The gas cylinder lock on my M1A standard tightens up a little over a 1/4 turn past the gas cylinder so there is some slop in the system. I know for maximum accuracy you want to have zero slop but what about a rack grade rifle that doesn't need more than minute of man accuracy, any harm by leaving the gas system with some slop?

You don't want a sloppy rifle. Order the shims, and time the lock properly.

Before shimming, try flipping the cylinder lock and thread it on with the opposite face.

I'll have to order the wrench for the FH to even flip the lock so I just as well order shims too, but I'll try flipping the lock first. Been watching some of Tonyben's videos on YT and saw the tip about flipping the lock.

I am building a national match. I watched Tonybens videos and shimmed my barrel. It was easy. Just follow his steps.

There are a few factors to consider when shimming the cylinder.

The military match barrels are now delivered without the gas port being drilled.

This allows centering the gas port with the gas cylinder, by marking the bottom of the barrel through the bottom of the gas cylinder with the spindle valve removed.

A big deal with match rifles is the the gas piston pre-load on the op rod.

This used to be adjusted by machining the front of the gas piston shorter, where in contacts the gas plug, but was later changed to shortening the rear of the gas plug, where it contacts the gas piston.

There has to be some pre-load for proper function, but for match rifles, the pre-load is minimized below the standard full pre-load range of the stock parts.

Most of the time, shimming won't hurt gas port alignment or gas piston pre-load with standard parts.

Even with tight shimming, the parts will loosen over time.

Before shimming, find the tightest and loosest sides of the lock.

First shim with the lock turned to loosest fit side, then when it loosens, the lock can be turned around to tighten up the fit.

Replacing shims might still be required though.

Also maybe check each side of the lock with the shims required for each side, the gas port alignment and the gas piston pre-load with the gas plug, using the op rod and its spring.

If the rifle wasn't match built,  then pre-load should be heavy enough to allow adding shimming.

How do you check preload and what are you looking for?

I forget the exact number of turns considered ideal, but around a half turn to a full turn of the gas plug, you should feel contact tension with the op rod and its spring.

That is minimum and is for match rifles.

Standard parts compress the op rod spring tension more turns than that.

Okay to begin,

Have the gas plug out and the action can be out of the receiver.

Install the bolt, op rod and op rod spring and have the gas cylinder tightened with its lock on the barrel.

Now the op rod is fully extended into the gas cylinder and held forward by its spring.

Drop the gas piston in the gas cylinder.

It should contact the op rod.

Then install the gas plug.

As you tighten the gas plug, near the end it should start pushing the op rod back with the gas piston.

This is the beginning of the pre-load.

If there is no pre-load at all, it won't function properly.

After the gas piston starts to push on the op rod, count the number of turns left to fully seat the gas plug in the gas cylinder.

With stock parts this could be two, three, or more turns.

As you shim the gas cylinder, you move it out away from the receiver.

This will reduce some pre-load on its own.

For match rifles, material used to be removed from the front of the gas piston, but since there were also other piston mods called for, later it was decided best to remove material from the rear of the gas plug, where it contacts the gas piston.

Some pre-load is always desired and required, so a half turn or one full turn usually the pre-load in match tuned rifles. (one and half turns maybe)

The op rod spring should be pushing the gas piston against the gas plug when the bolt closed and the op rod fully forward.

If isn't, there is isn't any pre-load at all and the gas system won't function properly.

When I was competing in matches, I knew one guy that TIG welded the op rod guide and the gas cylinder with stainless steel weld filler to the barrel, to keep those parts from moving.

When his barrel became too worn for matches, he would replace the barrel and weld everything again.

Thanks. There is a lot of stuff to learn with this platform, it isn't just plug and play like an AR. Guess it's kind of like comparing a 1911 to a plastic striker fired gun.

Got a stupid question here. Since a guy would use the castle nut wrench very few times on low use gun would it be possible to just use a bras punch to loosen and tighten the castle nut? With the little bit I'm going to shoot my rifle I might never have to shim the gas cylinder again so why spend the money on a wrench, especially when it seems like the readily available commercial units are junk.

Also, how tight does the castle nut need to be, just tight enough to hold the FH on without movement?

ETA. Checked preload and I have just short of one full turn. This is from when the piston touches the op rod to when it snugs up to where I'll have to put the wrench on it to apply torque.

It sounds like you don't have much to allow shimming anyway.

The contact between, op rod, gas piston and gas plug is where it starts and any turning past that is pre-loading the gas piston against the op rod.

There has to be some pre-load and no play.

The match tuning minimizes the pre-load, which in turn delays the timing, as does polishing down or grooving the gas piston to bleed gas.

The gas bleeding mods often only function with heavy match bullet loads, but if only the gas piston is modified, it can swapped for a regular gas piston for standard ball ammunition.

So you don't think it would be worth the time to shim my rifle to get the gas lock to tighten up in the correct position to remove the little bit of slop I have?

Rack rifle.....if you are content shooting 3moa.   Just keep it as is.


Don’t overthink it unless you want to tighten it up.  

My “ match” rifle is shimmed not welded and bedded in a M3a stock.  good enough for my needs.

I was always tempted to buy a unitized gas system but the rifle rarely sees daylight anymore.


To me it is a do it fully or don’t do it proposition.  A few light mods like shimming are a reasonable expense to play around with but unless you have accuracy issues or certain accuracy goals it may not mean much.

