The same thing using shims has to do with MDs designed to mount cans.
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Learning a lot. Thanks all.
Crush washers and lock nuts are not things to use when alignment counts. Precise shims and machining are for best alignment in all planes. A precise lock nut adds to a tolerance stack, that's not avoidable.
If the MD does not require that level of orientation, then the option may work out. Many others are better suited with precise shims and lower end torque values for their application.
I'll agree that shims might be important if concentricity is important. If it's just about timing, then crush washer is the ideal mechanism for attaching a muzzle device and shims do nothing but add an unnecessary level of futziness.
You can choose to do so, just depends on the alignment requirements for the muzzle devices. There is no set means, using a means that fits the user properly should be a goal over one that does not.
The ID of the crush washer grows smaller and its OD grows larger as it's being crushed. I've had a crush washer get stuck on the threads a couple of times when removing a muzzle device, even though the washer was not completely crushed. One was a factory installation.
Crush washers are a pain. Go a little past when trying to align a muzzle device and you have to start over again with a fresh washer- or go all the way around again. Too much work. Shims are easier. I find shims ate much more positive and repeatable than crush washers or peel shims. If the muzzle device clocks early, use a thinner shim. If it clocks late, add a shim or use a thicker shim