The receiver extension is metirc,you can use it as is,other rifle stocks such as magpul or any A1, A2's with fit,just have to reuse the stock screw thats on it.
You can replace it with a little work and time.
The barrel nut is metic and can be reused as well if you want to change barrels,the barrel on it right now has a 1 in12 twist rate which is great for 42- 55gr ammo,55gr being most common and easy to get.
And the grip screw will need to be reused if you swap out grips
and lastly,the handguard cap will only work with A2 or magpul handguards, not the A1's as it's round and an A1 needs the standard triangle cap.
Aside from that it's a great rifle for around $500-600.
Oh, and the paint job on them suck.lol