Long story short, I'm already fairly pleased with the accuracy out of my M1A.  Now seems like a great time to throw on some shims since its already in itty bitty pieces anyway.  What kind of improvement can I expect from putting on the shims?

From one of the M14/M1A Masters

Quote from: Art Lupino

Along with the ever present question, to having gas systems unitized or not, is the question of whether to use shims or not...I will argue that shims should be used on all rifles where some degree of accuracy is expected. In addition to the question of accuracy, is the problem of gas plug and gas cylinder thread stretch when a shim is not used. If a unitized gas system or non-unitized system is installed without the proper shim, the system is stabilized in the fore and aft direction by the torque pressure used on the gas plug.  This is called a "pull forward" system.

Torque pressure to hold the pull forward system is excessive and thread stretch will result over time if the plug is removed for routine service. If the gas plug has an index line with the cylinder lock, you will discover that the index line advances each time the plug is removed and put back in.  As the threads continue to stretch, it will become more difficult to install, and continued over-torque of the gas plug will result in having to replace both gas cylinder and gas plug because the threads will be pulled out of shape.

The force on the cylinder when the rifle is fired is to the rear, then the spring and rod drives the cylinder back toward the muzzle.  All the energy, except for a very small amount absorbed by the gas cylinder lock, tends to work the gas cylinder back and forth.  The pull forward system depends on the gas plug to keep the gas cylinder from moving or getting loose.  That is questionable to expect from a part not designed for that purpose.

My position is, with the proper shim installed, the gas cylinder lock will hand tighten to the 5:00 or 5.30 position, then using a wrench to bring it to the 6.00 position drives the cylinder back against the shim and band and through to the barrel shoulder.  This is called a "drive back" system.  The gas system is now captured between the barrel shoulder and the cylinder lock.  This prevents fore and aft movement.  The gas plug can be installed, I use 100 to 125 in-lbs, with NO damage to threads. This method never causes a gas plug removal problem, within reason.  I would not venture to say which system is the most accurate, as there are too many other variables. But with the drive back system, there is no barrel stress and replacement of parts will not result.

And that is one of the best descriptions I have ever read on the subject.
Thank you for sharing.

Helpful as always, thanks!

You are quite Welcome Sir, Art and a number of others are more knowledgeable than I will ever be

I guess I should have mentioned...I'm planning to put on a Troy Battle Rail.  As page 13 of the instruction manual here http://www.troyind.com/manuals/M14BattleRailManual.pdf shows, you have to put on a proprietary front band.  Does this change things?

I would not think so but you never know if shims will be needed with the new bad, my suggestion would be put on the the band and see where the GC lock times

Shims are a great addition.