My AnK M4 CQB

This is my very first airsoft gun, an A&K M4 CQB. I bought it in a black market in Quiapo Philippines for 4,500 pesos. It shoot at about 420 FPS out of the box, but when the spring settle down it go down to 390 to 400 FPS. I made a mock silenter for my gun, I made it from a couple of PVC pipes from our backyard.
After this gun have settled, I upgraded the internal part of it. I have changed the spring to SP140, I have change the spring guide, cylinder head, piston body and the gears. Right after it was upgraded it shoot up on whooping 530 FPS, it really hurts when you are hit with that. But wen it settled it go down to 450 FPS.