...Seems like a good way to reduce pressure on the op-rod while shooting hotter ammo. Obviously cycling milsurp should be less reliable in theory, but it is really an issue in practice?
I've used Schuster adjustable plugs on both M1A's and a Garand and the McCann plug on the M1. They both work when adjusted properly, but I'm no longer sure they offer any real benefit except with hotter ammo, and then I suspect there's still a limit on that. My rational when I tried them was that the M14 is a battle rifle, and must cycle under the worst environmental and maintenance conditions with potentially crappy ammo. As such, there's a good measure of latitude built into the rifle's design to allow for that, meaning
if you use good ammo and keep the clean and lubbed, the cycling process is much more violent than it needs to be. Using the adjustable plugs, you can bring cycling conditions of mil-surp ammo closer to the minimum, which gives additional headroom for somewhat hotter ammo.
Speaking from experience with the McCann on the Garand, very small changes in the size of the vent seem to make very large differences in cycling performance, to the point that mil-surp 30-06 LC needs a smaller port size than Korean mil-surp, which needs a smaller port size than Greek HXP in order to cycle. Also note that it's a lot harder to make a port smaller than larger (GI2) if you're drilling your own gas plug than if you're using the Schuster, which just involves screwing the adjuster in or out, or the McCann, which comes which different sized plug adapters (none of which worked on the Garand). I found it easier to just follow the ammo advice that's abundant on TFL and let it go at that.
