M4 shooting VERY low fps after rebuild

[Nathan.g]

Hey everyone,

I will be a thorough as possible because I am truly out of ideas.

A friend of mine had his KWA m4 take a tumble and resulted in A LOT of broken parts. As a result, we did a full rebuild. The problem is fairly simple; the gun is shooting at a constant 125-127 fps w/m110 spring.

When gun is fully assembled it shoots 125-127 fps; without the upper and when I just hold the hop up unit in place, fps jumps to the 270 fps range.

The gearbox has been ruled out; full cylinder, perfect seal, and shimmed w/ brand new high torque motor. I have changed out parts and started from scratch but no change in performance.

I am wondering if this might be a result of the 2 piece hop up pushing too hard on the gearbox, but after shunning the outer barrel out about 1cm, same issue persists. The inner barrrl is a brand new 455 madbull 6.03 and I installed a brand new 70d lonex bucking (no air leaks and squeeky clean)). Bbs do not roll out of barrel but can be pushed through with ease.

Could the bbs be getting caught on something? Has anyone experienced this before? Could this just be an ever so slight nozzle misalignment?

Thanks in advance for any help you can provide!

[RainyEyes]

The nozzle is leaking replace it with a double o-ring one and shim your hop-up barrel unit with o-rings instead of the spring if that's the original device.

When you hold the hop-up into the gearbox the fps goes up cuz the escaped air is being funneled down the barrel but when it's fully assembled the hop-up isn't seated fully against the gearbox.

Your gearbox isn't being pressed into the hopup unit hard enough in this case. Nozzle misalignment is indicative of misfeeding and low fps but since it's not misfeeding I have doubts.

[ThunderCactus]

Probably nozzle alignment. Look down the barrel with a flashlight. Then Check the air nozzle seal against the barrel by shing a light up the hop chamber.

[Nathan.g]

Thanks for the quick reply's - I will try this tonight.

I don't think it's an issue of how much the hop up is pressed on the gearbox because before I spaced the outer barrel outwards, it was being completely jammed on the gear box and did cause some minor misfeesibg issues (2 piece hop up was spreading wider and compressing, causing the lip to ever so slightly over hang). I should mentioned I had to change the outer barrel as well because the old one snapped in half.

[Nathan.g]

Update: I switched the upper with my kwa and it shoots perfectly, meaning I don't think the issue lies within the nozzle anymore.i am going to rebuild the hop up one more time but I am back to being clueless.

[Nathan.g]

Problem solved; after all of that, turns out there was a small burr on the inside lip that the bucking was up against. Filed it down and we were back to 380 fps.

Thanks for the help!

[ThunderCactus]

sweet