This is a 49/64ths" steel crush washer.  It's most common use is on .50 Beowulf ARs as well as other rifles and pistols.   Be sure to confirm the size of washer by measuring your barrel and/or what the maker tells you.

Pricing is per washer and there are volume discounts.

Approximate dimensions:

• Inner diameter: 0.785"
• Outer diameter:  0.957"
• Crush washer height without compression:  0.174"

Instructions in a nutshell:

1. Make sure the weapon is safe
2. Lube the crush washer on both sides to enable rotation
3. To install, the cupped end / large end faces the muzzle device.
4. Tighten the muzzle device down by hand
5. Inspect how you need to orient the flash hider
6. Do a quarter turn, and back off a bit, quarter turn and back off a bit - this avoids the crush washer cracking
7. Torque to the specification recommended fur the muzzle device and/or the weapon