Mid Length Gas Systems

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Mid Length Gas Systems

Postby Siringo » Mon Oct 01, 2018 9:12 pm

Been a long time since I posted on this site. I haven't used my 450 for a long time. BUT I recently purchased a custom barrel with a 1/16 twist. I wanted to revisit shooting resized 400 grain bullets. I disassembled my Bushmaster upper and installed the new barrel. Reusing the bolt (it was head spaced to the barrel extension) and gas block/gas tube. Went out and shot some factory ammo and some 405 Remington resized 405's. The action would not eject. I shot a few rounds of factory and the reloads (single shot), and the groups were better than what I had with the Bushmaster barrel. Especially with the 405's.

So I set my phone up in slow motion video and low and behold the bolt never moved. I looked into the action and no gas tub was protruding. DUH!. The barrel is a mid-length and I put carbine line gas tube in. Dumb move, but I just assumed it would have been made with a carbine system (I use a full length rifle hand guard). I purchased a mid-length gas tube and tried it again. It ejects, but will not lock back on an empty magazine. It actually locks in-between the bolt carrier and bolt head. It will not pick up a round from the magazine. The gas hole was way smaller than the standard 0.090 in the bushmaster carbine system. I opened up to hole to 0.089, but still it will not lock back. I opened the hole up to 0.0925 and will try it again this weekend. BUT I am curious if anyone else has run mid-length systems and what gas hole diameter works?! FYI I am using a standard carbine spring and buffer (3 oz).

This thing has to work at 20 below. If not it goes back to the barrel maker.
Siringo
 
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Re: Mid Length Gas Systems

Postby Hoot » Tue Oct 02, 2018 4:25 am

I heading to the airport as soon as the coffee works but wanted to say that a mid-length gas system should never have a gas port hole smaller than a carbine length system as the pressure is lower, the further down the barrel it is sampled. I don't have time to disassemble and measure my Bartz upper mid-length port size but IIRC, it was something like .096. Rifle length system 450b barrels can need up to .120 to cycle all bullet weights, Per Wildcatter. The one I used to have took .109 to start working. Another member has a thread going right now with the same problem.

Good Luck

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
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Re: Mid Length Gas Systems

Postby Siringo » Tue Oct 16, 2018 8:45 am

Opening gas hole to 0.096 inches allows bolt to lock back with factory ammo. Tested it in colder weather this weekend, leaving gun outside in 25 degree weather, yielded an ejection pattern at 3:00 and about 6 feet away. 400 grain bullets with LG do not work as the Port pressure is to low. Going to try VV120 when I have the chance.
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