CM16 GBB venting?

[CoveringFire]

Hey guys, hopefully there are a few people around familiar with the G&G/GHK GBB system that can help.

I've just received a CM16 GBB rifle that was advertised as having a feed issue. It was cheap so i thought i'd have a play as GBB rifles always interested me but the price of mags was prohibitive.

I've got a feeling the gas i'm using (ASG Ultra) is the problem. The gun cycles ok in semi dry firing, but switching to auto locks up on the second shot, venting gas until the mag is empty.

With a mag full of bb's, in semi i can usually get one or two shots off before the same locking up and venting gas, however occasionally the gun will fire two/three shots with one trigger pull.

Removing the mag or pulling the charging handle even a small amount stops the venting until the trigger is pulled again. With BB's and full auto i'm lucky if i get one shot before the lock up.

I've swapped the hop rubber/inner barrel, which improved the feed issue a little, but its still locking up and for the life of me i cant figure out why. All the seals are good, the action is smooth and returns fully, so the only thing i can think of is wrong gas?

If anyone can point me in the right direction here it would be fantastic. I've had the whole thing in bits, checked and oiled o-rings and with no real positive result.

[endo]

Re: CM16 GBB venting?

Could be a number of things, could be the gas (depending on which version of ultra it is) i've been using abbey predator so i couldn't tell you, but some other things spring to mind.

-has the bolt spring been modified? if the big spring has been cut down in anyway it can cause problems with the bolt carrier returning to battery and the nozzle entering the hop chamber properly (possibly stretch if ever so slightly to see if this helps push the bolt forward with a bit more force)
-o-ring on the base of the chamber where it connects to the mag (damage causes problems)
-overlubed? disassemble the top part of the gearbox and clean off excess lube from the chamber and inside of the bolt carrier, its possible its been overlubed this can cause issues in full auto
-see if you can add a spacer to the back of the gearbox between it and the reiever to force it as far foward into the hop up when the rifle is assembled.

Its a bit of a massive wall of text, but if it helps this is a rough guide to GHK / G&G gearboxes i put together.

[CoveringFire]

Re: CM16 GBB venting?

I've actually read that whole article before posting here (its pretty useful, given that my ultimate aim would be to replace most of the externals over time! ) and tried a couple of things without much success.

Can you tell me if the bolt carrier is supposed to be slightly bent outward at all? The forward section which covers the hop unit when the bolt is forward was pointing outward a degree or two and it was rubbing the receiver slightly. pulled it out and tweaked it back a tiny bit - It fired half a mag in semi without the jam and venting issue, but it still had some cycling issues in full auto. It could be down to my now extremely depleted gas bottle, so i'm waiting for a replacement for that before more testing.

Cheers for the help! I'll update if a full gas bottle resolves the situation (or not)

[MarkPulling]

Re: CM16 GBB venting?

Could the cold temperatures at the moment ,ie arround 5 degrees celcius be contributing to this ?,i have the same gun ,and sometimes it missfeeds aswell ....mark

[endo]

Re: CM16 GBB venting?

I
Can you tell me if the bolt carrier is supposed to be slightly bent outward at all? The forward section which covers the hop unit when the bolt is forward was pointing outward a degree or two and it was rubbing the receiver slightly. pulled it out and tweaked it back a tiny bit - It fired half a mag in semi without the jam and venting issue, but it still had some cycling issues in full auto.

Sorry i'm a bit late with a reply, glad i could be of some help
Yup the bit that covers hop unit should be straight, if it was bent that would cause some problems (which it has been giving it would seem, strange that it would be bent). Should be parrallel to the top portion of the gearbox if you pull it all the way back while its out of the gun.
If its still giving some problems with new gas, the only other things I could think of is wear to the base of the bolt carrier or sears so its not resetting properly and letting off the gas valve, possibly the bolt carrier is fouling somewhere else on the gearbox or receiver, or lastly a clean and a lube job on the trigger pack since it can get gunked up and the sears might not be moving as smoothly as they should.