G&P M4 GBBR low FPS?

[simon]

I'm sure this will have been covered before so apologies.

I've just picked up a boneyard whit I believe to be a G&P M4 GBBR. I've managed to rebuild the trigger group and sorted one of the mags which is now gas tight, lubed the bolt group which all loos like new, and turned the hop to about half on.

I've filled the mag with Ultrair Power Gas and loaded some 0.2 bbs but I'm only getting 200fps?

The colour of the bolt nozzle is blue but I do have a WE bolt nozzle which red from another rifle. I've tried both but makes no difference to the fps?

Everything seems to be sealed ok and no noticeable gas is escaping.

Any help please.