[Hectic]

It drops because you run out of compression befor the bb exits the barrel and so gthe bb starts to slow down while its in the barrel and will be bouncing around in the barrel befor it exits.
And yes also check the inner for play in tbe outter barrel and shim it with some tape or orings if needed. in the mean time you can get away with a 430 or 455 no problem on a 4/5 ported cylender as(most m4 these days have that. also going to 509 or 550 won't make much improvement over a 363 its the quality of the bore barrelthat make s the difference