10.5 inch Barrels in 5.56: Gas system length, twist rates, etc.

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valnar said:
I'm one who feels the rifle (barrel length) and caliber/cartridge (powder, burn rate) are generally designed to be symbiotic. I know tons of people build AR's with a <10" barrel, but I don't see the point. That being said, I wouldn't personally go lower than 11.5" for many of the reasons (and pictures!) listed above.
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The point is a more portable package. Sure, you'll lose some velocity but it doesn't totally neuter it and I don't need to stretch a 10.5" carbine (braced pistol) to 400yds anyway.
 
MistWolf said:
The Army released a study where they found the bullet has exited the bore and traveled several feet down range before the system pressurizes. . .
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I still want a cite for this if you have one.

Your explanation is plausible provided that the operating fluid is heavy enough to store the operating momentum. If the mass of the operating fluid was high enough to store operating momentum in the gas traveling up the gas tube, then you could uncork the muzzle before the pressure wave reached the piston. I think the gas is too light to do that.

So, citation please.
 
edwardware said:
I still want a cite for this if you have one.

Your explanation is plausible provided that the operating fluid is heavy enough to store the operating momentum. If the mass of the operating fluid was high enough to store operating momentum in the gas traveling up the gas tube, then you could uncork the muzzle before the pressure wave reached the piston. I think the gas is too light to do that.

So, citation please.
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The mass of the gas is only about 25 grs, depending on what powder is used. The energy comes from heat expansion. Unrestricted, smokeless powder expands at about 5700 fps, if I recall correctly. Regardless, it works.

I found the Army report on the Armalite Technical Notes website some years ago. Since then, Armalite has removed them due to ITAR concerns- https://www.armalite.com/Armalite/Support/FAQs

I've been searching my computer to see if I'd downloaded the report somewhere, but haven't found it. While I'm doing that, I have a video for you to study-

There are other slow motion videos that may be a little clearer, but at the risk of getting off the subject, I chose this one because the AR used is a cutaway and shows what goes on inside during the actual firing sequence.
 
Are we saying that the gas system becomes pressurized after the bullet has left the barrel or it starts to pressurize when the bullet passes the gas port?
 
So with availability issues and stocks in the product getting lower and lower, I think it's been getting harder and harder for me to be choosy about my selections. All I had initially was an Anderson stripped lower.

I decided to just go with my best options right now just in case I availability gets even more difficult. I did have to adjust my preferred parts list a bit, mostly due to so many products just not being available right now but also in a couple of cases, due to excessive costs. I've been looking at these parts for weeks trying to decide what I wanted for this pistol build and have seen prices shoot up on a lot of items. Here is the list of what I pulled the trigger on today and actually ordered:

  • barrel - TRYBE Defense AR-15 Barrel, Pistol 10.5in, .223 Wylde Government Profile 1:8 twist, carbine length gas system
  • Gas block - Aero Precision Low Profile .750 Adjustable Gas Block – Nitride
  • gas tube -TRYBE Defense AR-15 Gas Tube carbine length
  • handguard - Dirty Bird AR-15 SMRS Handguard – Slim M-LOK Rail System Gen 2 - black since fde was sold out - 9.7 inch
  • muzzle device - KAK Industry 1/2-28 Flash Can - outside diameter 1.36, length 3.25", weight 4.25oz
  • Upper Receiver- Dirty Bird AR-15 Stripped Upper Receiver - OD Green - wanted fde, but they were sold out on all, and this was the last od green one
  • forward assist - Strike Industries Forward Assist Kit – fde
  • dust cover - NBS AR-15 Dust Cover / Port Door Kit - all the ones I wanted were sold out, so went cheap on this one
  • lower parts kit - Aero Precision MOE SL Lower Parts Kit - FDE - didn't have the budget to get the trigger I wanted right now, so went with a kit that had one included instead of the one I had picked out, plan to upgrade in a couple of months
  • buffer tube - Trinity Force Pistol Buffer / Receiver Extension Kit - I'm not sure I got the right buffer tube...
  • pistol brace - SB Tactical SBA3

Didn't get yet, but still will need:
  • charging handle
  • bolt carrier group
  • Sights
  • Sling
  • Hand Stop

I'll have to post some pictures once the order arrives, hopefully, there won't be a large delay since I was careful to only order in-stock items. Let me know what you think of my selections if you have any opinions or advice.

Thanks.

g3ops
 
Long term plan for my 7.5” is a can and maybe a longer handguard. Should that become the new WOW! then I could double stamp it. I had originally intended to build a second upper using Aero parts to convert it back and forth.

I could always sell 1 now and realize that dream sooner but I’m patient.
 
