King Arms M4A1 Loss of Pressure/FPS

[AnthonyG]

I was having issues loosing pressure such that my gun, which should be shooting ~340fps, was shooting ~210fps.

With some help I isolated the problem to the metal ring that sits over the inner barrel creating a seal with the hop-up unit. First, does this little metal ring have a name? Second, at the moment my fix is that I've put some tape to stop this little metal ring from sliding out. I'd like a more permanent fix and just wondering what I should do?


Edit: I couldn't find an exact image, but basically it's the brass-coloured ring that slides in/under where I've drawn the arrow:

[L473ncy]

I've heard of it referred to as a "hop up barrel spacer" before. It should also just be "friction fit" into the hop up unit. It's odd though, is everything stock on it?

[Red Dot]

Some units use a spring to maintain pressure on that ring. You could try some Teflon tape to seal and secure the part.

[RainyEyes]

Are you sure it's not the hop-up rubber pressing down too much that causes fps loss? I've had that happen on one of my guns previously.

[Zfurlong]

Use Teflon tape to wrap the end of the hop rubber, that should solve your issue. I started doing it to all my guns and have never had an issue once I started doing that (providing of course that any issue I did have was compression loss caused by the rubber)

[AnthonyG]

Originally Posted by L473ncy

I've heard of it referred to as a "hop up barrel spacer" before. It should also just be "friction fit" into the hop up unit. It's odd though, is everything stock on it?

Stock.

Originally Posted by RainyEyes

Are you sure it's not the hop-up rubber pressing down too much that causes fps loss? I've had that happen on one of my guns previously.

I crono'd it before an after. It's definitely the spacer. I was at a field, so the employee crono'ing my gun suggested the same thing, and it didn't affect it.

[AnthonyG]

Originally Posted by Zfurlong

Use Teflon tape to wrap the end of the hop rubber, that should solve your issue. I started doing it to all my guns and have never had an issue once I started doing that (providing of course that any issue I did have was compression loss caused by the rubber)

Are you saying, you wrap this part in teflon tape?




Or the part inside the hop up unit I labelled B?:

[RainyEyes]

The END of the hopup rubber. So the place where the ring would sit on the hop-up where you labeled "where i put tape".

The goal is to prevent compression loss. So you would wrap the end part to help seal it, not inside.

[Wrath144]

Use an oring.

[AnthonyG]

Originally Posted by RainyEyes

The END of the hopup rubber. So the place where the ring would sit on the hop-up where you labeled "where i put tape".

The goal is to prevent compression loss. So you would wrap the end part to help seal it, not inside.

Got it, So I've got it wrapped in the right place as it stands. I guess teflon tape is better than the electrical tape that I have on there now.

[Zfurlong]

Under that brass ring, over the end of the hop rubber. It should tighten up that brass ring to the inner and allow it to sit where it should, and it should help the airseal

[Stealth]

If you have floss around, you need to do this mod:
https://www.youtube.com/watch?v=QInMxQPKyb8

This ensures a proper seal at the interface between your hop up unit and your hop up sleeve.

Since you mentioned KA, it doesn't really apply, but another source of air leaks is at the interface between your nozzle and hop up sleeve. M16 hop up units have a tiny spring which pushes the hop up unit backwards. This spring is weak and if tolerances are loose, the hop up unit can move forwards during shooting, thereby losing the proper seal. These o-rings keep things tight and maintain the proper seal.