Re: M1A Upgrade
The steps recommended above are spot on.
The order *I* would do them would be (note added steps):
1. Fit Op Rod Guide to barrel. Knurling seems to work best. Some recommend adding Loc-Tite. I agree. Use the removable type. Punching at least a dozen peen marks around the barrel side is a viable alternative, but pretty much demands either Loc-Tite or planning to refresh it every 1,000 rounds. Not most important, but first of three steps along the barrel.
2. Fit gas cylinder to barrel. See drill below (I changed the order so you don't put the FS back on too soon). There are some variations on the technique. Study them. Use what you think best. INCLUDE "timing" or "clocking" the figure-8 nut--it should bottom out not more than about 20 degrees past lining up with the gas piston's hole. If not, shims are your friend.
Unitize it with the barrel band if you feel like it. IME, doing the above steps are the most important and are enough. My welded unitized job (ArmsCorp of America didn't do a great job...) broke loose some time ago and it never made a difference in the accuracy.
3. Fit flash suppressor to barrel (peen barrel splines, eliminate ANY rotation so you are not dependent on the castle nut to keep the front sight from wobbling). NOW you can re-assemble all the stuff hanging on the barrel.
4. Bed the rifle to the stock. Only the most "handy" DIY types should try this. The trickiest part is routing out the stock without "going outside the lines". Don't "practice" on a USGI fiberglass stock because it cuts much more uniformly than wood.
The above steps are the most important.
Do what everyone else recommends that's not already done. However, don't expect any accuracy boost from the "NM" op rod spring guide. That is a *reliability* mod for the NM guns, not for tightening groups.
Finally, for reliability's sake, bend/tweak the op rod so the front end of the flat surface, forward of the locking cam hump where it meets the dogleg that goes under the barrel, is about 1/16-inch out from the side of the receiver ring, and 1/16 or a bit less above the receiver side ledge, when the UNSPRUNG op rod and bolt are all the way back, AND the bottom "belly" of the op rod *lightly* contacts the bottom of the barrel. If all is done correctly, the barreled action assembled without the op rod spring and without the trigger group clamped on, will fully open the bolt and travel all the way back when the muzzle is elevated to 45 degrees or less above horizontal, and will also fully close and lock and the op rod will continue forward to its rest position with the muzzle pointed not more than 45 degrees down.
If you are using iron sights, there is also "bedding" the aperture rack into the windage base, and a more arcane tightening of the elevation knob's internal components IF that knob has any backlash. Not every rifle needs this second step, every one I've ever seen needs the first one and my method (others discovered it too) is easier and more reliable than the old original technique of peening the metal parts.
But since you don't tell us whether your groups are using current match ammo, hunting ammo, commercial ball ammo, military surplus or your own match handloads, I really cannot opine as to whether ANY steps are likely to be needed or effective. If you're getting that with hunting ammo, none of this might make a difference.
On the other hand, if your report is for match ammo, AND there is no reliable/repeatable improvement after all of these steps plus what others recommend, you're definitely in need of a better barrel. Or you suck at shooting. Your question suggests that you don't suck at shooting, but I gotta cover all the bases.
