So I had this problem when I was working on a franken MP5 a couple months back...

You can do one of two things to fix this: swap out the hop up rubber because it is: too short or too thick for the bb to go through

Or
Swap out the nozzle with a longer one.

Both are a pita but I find swapping the rubber is simpler. I've had hit or miss with the g&g greens because g&g had a bunch of defective rubbers sent out to it's distributors which had to be replaced. I am not sure if this is related or not though.

Sorry about that. It is as far forward as it can get in the gearbox as of now, not in hop up, haven't tested that yet.

I could try a longer nozzle, such as a custom retro arms one, but the bucking in my KAC PDW is a Prommy Purple flat hop and is actually getting air leak through feed tube as well. Only difference in build is PDW has SHS aluminum o ring nozzle and M4 has G&P Stock aluminum o ring nozzle. Both are marketed for M4 length nozzles, so realistically they should work but maybe not. I think any further for either though may result in bb's not actually being able to feed in mag tube either. Had to do this as bb's weren't feeding due to tappet plate moving nozzle too far forward

Is it the stock nozzle?

Just noticed. Is the springs regular or irregular pitched?!

Stock G&P O ring aluminum nozzle. Spring was just changed to an irregular pitch M125 and new piston head o ring. Got it up to an AVG of 336.6 FPS w/ 0.2g. However, noticed my problem using the light test when nozzle was fully forward, I can see light down barrel. So, seems bucking isn’t long enough, and don’t think getting a 1mm linger nozzle would work as may cause feeding issues. Know any longer type buckings?

Try modify flathop.