AR Builds!

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Re: AR Builds!

Postby Archer » 18 Nov 2022 21:58

I've looked this up a time or two before but it's been a few years...

PSA is using midlength gas but putting the A frame at the rifle position. They claim timing is the reason for this (not enough dwell time) but other sources claim the tendency to use an oversized (or maybe over-oversized or OVERsized) gas port by some builders is the problem in producing a good build.
https://palmettostatearmory.com/psa-16- ... 49779.html

(added by edit)
PSA also makes a carbine length upper, (but they apparently offer about 3 to 1 mid length to carbine models at least for the uppers)
https://palmettostatearmory.com/psa-16- ... or-ch.html

Windham Weaponry (the old Bushmaster folks who started up after the non-compete with Remington/Cerebus expired.) is using a M4 profile but doesn't mention the gas length. They also marry their Dissipator upper with a carbine lower.
https://www.windhamweaponry.com/firearm ... ipator-m4/

I've seen mention of Del Ton using a rifle length gas system but I know the last ones Midway sold were listed as carbine gas systems.

LWRC makes a sightless mid length gas with a free float mid length handguard and a 16" barrel on a carbine lower that they call 'dissipator' by those specs I've probably built several on both carbine and rifle lowers but most of mine have at least had back up sights and some of them have had rifle length free float rails...

Then there's this: https://www.pewpewtactical.com/what-is- ... issipator/ but I've found the historical narratives to be somewhat less than reliable at times.
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Re: AR Builds!

Postby GasGuzzler » 19 Nov 2022 06:45

I've only built one 20". It had rifle gas if I remember.

Almost all the 16" units I have built have used mid-length gas systems.
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Re: AR Builds!

Postby Archer » 19 Nov 2022 23:26

I'm not sure I can count all the ARs I've put together since I did some of that at the shop as well as helping out with friends on top of my personal builds for my own personal use.

Carbine gas length was more or less designed around a 14.5" barrel for the M4 the military adopted.
The initial rifle designs were 20" barrels, it can be expected that rifle length gas systems are matched to that length.
Mid-length gas systems are lengthened from the carbine system to match the 16" minimum civilian length barrels.

That covers most of the standard length systems but there's more...

Some folks feel the need to make the AR even shorter and turn it into a pistol. In spite of loosing half of their velocity in most AR calibers they make a pistol length gas tube for barrels under 10".

Apparently they've made an intermediate length gas tube. I assume that's for 18" barrels since it is longer than mid length but shorter than rifle length. Every 18 inch barrel I've ever since is ported for a mid or rifle length system. I think I've built three or four 18" ARs.

I've seen + 1 and maybe +2 length gas tubes for custom builds with barrels over 20" long but they tend to be few and far between. The couple 24" builds I've put together used rifle length gas systems.
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Re: AR Builds!

Postby GasGuzzler » 20 Nov 2022 06:41

I built one 18". It had a mid-length gas system.
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Re: AR Builds!

Postby Arroyoshark » 20 Nov 2022 15:24

Archer wrote:I'm not sure I can count all the ARs I've put together since I did some of that at the shop as well as helping out with friends on top of my personal builds for my own personal use.

Carbine gas length was more or less designed around a 14.5" barrel for the M4 the military adopted.
The initial rifle designs were 20" barrels, it can be expected that rifle length gas systems are matched to that length.
Mid-length gas systems are lengthened from the carbine system to match the 16" minimum civilian length barrels.

That covers most of the standard length systems but there's more...

Some folks feel the need to make the AR even shorter and turn it into a pistol. In spite of loosing half of their velocity in most AR calibers they make a pistol length gas tube for barrels under 10".

Apparently they've made an intermediate length gas tube. I assume that's for 18" barrels since it is longer than mid length but shorter than rifle length. Every 18 inch barrel I've ever since is ported for a mid or rifle length system. I think I've built three or four 18" ARs.

I've seen + 1 and maybe +2 length gas tubes for custom builds with barrels over 20" long but they tend to be few and far between. The couple 24" builds I've put together used rifle length gas systems.


Archer, methinks some of your ballistic understandings are un poco loco ...
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Re: AR Builds!

Postby Archer » 21 Nov 2022 00:32

Arroyoshark wrote:
Archer, methinks some of your ballistic understandings are un poco loco ...


Could be. I guess you could call the gas system part of the internal ballistics.

Rainier sells ALL the gas tube lengths I mentioned before including the 'rifle + 1', 'rifle +2' and a 'pistol -1' that I didn't know about.
https://www.rainierarms.com/gas-tube-stainless-steel/
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Re: AR Builds!

Postby Arroyoshark » 21 Nov 2022 15:54

RE: 'Some folks feel the need to make the AR even shorter and turn it into a pistol. In spite of loosing half of their velocity in most AR calibers they make a pistol length gas tube for barrels under 10"."
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Re: AR Builds!

