The final part of doing the accuracy work is the gas cylinder. Before starting on the gas cylinder and with the gas cylinder installed, check the front sight base to see if it wobbles. If it wobbles, then the splines on the barrel will need to be peened a bit. You simply cannot expect good accuracy if the front sight is wobbling around as you shoot. To peen the splines, take the round end of a ball peen hammer and rest it on the splines, and then strike the face with another hammer. You don't need much force, just enough to slightly peen down the sides of the splines. Do that on each of the three splines and peen at the same distance from the muzzle each time. Check the fit again and peen again if it is still loose. If you use too much force, you can damage the lands inside of the barrel and it is recommended you use a bore spud for this work to minimize the possibility of damage (Brownells # 034-100-299 $9.95).
You will next need to file a 30° bevel on each side of the rear lug of the gas cylinder to prevent it from contacting the front handguard ferrule. There is nothing magic about this, just use a good file and keep the bevels equal on both sides, periodically checking. Don't expect to use touchup bluing on this, because the cylinder is stainless and the touchup won't work.
Inspect the rectangular gas port hole in the cylinder. Remove any burrs that may be present.
Next, you will need to remove a bit of metal (shorten the cylinder) from the rear of the cylinder and remove some metal from the inside of the rear barrel ring of the cylinder. Unless you are very adept at filing, this is a project you should have done at a machine shop. Your goal is to prevent contact between the front handguard ferrule (your previous filing of the face of the handguard ferrule will have helped in this regard) and the rear of the gas cylinder by milling off about 1/32nd from the rear of the gas cylinder so that when the rifle is assembled, the rear of the gas cylinder does not come in contact with the front of the front handguard ferrule.
With respect to the rear barrel ring (and not the gas cylinder piston ring), you should open up the rear barrel ring from .620 inch to .640, which is a radial increase of .010". You have to use considerable caution to prevent it from becoming too thin where it contacts the gas cylinder itself. And, you want it to be directly in line with the front opening of the gas cylinder.
Next, shorten the gas cylinder (as noted above) by removing about 1/32" from the rear of the gas cylinder and barrel ring. This can be done with very careful filing (you can use your vise jaws and a straight edge as a guide if you are careful), or take it to a machine shop. You want it to be square.
You are almost done. Next, you need to index the gas cylinder to the operating rod. To do this, you should follow the directions of JB Roberts Jr. found on page 18 of the M1 Rifle reprint. To paraphrase, you first drive the gas cylinder on the barrel until the gas port is clearly visible at the rear of the clearance hole. Then, you reassemble the rifle and leaving the bolt open, you then will use the gas cylinder lock to swage the gas cylinder rearward until two conditions are met. First, the chrome button on operating rod (gas piston) must be completely inside of the cylinder when looked at from underneath and the bolt open. Secondly, the gas cylinder lock must not be more than 60° past dead center (and the closer to being dead center or slightly before dead center is better). When those conditions are met, you will stake the rear of the splines on the barrel to lock down the gas cylinder (again, using a barrel spud will help prevent damage to the lands). To stake, I use a center punch and just tap both sides of the barrel splines where the rear of the sight mount joins the barrel. I stake all three splines. Then, you should turn the gas cylinder lock back to the center (if it is hard, tap it slightly with a brass hammer), install the plug and turn it tight. Before you spike, you should also make sure there is adequate clearance between the rear of the gas cylinder and the front of the front handguard.
Reassemble the rifle, test for any areas that the operating rod is rubbing against, file or sand those areas until the operating rod rides through them free and then you are ready for final accuracy and load testing. There is one final enhancement that you can do and that is to have the muzzle re-crowned by a good gunsmith. Or, if you are brave or foolish or both, you can try to do it with a hand reamer. (Brownells Part # 080-930-308 $43.74 or alternate -- speak with Brownells Technical staff as to the options.) Once that is done, the rifle will shoot as well as it will ever shoot, without a new competition match barrel. Complete your project with a good numbered competition leather sling from Brownells (Part # 084-270-120 $48.20 and worth every penny). Sling up tight, shoot straight and be happy!