From the -23&P technical manual:

3-10. UPPER RECEIVER AND BARREL ASSEMBLY (CONT).
d. REASSEMBLY (CONT)
NOTE
The peel washer may be heated to remove thin sections. Always place thin section to rear.

14. Install peel washer (11) and compensator (12) on rifle barrel assembly (2).

15. Torque compensator (12) to 15-20 ft-lb (20-27 N-m) using combination wrench (10) and torque wrench. Torque is measured when both wrenches are used together. Some portion of the opening of the third or middle slot must be straight up (align with the front sight post) (12.1) at proper torque level. Thin sections of the peel washer may be removed or added as required. Save unused sections

Quoted:
They are layers of thin shim stock pressed together under very high pressure. You can peel them with a knife or screwdriver or sometimes a fingernail.

Using a match or lighter on it seems to help loosen some of the layers.

Quoted:
Was the peel washer only used on the A2?

Ron

Quoted:


From my understanding, yes.

Quoted:
There still are/were a number of M16A4s (mostly the very early ones & a few that received early replacement barrels) that have peel washers installed rather than crush washers. Also, a number of early M4s used them as well. There were A LOT of left over peel washers around when the A4s first came out - hell - there were still peel washers in some units inventory bins even in 2004.