Postby Archer » 21 Nov 2022 22:40

Arroyoshark wrote:RE: 'Some folks feel the need to make the AR even shorter and turn it into a pistol. In spite of loosing half of their velocity in most AR calibers they make a pistol length gas tube for barrels under 10"."


Ok, maybe a small exaggeration... and possibly thinking more in terms of energy than velocity

What I was thinking of WRT .308 Win energy:
SBRs in 7.62x51 vs. 7.62x39...
https://www.youtube.com/watch?v=U2dtJsXLIOI

Still going from a 20" or 16" to a 10.5" in 5.56... 390 to 580 FPS delta depending on ammo
https://www.youtube.com/watch?v=sNZv5Z1LpqI
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Re: AR Builds!

Postby Arroyoshark » 28 Nov 2022 15:35

Archer wrote:
Arroyoshark wrote:RE: 'Some folks feel the need to make the AR even shorter and turn it into a pistol. In spite of loosing half of their velocity in most AR calibers they make a pistol length gas tube for barrels under 10"."


Ok, maybe a small exaggeration... and possibly thinking more in terms of energy than velocity

What I was thinking of WRT .308 Win energy:
SBRs in 7.62x51 vs. 7.62x39...
https://www.youtube.com/watch?v=U2dtJsXLIOI

Still going from a 20" or 16" to a 10.5" in 5.56... 390 to 580 FPS delta depending on ammo
https://www.youtube.com/watch?v=sNZv5Z1LpqI



Archer, didn't see your response. Perhaps I was only needling a little, but I recall from engineering that it is sometimes risky arguing a point relying only on generalities.

For example, I actually avoid the 5.56 round because, for me, it is a snotty noisy round from anything under 18" barrel. Hurts my ears, even with ear protection and it annoys other nearby shooters at range. It is a rifle round, loaded with rifle powders, same by the way as your .308 reference. From short barrels, there is resultant muzzle flash and, ugh, unbearable for me, concussion. I find more enjoyment from some of the boutique calibers such as .300 Blackout or .350 Legend, even .458 Socom, because the rounds get loaded with either magnum pistol or really fast rifle powders. Those ballistics don't suffer as much going from, say, a 16-inch barrel to even a 7.5-inch barrel, as long as proper components are selected (and I am referring here to AR15 components).

The short barreled AR pistols function properly with pistol length gas system.

There are those that go hunting with shortened .308 carbines, even 12 -14-inch barrels. Those shooters have to deal with the ballistics of the shorter barrels with rounds loaded with medium burn or even slower burn rifle powders. Resultant muzzle flash, concussion and lower velocity/energy become a reality that has to be compensated for, and those shooters do compensate. They also kill deer, pigs and even elk.

Here is ballistic chart for .300 Blackout, for example, a round which I find generally superior to the 7.62x39 round, particularly in bullet selection, downrange ballistics and noise.

https://firearmwiki.com/wiki/.300_Blackout_Velocity_by_Barrel_Length

You referenced on a scale you were familiar with, no doubt, and I am referencing a different scale. With proper round/powder selection, the "delta" between barrel length/ gas systems becomes smaller.
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Re: AR Builds!

Postby Archer » 29 Nov 2022 00:07

Yeah, loads and performance are relative to what you are thinking about and there's lots of variations.

I'm always struck by the claim that's attributed to Paco Kelly that once you hit 16" on a lever action you don't get any extra OOMPH and will supposedly actually loose velocity. Doesn't seem to be born out if you load with magnum pistol powders.

I do step outside the standard calibers a bit with the AR but outside of 50 Beowulf and maybe 7.62x39 the ballistics are likely similar to the 'standard' calibers when you are looking at .243 Win and 6.5 Grendel. AND after the recent unpleasantness of the lock downs and ammo shortages there's a certain comfort in having (a) case(s) of 'standard' ammunition and knowing most of your friends have the same on tap.

----------
Speaking of oddball rounds...
Clerk at the gunshop this evening didn't have any .44 Remington Magnum, No Grendel, No Beowulf, No .300 Win Mag, No.444 Marlin.
He had a couple boxes of 6.8 SPC. Said I looked like a .45-70 guy and when I admitted it said the store wanted over $51.00 for a box of 20 COWBOY .45-70 rounds. Said he stuck to 5.56 and couldn't afford the expensive stuff. Wanted $44.00 per box of 50 .357 Magnums and the store only had 3 boxes.

Clerk at the sporting goods store a couple weeks ago got in a couple boxes of .300 WM but wanted over $60/box for Federal blue box leaded that can't be used for hunting here. She had some .44 Mag HPs at something like $64/20 rounds.

Something wrong when the manufacturers in the U.S. spends a couple years making almost exclusively the 4 or 6 most popular calibers and can't keep up.
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