I purchased a AMD-65 parts set and a new receiver.  The new receiver is thicker then the stock receiver so I'd like to machine down the trunions to fit the new receiver.  I asked a friend who is a machinist and he needs to know what type of material it is made from and the hardness.

From all the research I've done I believe it's hardened stainless steel, possibly 8620.  Does anybody have any more information I can give my friend.  We can tell it's hardened by taking a file to it and it won't hardly touch it.

Any replies will be helpful.  Thanks in advance.

i dont know the particualr type of steel, but a belt sander will work.
just do like 5 seconds on each side and then test-fit...
if its still too big, repeat.

8620 is an alloy steel, not a stainless steel. It is used a lot for case hardening applications.
If it is case hardened, you could be looking at 59-60 HRC.
A belt sander will work fine, or if you need more precision, use a surface grinder.
Jaymor