This is a 5/8" steel crush washer.  It's most common use is on 7.26/.308 and a number of big bore 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: 0.625
• Outer diameter:  0.80"
• Crush washer height without compression:  0.150"

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