Large frame (.308) AR receiver extension? Same as AR15 or no? - Page 1

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[ARCHIVED THREAD] - Large frame (.308) AR receiver extension? Same as AR15 or no?

Posted: 2/17/2017 2:05:39 PM EDT

Good afternoon folks. I'm speccing out a .308-based AR and I'm having some difficulties on the stock front. I am assuming .308-based ARs use a different receiver extension? Would anyone be able to detail this for me?

Posted: 2/17/2017 2:26:26 PM EDT

[#1]

Some do some don't. The better guns often use longer ones. Which parts do you have now?

Posted: 2/17/2017 2:33:49 PM EDT

[#2]

I was making the list first so I can buy most everything at once. It will be based around a 2A Armament XANTHOS-lite receiver set. I had thought to use the Mission First Tactical minimalist stock and JP captured spring, plus an adjustable gas block to really tune everything.

Posted: 2/17/2017 2:37:13 PM EDT

[#3]

Quoted:
Good afternoon folks. I'm speccing out a .308-based AR and I'm having some difficulties on the stock front. I am assuming .308-based ARs use a different receiver extension? Would anyone be able to detail this for me?

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There are at least 3 different RET lengths:

AR15 carbine RET, used with super short LR-308 carbine buffer
ArmaLite/A5 RET, used with AR15 carbine buffer length
Rifle RET, used with AR10 or LR-308 rifle buffer length (shorter than AR15 rifle buffer, longer than carbine)

Posted: 2/17/2017 2:39:31 PM EDT

[#4]

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Quoted:
I was making the list first so I can buy most everything at once. It will be based around a 2A Armament XANTHOS-lite receiver set. I had thought to use the Mission First Tactical minimalist stock and JP captured spring, plus an adjustable gas block to really tune everything.

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Quoted:
I was making the list first so I can buy most everything at once. It will be based around a 2A Armament XANTHOS-lite receiver set. I had thought to use the Mission First Tactical minimalist stock and JP captured spring, plus an adjustable gas block to really tune everything.

If you're using a standard AR15/M4 carbine tube (not an Armalite AR10 tube), you will use the JP .308-specific buffer/spring.
https://www.jprifles.com/buy.php?item=JPSCS2-10

The JPSCS2-10 is designed to work in standard 7-inch M16/M4-length buffer extension tubes on .308 AR-type rifles like the DPMS LR-308. Armalite .308 rifles, likewise, use these standard M16 extension tubes with rifle-length stocks, so this unit will function normally when installed with the supplied spacer. However, Armalite’s .308 carbines use a slightly longer, non-standard carbine extension tube. The JPSCS2-10 can still be used with these extension tubes by installing the spacer, which must be modified to fit. Alternately, the system will function readily if the longer Armalite carbine tube is replaced with a standard 7-inch .223 carbine extension tube.

Posted: 2/17/2017 3:09:33 PM EDT

[#5]

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Quoted:

If you're using a standard AR15/M4 carbine tube (not an Armalite AR10 tube), you will use the JP .308-specific buffer/spring.
https://www.jprifles.com/buy.php?item=JPSCS2-10

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Standard AR15 carbine tubes it is then. Thank you sir!

Posted: 2/17/2017 3:10:37 PM EDT

[#6]

It's not what I would do but it might work.

Posted: 2/17/2017 3:43:31 PM EDT

[#7]

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Quoted:
It's not what I would do but it might work.

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I believe the vast majority of .308 AR carbines floating around use a standard M4 receiver extension. Armalite is pretty much the only big exception.
The only "downside" is a shorter buffer with less room for dead-blow mass.

Posted: 2/17/2017 6:41:15 PM EDT

[#8]

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Quoted:

I believe the vast majority of .308 AR carbines floating around use a standard M4 receiver extension. Armalite is pretty much the only big exception.
The only "downside" is a shorter buffer with less room for dead-blow mass.

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This

Posted: 2/17/2017 7:34:46 PM EDT

[#9]

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Quoted:

I believe the vast majority of .308 AR carbines floating around use a standard M4 receiver extension. Armalite is pretty much the only big exception.
The only "downside" is a shorter buffer with less room for dead-blow mass.

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KAC uses longer also.

Posted: 2/17/2017 8:07:34 PM EDT

[#10]

Shifting gears slightly, can I assume the same about gas tubes? .308 AR mid length gas tube the same as a standard AR15 mid length gas tube?

Posted: 2/18/2017 3:55:18 AM EDT

[#11]

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Shifting gears slightly, can I assume the same about gas tubes? .308 AR mid length gas tube the same as a standard AR15 mid length gas tube?

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Correct.

Posted: 2/18/2017 4:06:45 AM EDT

[#12]

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Shifting gears slightly, can I assume the same about gas tubes? .308 AR mid length gas tube the same as a standard AR15 mid length gas tube?

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You want rifle gas for 16" barrels.

Posted: 2/20/2017 7:04:30 PM EDT

[#13]

Why is that? Proper pressure curve? JP seems to think midlength on a 16" .308 barrel is okay.

Posted: 2/20/2017 9:55:23 PM EDT

[#14]

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Quoted:

Correct.

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Quoted:

Quoted:
Shifting gears slightly, can I assume the same about gas tubes? .308 AR mid length gas tube the same as a standard AR15 mid length gas tube?

Correct.

Incorrect.

Some are the same, and others are longer, like on the ArmaLite carbines.

There are at last 2 different MLGS tubes on the market, and at least one additional specific length for another manufacturer who doesn't sell parts, only complete rifles.

I do believe most barrel makers use an AR15 MLGS length for their port locations on AR10 barrels with MLGS.

Posted: 2/20/2017 9:56:37 PM EDT

[#15]

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Why is that? Proper pressure curve? JP seems to think midlength on a 16" .308 barrel is okay.

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You can set the gun up for either.

If you're going to be shooting lots of 175gr suppressed, I would go 16" RLGS or ILGS.

Posted: 2/23/2017 10:54:38 PM EDT

[#16]

It'll shoot whatever my department issues, really. This would serve the role of patrol rifle and sub in as precision rifle when needed. I might even go for JP's 14.5" barrel and pin a nice Ti comp / suppressor mount. As long as I can run it through a couple three-day classes (our patrol carbine quad class is three days) then that's what I'll go with.

Posted: 2/24/2017 10:46:06 AM EDT

[#17]

Just eyeballed a DPMS LR308 carbine vs a 6920... The 308 Re is about 3/4 " longer than the 6920.

Posted: 2/24/2017 12:57:07 PM EDT

[#18]

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Just eyeballed a DPMS LR308 carbine vs a 6920... The 308 Re is about 3/4 " longer than the 6920.

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From the factory, they use the same RT length.

Recalibrate eyeball, or check to see if someone installed an A5.

Posted: 2/24/2017 1:08:20 PM EDT

[#19]

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Quoted:

From the factory, they use the same RT length.

Recalibrate eyeball, or check to see if someone installed an A5.

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This. They should be identical. The stock lock holes may be different between brands so if you're comparing them with stocks on that may be the source of your confusion.

Posted: 2/25/2017 8:43:37 PM EDT

[#20]

disclaimer: I do not want to represent my self as someone that knows anything about 308 ARs, just saw this thread and happened to have my 308 out of the safe for the 1st time in years. Had it for about 8 years and never even fired it, until today.

I measured the 6920 and the DPMS today, the 308 is about a half inch longer about 7.2" vs 7.75", what's going on? Now I wish I had measured another carbine before I put them in the safe.