Installing a flash suppressor, muzzle brake, or compensator is usually pretty straightforward. But sometimes you have to "time" the muzzle device, such as when you're installing a flash suppressor on an AR-15. Muzzle devices often have a distinct top and bottom and must be oriented correctly to work properly. For example, the classic A2 flash hider for the AR-15 has open slots on the sides and top, and a solid bottom that prevents kicking up a lot of dust when you're shooting prone. When the device is oriented correctly, it's said to be properly "timed." We know Brownells Gun Tech™ Caleb Savant has impeccable timing, which he exhibits here by showing us how to time a flash hider on an AR-15 using shims. Hint: the Brownells Master AR-15 Shim Kit is a big help in this process.
Clamp the upper receiver securely in your bench vise, using an upper receiver vise block. Screw the flash hider onto the muzzle until it's slightly snug. How far off is its orientation? UNSCREW it until you have it oriented the way it should be. We'll be adding just enough shims behind it so that when we torque the muzzle device tight, it'll end up in that correct orientation. You can get out the feeler gauges and measure the gap precisely.... or you can test-fit shims from the kit until you find the right combo to fill the gap. You may need to stack multiple shims, or maybe you'll get lucky and find one shim that does the job.
When you have the right shim combo, the top of the device should be a bit off the 12 o'clock position (maybe 10 o'clock), so that when you torque it tight, it'll end up where you want it. If you're so far off the 12 o'clock mark that you'd have to really whale on the wrench to get the muzzle device torqued, get a thinner shim. (That's why the Brownells shim kit is so handy!)
If the muzzle device also serves as a suppressor mount, you'll probably have to secure it with a heat-resistant thread-locker such as Rocksett. But don't apply the Rocksett to the threads until you're sure you have the muzzle device perfectly shimmed up to 12 o'clock! After installation, wait 24 hours for the Rocksett to fully cure.