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Posted: 10/8/2025 2:59:27 PM EDT
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Anyone shim a unitized gas cylinder and have any positive results? Have a M1A NM with a little bit of movement back and forth. |
| I had to shim mine when installing a unitized GC on a Criterion barrel for my Sage EBR build about 10 years ago. I don't recall where I bought the shim pack, though. I do recall that it came with several different thickness shims so it was pretty ready to find the combination that gave me the lockup timing I was looking for. |
| Fulton Armory sells a shim pack for cheap. Used it on an M1A and the groups really tightened up. |
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Originally Posted By TOTHEMAX: I have not put the results on paper, but every post I read about it, shiming improves the groups. This, but it's better to unitize the gas cylinder, and torque the gas plug to NM spec, and install a NM srpring guide at the same time. They all work in conjunction. |
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Originally Posted By CPT_CAVEMAN: Anyone shim a unitized gas cylinder and have any positive results? Shims are used to "unitize" the separate standard gas cylinder and front band together when firing. There is no "benefit" added by shimming the NM gas cylinder. The back and forth you notice when assembling gets locked when the cylinder lock and gas plug are tightened. 
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Originally Posted By BULLDAWG_556: This, but it's better to unitize the gas cylinder, and torque the gas plug to NM spec, and install a NM srpring guide at the same time. They all work in conjunction. What's that? I've just been locking the bolt back and tightening it until it feels good |
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An M1A with a unitized gas system still needs to have the barrel staked to keep the gas cylinder precisely positioned relative to the op rod at the rear with a properly fitting gas cylinder lock, flash hider and castle nut correctly securing it at the front. The castle nut has its limitations, but will properly preload the stock with minimal fitting. It's easier to execute on an M1A/M14, but it is no different in concept than accurizing an M1 Garand. With the Garand, the gas cylinder is properly positioned relative to the gas piston on the end of the op rod, with the barrel staked behind it and the gas lock securing it at the front with zero play, via a properly fitting gas cylinder lock that clocks correctly to properly preload the stack of parts. Back in the day building a NM Garand created a mix master by definition as you'd fit parts like the gas cylinder lock using a box of them, swapping until you got one that clocked properly. later, building a DCM match rifle without that luxury of boxes of parts, meant getting a few and finding the one you had to mill the least off of to get it to clock properly. Shimming the M1A gets you about 80-90 percent of the benefit of unitizing the gas system with a *lot* less effort, as it removes the play between the op rod guide, barrel band, and gas cylinder. But that assumes reasonably correct gas cylinder positioning relative to the o[ rod. Short of a Match or Supermatch M1A, where the gas system is already unitized and properly positioned, there is no reason not to shim an M1A "standard" or "loaded" M1A as the cost is minimal, the tools are not that expensive, and it does a great job of reducing vertical stringing. --- Another area of concern is the fitting of the handguard behind the barrel band. As on the Garand, you wan to ensure there is some clearance (1/16" minimum) between the rear of the handguard and the front of the receiver, so that its not putting pressure on the barrel band. If there is no clearance, the handguard will put differing degrees of pressure on the barrel band as the barrel heats and cools. M1A or Garand, most of the metal work, the fitting of the handguard(s), and creating proper downforce on the stock ferrule addresses vertical stringing. The remainder of the bedding the receiver primarily addresses horizontal stringing. |
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I did it with mine; I noticed some slight improvement. Think I used a Sadlack kit. Cheap, doesn’t hurt anything. |
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