[fuzzywolly]

I had a similar set up, only I was using .4 gram bbs, and a SP 130 spring. Put my FPS at about 400 or so. You should be able to carry the heavier bbs, and I could even adjust the setting to carry them almost straight up with a ridiculous amount of hop. I assume you cannot get your heavier bbs to get this kind of hop?

What bucking and nub are you using? Perhaps you are using a hard, which could be the cause of this.

 

[fuzzywolly]

Sounds like you are using a PDI nubbin, the stock nubbin that comes with the chamber, one of my favorites.

I would check the bucking, inspecting it for anything deformed, ripped, etc. That is the first thing you should check.