9mm said:
How many shims do you apply at once? how often do you have to change them out? every time you unscrew it after a range visit?
I'm not sure I understand what you mean by this, and I guess that means I didn't explain it very well in my previous post.
There's nothing to change out. Once you install your muzzle device you never need to take it off or change it. The shims are just very thin rings of metal that fit between muzzle device and the shoulder of your barrel's threads:
The sole purpose of the shims is to change the orientation of the muzzle device when you tighten it down. Without shims, when you screwed on a muzzle brake or compensator it almost certainly wouldn't be oriented properly; the top of the brake wouldn't be in the up position and the brake wouldn't work as well.
This is called "timing"; you're installing shims to act as spacers to change the way the muzzle device is oriented when it's tightened down on the barrel threads. Most people don't bother with shims when installing flash hiders because flash hiders don't need to be rotated in a specific direction to work properly.
The muzzle brakes and compensators come with several different shims of different thicknesses. You only use as many shims as you need to get the correct spacing to orient the muzzle device in the correct position. You don't want to use more than three shims because the more you use the more potential there is for the muzzle device to be out of alignment due to tolerance differences in the shims. Silencer shims are precisely machined to avoid this, but they're never perfect, and the more you use the more you risk misalignment. So basically it's just trial and error until you get the right combination of three (or fewer) shims that time the muzzle device perfectly.
Just for reference, these shims do the same thing that the crush washer does on a standard AR-15. On a standard A2 flash hider, the slots are on the top and the solid portion is on the bottom. And the way they get it to orient this way is with a crush washer:
The crush washer does just that: It crushes when you tighten down the flash hider so you can time the flash hider by tightening it down until it's oriented with the slots up and the solid part down. But a crush washer doesn't allow the flash hider to line up 100% evenly with the line of the bore, which makes it a poor choice for use with a silencer (bad bore alignment can mean baffle strikes). So instead of a crush washer, silencer mounts use shims to time for precise alignment.
