Anyone running a 7in barrel on their ar15?

[mak91]

Is anyone running a 7inch barrel on their ar15 in 300 whisper/fireball ? If so what loads are you using? Do subsonic loads cycle with a barrel this short? If you know what size gas port you are running please let me know.



Thanks.

[LouBoyd]

My only 7" ARs are in 223. My shortest 300 whisper is 10.5 with the gas port at the same position as a 7" pistol. No doubt you can move the gas port closer to the chamber and still use a reasonable length gas tube to prevent early bolt opening. The gas impulse from the gas port is at the average pressure from the time the bullet passes the gas port until it exits the muzzle. The energy available is the time x pressure product. gas port area is a multiplier. Sure a larger gas port helps, but 300 Whispers have about 1.9 times the bore volume and about 1/3 the propellant of a 223. The 300 Whisper propellent is (usually) much faster burning so the pressure drops very rapidly with bullet travel down the barrel. The slower bullet plugging the barel helps the time factor, but it's still won't be as much available energy at the port as a 223.

Ever look at a Desert Eagle gas system? Their gas port is about 1 mm in front of the case mouth. The gas still travels the length of the barrel (in a small tube parallel to the bore) to reach a gas piston near the muzzle.

If you're planning on suppressing and just want the overall length reduced you might consider this Finnish design where over half of the suppressors expansion volume wraps around the barrel instead of sticking out in front.

[mak91]

Reason why I ask is that I was going through some of my old parts and came across a barrel I had picked up about 6 years ago. Its a 300 fireball 1x8 twist, 7 inches long and threaded 1/2x28. I found enough parts to build a complete upper including a new bolt but don't have a upper receiver. I was wondering if it would run and be worth buying the upper to build it. Any thoughts on only 7 inches stabilizing heavier bullets ? I already have a 16 inch and 10 tp555 running and have a sbr and pistol lower so a shorty would be cool if I could get it running.


Thanks.

[Spook]

I've made them down to 8 1/2 but the guy said it wouldnt cycle without the can on... .120 gas port in the pistol position.

[ds762]

Originally Posted by Spook

I've made them down to 8 1/2 but the guy said it wouldnt cycle without the can on... .120 gas port in the pistol position.

my gas port is .125, 8.5" Bartlein tightbore, and it will not cycle properly without the can in place

[JFettig]

Originally Posted by Spook

I've made them down to 8 1/2 but the guy said it wouldnt cycle without the can on... .120 gas port in the pistol position.

Sounds like you need a custom length gas system.

What you can do is make a high gas block, take a rifle length tube, cut it and turn the end and have a straight tube. I actually used a piece of 3/16" brake line on my TOK, the gas system is less than half the length of a pistol gas system.


Jon

[Spook]

Great idea, any chance we could get some visual aids.... for the creatively challenged ?

[mak91]

Yes, I was thinking about moving the gas block back some. Any one have any ideas on how far or what size gas port to start out with ? It will take me a little while to do this but I'll report back on how it goes.

[JFettig]

Visual aid added.

Also in the picture is a rifle length gas tube, cut it off at the length you need less than pistol length and profile it, along with a taller gas block you can use a straight tube.

I wonder if there is a demand for tall gas blocks, I could have some real nice ones whipped up at work.

See this pressure curve, if you could capture at the peak or just after and regulate the port size(start at about .060") you could probably get a really short barrel out of it(5-6").



Please don't post the pic below on other boards.

Jon

[mak91]

Jon, Thanks, this is good info. I am not sure how you did your chart but could you do one of what a full charge .223 looks like. I know if I try to unlock the bolt with the pressure to high I'd have extraction issues. I am trying to figure out at about what pressures a .223 does it.


Thanks again for the info.