJ's..... and I have found numerous copper flecks the size of large metal flake when having to service Carbines that have shot many of the Plated rds.
This summer I had a helper and serviced our Rifle Clubs Carbines. We ran in to about 12 ~ that had to have been using corrosive ammo. We found a few crates of the French head stamped ammo stored with them. A story in it's self, but don't shoot corrosive, if you seen the gas system afterward without being cleaned you'd know why.

I posted about a year ago asking about the lead bullets with that washed coating on them. At the time tossing around the idea of buying some to try.
Then got back to thinking about the heated up barrels after burning/melting a hole in a Caldwell sand bag rest that I'd set the barrel on after numerous mag drops while going down range to see and set targets.

Someone posted a bore scope picture of their Carbine barrel that showed a burr inside the barrel from the gas hole being drilled that was catching lead. The burr was just past the gas hole, maybe deflecting some fine shavings in to the cylinder? I don't recall if they were having any issues or just caught it playing with their new bore scope. Or if they had reamed a bigger hole or if it was the original hole.
I've used lead removing solvents on some other arms with good luck. I believe Kerosene 'May" be the main ingredient but would have to double check.

Eb the OP,
Made a good point above in that he had the proper tools. Don't attempt pulling the gas piston and nut without the correct wrench. Often you'll need to chase the threads because of disturbed staking.

Stay Safe All,

---------- Post added at 10:17 PM ---------- Previous post was at 10:13 PM ----------

When the gas port was drilled, they left a tiny burr on the mouth of it.

Jim,
I was just typing about seeing a bore scope picture with a burr on the down barrel side of the gas hole that was doing just as you said... Shaving a little lead.
Maybe blowing it in during the back flush?

[eb in oregon]

On your test fire, I'd of had to of spun the nut on a couple of threads, just to knock down the chances of any damage from the flying piston.

Actually, in my opinion, the gas piston can do little damage from "flying" around. By the time it's through it's stroke the bullet has already left the barrel, it will impact the slide like it always does, and it will either bounce into the barrels channel or possibly back into the gas block. As the bolt was held to the rear by the magazine I carefully removed the magazine and gently let the slide go forward and then took the rifle apart. The piston was pinched between the slide and gas block. No marks or other damage was noted and after throughly cleaning the gas cylinder, nut, and piston it went back together with no issues.

As for burrs in the barrel from drilling the gas port I'm skeptical. As a machinist for years I've dealt with plenty of burrs in metal from machining and it would take a heck of a burr to withstand hundreds of bullets being shot over it. The bullets themselves would (should) wipe that burr off pretty quickly, unless it is a very stout burr. Like I said I'm not about to tell people what they can and can't shoot, but I'd prefer to not take the chance. It doesn't save all that much money in the long run and buying bullets in bulk works for me.

[USGI]

I bought an Underwood 2-3 years ago that had a loose piston nut - was unscrewed about 1/2 way. I've had several of my other carbines apart, but this Underwood is the only one I've ever found to have lead built up inside - just like it was plated with it. I removed it carefully just like Eb described and checked for burrs in the barrel too, but looked fine to me. - Bob

[floydthecat]

Not only lead fouling, but underpowered ammo (commercial or reloads) can foul a gas system. One might have the urge to shoot what barely works to prevent beating-up a gun, but underpowered low-pressure loads leave unburned powder and carbon residue that can gum-up the gas system. The propellant burns cleaner under higher pressure. I have one carbine toy that I play with for the specific purpose of testing reloads. Usually, after a couple hundred rounds the orifice requires cleaning. Shoot the good stuff, it’ll run better longer 😁.

[browningautorifle]

I shot almost exclusively lead gas check bullets and usually 2400 powder. I think gas checks keep the lead from melting or shaving as I never had issues and had about 25 different carbines including one M2...an Inland.

[jimb16]

The cast bullets that I've shot are straight clip on wheel weights. I cast the Lyman 130 gr plain base, 120 gr Lee RN with a gas check and the 115 gr Lee with a gas check. As far as the burrs go, please remember that drilling a hole at 90 degrees is a lot less likely to leave a burr than drilling at an angle. The gas port is drilled at an angle. Even so, it is the exception, not the rule when it comes to leaving a burr (maybe 1 out of 50). BTW Painter, pure soft lead melts at a higher temp than alloys. So alloys would be more likely to melt at shooting temps than pure lead! You would think that harder bullets would melt at a higher temp, but they don't!

[eb in oregon]

As far as the burrs go, please remember that drilling a hole at 90 degrees is a lot less likely to leave a burr than drilling at an angle. The gas port is drilled at an angle.

Actually drilling a hole on an angle isn't subject to leaving a burr any more than a 90 degree hole as the real reason for creating a burr is more related to drill pressure (how hard are you leaning on the drill bit) and the sharpness of the drill bit. Leaning on a dull drill bit is a sure way to leave a nasty burr regardless of the angle. There will always be a small burr or two, but if the drill was sharp and the operator wasn't putting undue pressure on the drill bit they will be minimal and easily removed. Just cleaning with a bronze brush should be enough in most cases.

[DaveHH]

One of the most difficult metal working lessons is using drill bits. The way it is supposed to be done is a slow turning sharp bit with a dab of oil. You can literally watch a ribbon of material cut off the work. Sadly most people take an already dull bit at high speed and push with everything they have, resulting in an even more dull bit and scobbed up work material. Hopefully after many years the light blinks on and drilling holes in steel becomes an easy task.