For a match accurate setup I think

1 you have to bed the gun
2 do a trigger job
3 unitize the gas system, the shim is halfway not permanent approach


these are just my opinions.

This.........

It's just a plinker for me, as long as I can keep the shots on an 8" gong out to maybe 300yds I'll be happy. I guess I misunderstood and thought that some slop the gas system would cause damage to it.

Yesterday I went out and did some prone shooting at 140yds with 175gr GMM(only ammo I have for it right now) and my groups were around 4-5". I had a crosswind to fight a bit and I don't do the best with irons these days so probably won't get much better than that.

I made a castle nut pliers out of a C channel vice grip. The idea came up on a search and Tonyben used the same kind of setup so I figured I'd give it a shot. Mine doesn't look as pretty as the others I saw but it did the trick with just a minor scratched finish inside one of the nut grooves, nothing that's really noticeable. It helped that the castle nut wasn't very tight to begin with, just a little bit past finger tight to get the grooves to line up for the set screw. Going to the next groove was way too tight. I did make the mistake of not marking the nut before removing it and it took a little bit of moving it back and forth to get it timed correctly.

I used a feeler gauge and it took .024" to get the gas cylinder snugged up properly and I ended up with 3/4 turn of preload with the piston plug hand tight and slightly more with it torqued with the wrench.

Only measure the turns for pre-load after spring tension begins to where the gas plug is seated, before torquing the gas plug.

Just to where the shoulder of the gas plug seats and before torquing.

I don't remember any exact castle nut torque, but I'm sure there is one.

The gas piston always requires some pre-load, so never shim so far that there isn't any pre-load.

With the spindle valve removed, the alignment of the gas port can be checked.

The gas port in the gas cylinder is larger than the gas port in the barrel, so there is so working room.

I want to say the castle nut is just draw tight a bit and then to the next notch so you can lock it.


No I don’t believe any damage would occur.  You’re just trapping thing together for consistency......making a fixed “tuning fork” instead of an adjustable one that might vary if overly loose.

With the cylinder lock and gas plug, the gas system shouldn't have slop, but you don't want the gas piston to not have any pre-load.

You can use a tapered punch and, "peen" the lugs inside the cylinder to fit the barrel more snugly, or peen the grooves on the barrel, but nothing severe and if unfamiliar, it's likely best not to mess with it.

The gas cylinder is much harder than the barrel and difficult to peen its lugs, but the barrel grooves are easy to peen, or over peen.

Again this is an extreme match modification and makes service and replacing parts more difficult and risks damaging parts.

I understand.


My bedded gun with Springfield inc NM  barrel (ok not really match grade) ....shoots about 1.5-2 moa iirc.  70-80% in the 1.5” and the balance in a cluster of its own high and a touch right.    Never figured that out why there is a consistent two group thing going on.  That is scoped.  Edit .....my reloads with 168 smk’s and Rl15, Imr4064, or one of the 4895s.  

For your purposes the fggm is overspending as you likely won’t get the accuracy gain on paper beyond good 150 fmj.  BTDT.

Most all of the commercial flash hiders are now match reamed.

Commercial barrels almost always have tighter chambers than military barrels.

The commercial rifles these days do have some head starts on accuracy improvement.

Even the old match mod of fixing the front band to the gas cylinder is going out of style, because it has to be perfectly centered and stay that way, and it is impossible to get perfect.

Both peening and shimming can and do work loose with use.

Most of the internal gas system mods are to allow using heavy match bullets without overloading the gas system, even if they claim some of those mods are to improve accuracy.

There are the heavy op rod spring guides, that often require fitting to the receiver, for a close fit.

The match trigger mods are the least effective method of improving accuracy and can allow the parts to wear more rapidly.

Some standard rifles can be accurate.

Shimming can work to tighten up groups along with a stock that puts the right  amount of tension on the front band, also relieving the contact between the hand guard  and stock. We picked up 1.5 moa on one rifle.

If you do decide to shim get a real pair of castle nut pliers. The import ones are soft and will bend or possibly shear and screw up yer castle nut or flash hider...

Since I like to tinker I went ahead and shimmed the gas block. I played around with the different shims until it got snug around 5 o-clock or maybe a touch more then pushed it a little more to line it up. No play at all in the front band now, when installed the band has pretty good tension against the stock also. Gas plug snugged up 3/4 turn after the piston contacted the op-rod so I should have enough preload. Then I removed some material off the bottom of the handguard so it doesn't touch the stock. Hopefully I'll get out in the morning before the wind picks up to test it out.

If I get more into shooting this I might get a unitized gas system down the road and maybe the correct FH with the bayo lug.

The work didnt' go without it's problems though. I had the FH off last week and the homemade wrench(made from a C clamp like tonyben made)worked great but when I went to take it off tonight it was like the nut was stuck on and I used a bit too much force and the wrench slipped and dinged up the castle nut and scratched the backside of the FH. I never put the thing on that tight last week and then I realized I forgot to remove the locking screw. I had ordered a spare nut so I just used that and will live with the scratched FH until I get one with a lug on it.

It’s easy to forget the castle nut set screw so I keep the allen wrench with my pliers so I’ll see it and go “oh yeah”! Lol.

You’ll have to give a range report.