I bought a like new SA M1A a few weeks back and it has been fired maybe 200 rounds. I tore it down to clean and look at it and I have a problem. On the trigger group housing on the right side there is a small verticle rail like peice of metal about 1/8 X 1/8 inch  by 1 inch long and is milled right into the housing, that slides into a slot in the receiver when the rifle is put together . I don't know what this rail fuction is but I think it may add stability to the trigger housing. Anyway, when I looked at this I found it has really been taking a bashing, it is battered up pretty bad on the top half of it, But worse is that the slot it fits in in the receiver is also statring to get deformed from the battering. My question is, can anyone tell me why this is happening? and what can be done to correct it? I have checked the stock to housing fit and it seems ok but it is really hard to tell. The rifle is a Springfield Armory Loaded with the SS bbl. Any help from anyone will be appreciated .

Though you state the housing to stock fit is ok, it sounds like the housing might be loose from the battering you describe.  If the housing wobbles with the trigger guard locked you will need to increase the tension of its fit in the stock.

Build-up the upper surface of the stock with expoy at the rear wear it contacts the receiver is one possible solution.  Another is to build-up the surface of the stock with epoxy where the rear of the trigger unit touches it.  Try to determine which area would be best to build-up, if there is a visible space between the rear of the receiver and the stock then build-up that area with epoxy.

After you have finished the build-up, test the trigger unit by holding back the trigger while you work the bolt back and forth at the same time.  If the hammer follows the bolt while the trigger is held back, then you must thin the built-up area until this no longer happens.  This condition of hammer follow is very dangerous and can lead to an out of battery detonation which is extremely hazardous to the health of the shooter.  

Tommo,  I put the rifle back together and checked as you said in your reply, I found, the trigger group is tight with no movement with everything locked down, also I checked the rear of the reciever to wood and on the rounded rear part of the reciever a .002 feeler gauge will not fit between metal and wood, also on the left side it will not fit between reciever and wood for about the first inch or so, on the right side a .002 will fit between metal and wood forward to the selector slot but a .003 will not slide in. Also I checked the trigger/bolt movement and it seems ok.
 

If the trigger unit is a Springfield Inc. cast one the metal may be soft or out of spec.  I have also seen USGI units that are battered too.  The only reason I can think of is the unit or the slot in the receiver is out of spec letting the unit move slightly as the bolt recoils causing the battering.  You should post this question on the www.m14-forum.com and the www.battlesrifles.com forums.  Their members have allot more knowledge of M14 type rifles than I do.