GBBR - Inner Barrels and FPS boost
 

I have an M4 GBBR that is now dialed in perfectly at 385ish FPS with .20 (just under field limit)

Just installed a KAC suppressor and the stock inner barrel is about half way up the pipe. I would like to extend the inner barrel so that it comes to the end of the suppressor but in my experience with upgrading GBBR inner barrels (especially added length) the FPS goes through the roof and no amount of npas tweaking gets me below 400fps

are there options (larger bore maybe?) to upgrade inner barrel length and overall performance without blowing out the FPS past 400 on propane..

thanks

You would almost have to test. You are correct going longer (or tighter) will up the fps. Shorter (or wider) will lower. By how much I don't know for sure for your setup.
why not take advantage of the sound dampening and protection offered by the suppressor?

That was my original plan however, I've grown concerned with a BB hitting the inside of the suppressor.. i see a couple of white marks on the inside wall so bb's have hit it a few times at least..

There is a 'slight' wobble in the inner barrel so i could address that and hopefully the bb's shoot straight out of the suppressor.

There is about a 3 inch space from the end of the inner barrel and the exit of the suppressor...

Alternative solution for inner barrel.. get a better KAC suppressor, since the one got is probably junk, assuming it's the 5KU, etc.

Need the get the VFC one, around $100 though from Mach1.

If you want to reduce the FPS and go longer inner barrel, you can go with a 6.10 orga or 6.23, that'll reduce your fps quite a bit.

The air volume for a gbbr is actually controlled by the poppet valve or npas. So simply going to a longer barrel may increase fps, but it might also actually cause you to joule creep in reverse.

Start off by chronoing your gun on .28s or whatever field bb you use to determine if you really are under the field limit, then extend the barrel and perform the same test to double check.
i know with the kriss vector, when you use a longer barrel it actually REDUCES the muzzle energy on .30s quite a bit due to the tiny gas volume used to fire the bb.

Second, your bbs shouldnt be hitting the suppressor at all, very especially within only 3 inches. Removing that wobble in your inner barrel will also increase your accuracy at range. If you test your accuracy with and without the suppressor it will be very noticable if the bbs are hitting the aperture or walls on it.
BBs hitting the inside of a suppressor are usually due to misfeeds and suppressor wobble. If you dont have at least a 6mm grouping 3 inches from the end of your barrel you've got some serious issues. I once had a suppressor with a 6.5mm aperture 6" from the end of the barrel, never had a strike on it.

Yeah I was gonna say I have a 12 inch or so silencer on one of my guns and only hide about w inches of barrel. Never had an issue. Conversely my buddy bought a gun that came with a short silencer, it was threaded on crooked and thus caused bb strikes.however just looking at the gun you could see the front of the suppressor was about an inch to the left of the center line