Tightbore & FPS Issue

[dfrear]

Bit of an odd issue I'd like to address if possible:

Background: G&P M4 Magpul model. Chrono'd with erratic FPS between 220 and 260, no consistency at all. Range was poor but useable, quite often double fed.

Got it home, attempted some test fires just into some 5mm cardboard, normally bb's tear through this no worries - wouldnt go through at 2inches away. Took the hop unit out and TB, changed hop rubber and nub just incase. Put it back together, same issue. Tried it without the front end on and the hop unit/barrel directly onto the gearbox - same issue.

Replaced the TB with a standard brass, everything seems to be back to normal - blasting through cardboard like butter.

I've inspected the TB, it's not dirty and doesnt appear to be bent in any way shape or form.

Any ideas?

[happyal]

Re: Tightbore & FPS Issue

It sounds like there is an air leak between the air nozzle and hopup rubber.

The first thing I would suggest is to replace the hopup rubber to a different brand.

[dfrear]

Re: Tightbore & FPS Issue

Yup first thing I did was replace the hop rubber as stated, upon replacing the tightbore itself I regained what I'd call "normal operation"

[No1_sonuk]

Re: Tightbore & FPS Issue

For a start, he said a different BRAND, not a second one from the same source. You didn't specify, so he was making sure.

The problem might be leakage around the hop rubber/barrel junction. If the outer diameter of the TB is slightly smaller than the brass barrel, there could be a leak there.
I've seen some tutorials which suggest binding the rubber onto the barrel with thin twine to improve that seal, then insert it into the hop unit. PTFE tape (like plumbers use) is another method I've seen in write-ups.

[happyal]

Re: Tightbore & FPS Issue

In that case, the next thing I would change is the air nozzle.

[Panoptes]

Re: Tightbore & FPS Issue

Is the Chrono usually stable? If it's a Madbull one, wild readings aren't unusual in dull lighting conditions. Set the controls for the heart of the sun.

[dfrear]

Re: Tightbore & FPS Issue

@Pano:

Yea the chrono is usually stable (not my own, regular sites chrono), it was stable for my other guns too just to be sure.

@No1_sonuk

Nice idea with regards to the diameter of the TB - I did note that it looked quite small last night when it was sat next to a brass barrel, I'll give some PTFE tape a try. Now you mention it, I've also seen the twine idea.

@happal

Cheers, I'll have a look at that. Its the metal G&P type which Ive not had much luck with before.

Thanks to everyone who replied, I'll have a look at that tonight

[Boo-Sabum Ben]

Is there room for the barrel to wobble in the hop unit? Is the little gold ring that sits on the ed of the hop unit inserted?

[laurence]

Re: Tightbore & FPS Issue

OK, well after you install the bucking, lust look down the barrel to check its properly aligned, and then adjust hop-up on and off, check that everything is seated correctly. Also, what barrel and bucking are you using?
thanks

[dfrear]

Re: Tightbore & FPS Issue

Is there room for the barrel to wobble in the hop unit? Is the little gold ring that sits on the ed of the hop unit inserted?

No room to wobble as far as I could tell - the little ring was inserted and everything was tight up in there. Not had chance to try any of the solutions yet however.