When I built my 10.5" SBR, I went with a Noveske barrel, after a bit of back and forth with them over the particulars. This was a few years ago, so don't ask me for details. But, they pretty much explained that its the gas pressure at the port as the bullet passes, but also how long that pressure is applied, aka, distance from port to muzzle. I could probably weigh the buffer they said to get, and say which it is but it would all be valid only for a standard spring and then Noveske port. What they said was pretty much in line with another page I saw that had all the different buffer configurations, weights, innards... It was a heavier buffer than the carbine type. The spring is a standard carbine spring. Anyway, Noveske said "do it this way" and I did it the way they said. No issues, no battering, no nuthin, the rifle just works as it is expected to work, and shows good accuracy with the Aimpoint Pro. Basically the rifle will do better than I can with that set up, and I don't think I'd have any issues hitting say a standard "torso" target out to more distance than I'd have any reason to shoot during a defensive situation.

(edit in)
Forgot to say a "Flaming Pig" takes all of the flash, and a lot of the bark and makes 'em disappear. They say the sound is "directed" forward. Dunno, but its not any worse to shoot than the carbine length. I've also got a Noveske 7.5 inch on the other SBR, different buffer, same spring. That one also is pretty well tamed with the Flaming Pig. They're big, heavy, but work.
 
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Project355 said:
When I built my 10.5" SBR, I went with a Noveske barrel, after a bit of back and forth with them over the particulars. This was a few years ago, so don't ask me for details. But, they pretty much explained that its the gas pressure at the port as the bullet passes, but also how long that pressure is applied, aka, distance from port to muzzle. I could probably weigh the buffer they said to get, and say which it is but it would all be valid only for a standard spring and then Noveske port. What they said was pretty much in line with another page I saw that had all the different buffer configurations, weights, innards... It was a heavier buffer than the carbine type. The spring is a standard carbine spring. Anyway, Noveske said "do it this way" and I did it the way they said. No issues, no battering, no nuthin, the rifle just works as it is expected to work, and shows good accuracy with the Aimpoint Pro. Basically the rifle will do better than I can with that set up, and I don't think I'd have any issues hitting say a standard "torso" target out to more distance than I'd have any reason to shoot during a defensive situation.

(edit in)
Forgot to say a "Flaming Pig" takes all of the flash, and a lot of the bark and makes 'em disappear. They say the sound is "directed" forward. Dunno, but its not any worse to shoot than the carbine length. I've also got a Noveske 7.5 inch on the other SBR, different buffer, same spring. That one also is pretty well tamed with the Flaming Pig. They're big, heavy, but work.
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Pretty much exactly ! Or with a suppressor instead of the great Flaming Pig !
 
I'm open to being schooled! Novekse said the FP held some pressure back, like a suppressor does, and its why they said to use that on their short barrels. Since I don't know zip about suppressors except having seen an oil filter on a 10/22 barrel..... Do all suppressors have similar gas/pressure release properties regardless of physical size? Maybe not the right thread for this... if so, sorry. Noveske urged caution about just "using parts" as opposed to using the "right parts" when installing those shorty barrels. So... I dunno... clue me in.
 
Suppressors slow and cool the gasses before they exit. That means it takes longer for the pressure in the bore to drop. The result is greater gas flow in the system. This increases BCG speeds. A smaller gas port is needed when a suppressor is added. Also note that the shorter the gas system, the greater affect changes in gas port diameter has on the action.
 
3 prong flash hider works pretty good on my 10.3.
I did try a 3 port brake one time telling myself it won't be too bad, lol.
Well, I was lucky I still had my eyebrows after that :oops:
 
So most of the parts came in. Looking pretty good so far.

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Got it mostly put together. Still waiting on some parts.

6fK31EP4Zh12VMXuEZ4QiI_gkr9CDeQ9hWesF4fzdNKapSQWkBnAxo4UEOtl9LM64TdzsGw=w1920-h784-no?authuser=0.jpg
 
The Flaming Pig does hold some back pressure and the 7.5" Diplomat pistol was advised to allways use it for good functioning, and in my experience they were right ! However 5.56 barrel generally suck IMHO so that is the one Noveske Item I sold off, the Diplomat 7.5 upper. I kept it's lowerand the Flaming Pig , added a brace and put it on a gas trap upper 9.5" rifled, the gas trap chamber is another 1" longer before barrel threads (another and more radical way of reliably slowing down AR cyclic rate ) and it works super duper with not that much blast , except forward :) Your build looks good. And for the record 8" barrels and longer work OK on .300 BO ! Go figure ! I also have a Infidel carbine , the newer 2nd type type version and have a Flaming Pig pinned and welded to the barrel to just hit 16" when the Pig is dissembled for cleaning. Because of the polygonal gain twist rifling in the gas trap 9.5" rifled section the chronographed velocities of it are only 50 fps slower, at the most. from the 13.7" Infidel barrel !!!
 
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