When the M1's gas cylinder lock is screwed onto the barrel, it applies a light pressure against the gas cylinder. My question is what stops rearward movement of the gas cylinder? I'm thinking the gas cylinder and forward hand guard assembly are equally captured between the gas cylinder lock and steel "Lower Band". That doesn't' sound right because it would compress parts that would influence vertical alignment of the barrel.

Lots of loose parts here. I also need to peen my barrel splines. Lots of work to do.

Thanks

There is a boss on the barrel that should stop the gas cylinder and front flange group. That is, the flange that holds the front of the stock to the barrel, the gas cylinder assembly are compressed onto a circular stop on the barrel by the action of the figure-8 nut. The "timing" is such that you want to figure-8 nut to start being tight at the 5:00 position looking down the barrel, and such that you can turn this nut to the standard 6:00 position by using your fingers with increasing tension. This tension holds the back of the gas cylinder group and the front flange to the barrel rather tightly but not as if yo had torqued down a cylinder head. Firm but not stiff, as it were.

May times new guns are not "timed" correctly and a shim kit ($12) is required to make all the parts fit correctly.

The previous post is for M1A or M14, which uses shims to align the barrel gas port with the gas cylinder port.  

Are you asking about a M1 Garand?  If so, the thing that prevents the gas cylinder from going further is the gas cylinder lock.  It essentially locks the gas cylinder in place when the plug is threaded through the lock and into the gas cylinder.

On the M1 Garand you really don't want the gas cylinder touching the front handguard.  Guys that tweak M1's for accuracy will try to match hand guards to gas cylinders so there is just a little cclearance.  Otherwise you put some additional load into the barrel that can affect accuracy.

The front gas lock will push the gas tube / front sight assembly back into the splines cut in the barrel and more or less lock it in place.  There are some good tips out there about how to check the fit of the gas tube, gas lock, and front hand guard.

http://m14forum.com/m1-garand-accuracy/135677-garand-gas-cylinder-questions.html

On the Garand barrel there is a small chamfered shoulder that a properly timed Gas Cylinder Lock should contact. Even if the GC Lock does not contact the chamfer the nearly twenty 9/16X28 threads on the barrel are more than enough to secure the GC Lock to the barrel and the GC Lock Screw secures the Gas Cylinder to the GC Lock.  As Stated the front Hand Guard should not be placed in a bind with the Gas Cylinder, it has a negative effect on accuracy.

Yes, M1 Garand

the gas cylinder wants to slide a long way down the barrel, even after the figure-8 lock nut has bottomed out on the shoulder at the bottom of the threads. It's nowhere close to being wiggle free.

I'll have to think about a solution to this. I also understand the splines on the barrel and gas cylinder cannot be a true taper otherwise it could be difficult to remove.

I should take a closer look at the barrel anyway, as it is kinda rough. I have another gas cylinder to compare this one with.

When Gaands were originally assembled they were not timed with gas locks.
Peen your splines, tighten gas lock until there is a very small gap between the handguard and gas cylinder, install gas screw

If you go through the trouble to exchange cylinder locks to get one to bottom out against the shoulder on the barrel when lined up properly with the gas cylinder,  then the cylinder lock will have no play.  When you screw the gas plug through the cylinder lock and into the gas cylinder, the gas cylinder will pull forward to the cylinder lock eliminating any movement.  Worst case with a cylinder lock that does not "time" correctly the movement should only be the clearance between the threads of the gas cylinder lock and barrel.  There is no positive stop for the gas cylinder, it is only through assembly.

Peening my barrel almost eliminated any play,  but if it loosens up I may try a few cylinder locks.  The looseness just bothers me, but didn't affect function as far as I could tell.