From the manufacturer:
The most precise and easy to use muzzle device alignment system on the market.
Simply choose the correct size Accu-Washer™ shim to accurately index your muzzle brake or flash suppressor while maintaining perfect bore alignment. Since only one shim is required, the tolerance stackup’s and complexities associated with peel washers and other multi-shim systems are eliminated. This minimizes the risk of baffle strikes, ensuring maximum accuracy and safety from your muzzle brake or suppressor. Includes 18 individually marked Accu-Washer™ shims (timing adjustable from 0-360 degrees in 20-deg increments) and easy to follow instructions. Precision machined from high strength 400 series stainless steel to a guaranteed thickness uniformity of ±0.0001″. All shims feature an oxide blasted non-reflective surface finish for low visibility. One Accu-Washer System will align approximately nine brakes (on average) before replenishment shims are needed. Designed to fit 1/2-28 or 5/8-24 threaded barrels.
We switched to these timing washers a couple of years ago and never looked back. Simply put the Accu-Washer™ system from Precision Armament is second to none. Each set can time 8-9 muzzle devices. These are also fantastic for muzzle devices that don't require alignment. The ID marks on the washers make them very simple to organize and use.