High FPS on some shots

[RKG]

Hello everyone. I've been doing some work on my specna arms M4. I've put a new hop rubber, tightbore barrel, new piston head and corrected the AOE. The compressions on the piston is really good now whereas before it was non existent.

It's shooting at about 300fps with 0.25g BBs with a sp100 spring. The only issue I seem to have is that every few shots the FPS seems to jump to between 340 and 320, which is obviously over the limit for a 0.25.

Does anyone know what the issue could be?

 

[Emergencychimps]

are you 100% sure, absolutely positive that you haven't got mixed weights of bbs? 

 

[SSPKali]

Weak tappet plate spring not moving the nozzle forward consistently?

Mid-cap syndrome where the mag spring pushed the BBs hard against the nozzle causing it to sit at and angle and leak air?

Did you change the nozzle? Sometimes square ended or tapered ones can play up with the bucking if they are different from stock.

Or mixed BB weights ?

 

[Leo Greer]

What manufacturer is the spring? It sounds to me like you're getting bad air seal most of the time, and then the jumps are the only correct readings. This differs by manufacturer, but for Guarder SP springs, which are the most common, you basically add 20 to get to the "M" rating equivalent. 340 is about normal then, and 300 is low.