PDA

View Full Version : .300 BLK gas length question.


Slow
February 04, 2017, 20:52
how does a pistol length gas help on a 16" Barrel ?

primary use will sonic with suppressor ATM, may go sub sonic later.



thanks
.

4x401
February 04, 2017, 21:05
Pretty much the same way it would help on any gas operated firearm...dwell time is the key to good functioning of a gas operated system...I.E., the longer the bullet stays in the barrel AFTER passing the gas port, the more GAS pressure is vented backward to function the system...That in turn increases reliability.

Sig220
February 04, 2017, 22:49
The word I hear is that if you want to shoot subs to use pistol length.

I think the thought being is that with reduced power charges, you don't produce as much gas/reduced pressure.

hueyville
February 05, 2017, 01:18
Just got my new 6.8 spc II barrel stabilizing heavy and slow subsonic rounds. It's a four groove 16" 1:7 twist with carbine length gas. Pushing a slow heavy bullet down a fast twist barrel am expecting some serious dwell time to pump gas back to bolt plus heavy push on rear of case onto bolt if unlocks early. Already have an as justice gas block, pigtail gas tube to increase gas to rifle length and an H2 buffer planned to slow down the bolts rearward push. Found a Woodliegh 180 grain gas check bullet mold for the 6.8 cast bullets thats reputed to work great in 6.8 subsonic loads. Plan for a 1,000 FPS goal hoping to high side just a tad without going transonic/supersonic. Big heavy slow bullet will be lumbering along for nine inches once pass gas port. Compared to most ammo it will be feeding gas almost three times as much time before exiting bore plus fact takes more powder to move the big heavy 6.8 bullets. Have two 7.62x40's that are purpose built subsonic rifles. Don't have any issues with lack of gas, both have pigtail tubes and adjustable gas blocks to slow gas down.

If building an AR pistol might worry about gas with low powder charges in 5.56 as 80 grain bullet is about as heavy as can push. Especially if using 10.5 or shorter barrel, would have to feed it as much gas as possible. Since your planning both super and subsonic rounds would look at a Noveske Switchblock, Superlative Arms Bleed off or MicroMOA Govnah where drill gas ports to match ammo selections. The Govnah have series of holes you slowly increase size with drill till get amount of gas need for each load then use point of bullet to push into position needed for ammo using. Lot of factors come into play according to bullet weight and dwell time. Good luck sir. If get this 6.8 to run well my two 7.62x40's will be gone and one less cartridge to keep rifles in. Was trying to get to where only use 223/5.56 and 6.8 but now have the 22 Nosler barrel on way. With any luck won't like it so can sell and use a turn bolt when need to reach that far.

HankC
February 05, 2017, 17:56
If you shoot mostly supersonic and full power, pistol gas helps you to spend the money to buy adjustable gas block.

Below is from 300BLKTalk Q&A

Q: Should I look for an AR upper with carbine or pistol gas?
A: First, there is no barrel length which works with mid-length or rifle gas. The choices are carbine or pistol. For barrels under 16 inches, pistol is needed - the bore is larger than 5.56mm so the gas pressure drops off faster. For barrels 16 inches or longer, carbine or pistol gas will work.

Q: Well then - for a 16 inch barrel, why would someone chose one type of gas vs the other?
A: If you are the type of person who prefers mid-length gas on a 5.56mm because it has less gas port erosion and less stress on the bolt and extractor, then choose carbine gas. If you want to experiment with extra fast-burning powders and are willing to either not shoot full power ammo or have an adjustable gas block - then use pistol. Remington subsonic ammo generates enough gas pressure to be compatible with carbine gas in a 16 inch barrel.

http://www.300blktalk.com/forum/viewtopic.php?f=128&t=73274

Also a good article
http://micromoa.com/300-blk-port-size-testing/

Right Side Up
February 05, 2017, 18:16
If the gun will ever see subsonic rounds I would go with the pistol length setup. My 300 BO has a Noveske barrel so I didn't have a choice in the matter. They're all pistol length (at least at the time).

I plan on doing what HankC said above, putting an adjustable gs block on it. I tried to get one a while back but it was sold out.

This is the one I want:

http://micromoa.com/govnah-gas-blocks/

Slow
February 06, 2017, 09:31
If you shoot mostly supersonic and full power, pistol gas helps you to spend the money to buy adjustable gas block.

Below is from 300BLKTalk Q&A

Q: Should I look for an AR upper with carbine or pistol gas?
A: First, there is no barrel length which works with mid-length or rifle gas. The choices are carbine or pistol. For barrels under 16 inches, pistol is needed - the bore is larger than 5.56mm so the gas pressure drops off faster. For barrels 16 inches or longer, carbine or pistol gas will work.

Q: Well then - for a 16 inch barrel, why would someone chose one type of gas vs the other?
A: If you are the type of person who prefers mid-length gas on a 5.56mm because it has less gas port erosion and less stress on the bolt and extractor, then choose carbine gas. If you want to experiment with extra fast-burning powders and are willing to either not shoot full power ammo or have an adjustable gas block - then use pistol. Remington subsonic ammo generates enough gas pressure to be compatible with carbine gas in a 16 inch barrel.

http://www.300blktalk.com/forum/viewtopic.php?f=128&t=73274

Also a good article
http://micromoa.com/300-blk-port-size-testing/



Thanks for the info.

i did not choose this barrel it kinda fell into my lap, and im trying to decide to use it or if it is a finicky problem child it might see my table at the Gunshow.





;)

jhend170
February 07, 2017, 09:58
The light powder charges associated with subs essentially necessitates a pistol-length system. The system requires both volume and pressure, and if the entire system volume were larger (i.e. a carbine-length) sufficient pressure to cycle the system would not be available.

I think most BO manufacturers work to the assumption that subs are likely the reason you're using the caliber anyway, and build to that end.

Think I'd cut it to the 14.5 and permanently attach a FH, if the goal is a rifle. 16" of barrel with a BO is about 5 or 6" more than necessary for the caliber to achieve complete powder burn anyway, which is why mine is a 10.5" pistol.

HankC
February 07, 2017, 19:41
"The light powder charges associated with subs essentially necessitates a pistol-length system. The system requires both volume and pressure, and if the entire system volume were larger (i.e. a carbine-length) sufficient pressure to cycle the system would not be available."

That is why .125" gas port for carbine gas system. .125" gas port will handle factory subsonic ammo fine if you have standard buffer (and lighter AR15 semi auto bolt carrier helps also). If have to, switch to a softer recoil spring. On the other hand, pistol gas system has smaller gas port, otherwise will be over gas when shooting supersonic, uses heavier buffer to slow bolt down and better uses an adjustable gas block. AR adjustable gas blocks are not like FAL with click gas setting. Personally, I would rather change recoil spring to a softer one, instead of messing with adjustable gas block, if I ever want to shoot subsonic. It all depends on what you shoot most, supersonic or subsonic, and make sure the gas port size supports the gas system.

"I think most BO manufacturers work to the assumption that subs are likely the reason you're using the caliber anyway, and build to that end."

Definitely more supersonic 300BLK ammo in the market than subsonic and cheaper! I would say BO barrel manufacturers assume folks shooing BLK with cans and therefore many carbine gas system barrels have smaller than .125" gas ports!:biggrin: