Compression issue after barrel swap

[Adam3088]

So I've gone and put a madbull 6.03 tightbore and G&G green rubber in my King Arms M4, but now the compression appears to be total crap.

I've not got a chrono to accurately test the actual fps but when firing it seems to be well down, at a guess around the 200 mark.

 

Anyone have an idea as to why this could be? From what I can tell nothing's out of place and I've not touched the gearbox.

[Lozart]

If all you've done is change the barrel and hop rubber, I'd suggest that you've buggered up fitting the rubber. Check it isn't getting caught going into the hop. If it isn't sealing properly there the nozzle could be catching on the lip and screwing up the airseal.

[Samurai]

Is the new barrel much longer/shorter than the original?

[Adam3088]

No it's only a few mm shorter, not enough for the cylinder to be a problem.

 

 

I'll take it apart again tomorrow and have a look at the rubber, I did check it was aligned and the hop was going on + off properly before putting everything back together but maybe it's shifted somehow.

[BrightCandle]

Could also be as its gone back in the gun the hop unit hasn't made good contact with the gearbox. You loose a lot of FPS there unless that is nice and tight and on some guns there is a bit of a knack to getting them back together properly. My CM16 has that issue, the hop just doesn't push back well whereas on the top tech it always does.

[straffham]

M4 hops usually have a spring attached to the hop or around the barrel pushing them against the gearbox, did this get refitted? As Brightcandle says if the hop unit isn't seated into the gearbox properly it will kill the airseal. Also the hop rubber should be a good snug fit, if its too loose in the hop unit air can escape around it.

[Adam3088]

Finally got around to having another go at fixing this, chrono'd it last weekend at ~150fps.

 

Removed barrel + hop, checked was airtight, everything fine there.

Stripped it down anyway, double checking fitment and PTFE taping the barrel - hop to really make sure it's airtight and to stop the spacer from shifting about.

 

Put it all back together, no change.

 

There's no forward / back movement at all on the hop unit when installed, so I'm not sure what else could be causing the problem.

[RR01]

If you fit the hop into the top receiver, does it move in and out against spring pressure when pressed in by hand? If not then it may not be pushing back fully against the gearbox when you refit the receiver halves. If it is a very tight fit in the top receiver then you may need to lightly sand down the wings on the hop unit, or check that nothing has got dropped into the outer barrel to stop the hop unit seating fully.

[Adam3088]

It does, but once the gun is fully assembled it appears to have no function at all as the hop unit cannot move whatsoever. It sits right against the gearbox.

[sp00n]

have you tried swapping the barrel back to the original one? There *could* be a defect in the new one

[Adam3088]

I suppose switching back to the original barrel + rubber is the next logical step

 

Will give it a go tomorrow and see what happens. Starting to really hate this gun, I fecking hate taking M4's apart, F2000 on the other hand takes seconds to strip down..

[sp00n]

try the original barrel with the new rubber

[Adam3088]

It's now got to the stage where I think something's shitted up in the gearbox, this is where I throw in the towel and pay someone else to do it for me