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1/14/2017 8:11:35 PM
Posted: 8/29/2011 7:28:03 AM EST
What is the largest caliber upper that will fit on a standard AR-15 Lower (Sig 516)? I just got my Sig, and love it! But I was looking to go elk hunting with my brother next year, and was wondering about what my options were. I have been leaning towards a 7.62 as I have heard that it will fit, and there is no mods needed to the lower.
Link Posted: 8/29/2011 7:40:20 AM EST
Negative. A 7.62 upper does not mate with a 5.56 lower. Whole different beast.

From the FAQ section of the AR15 discussion forum:

Q) How many calibers can I get uppers that fit my .223 (AR-15) lower? What are they?
While the list is nearly endless with all the wildcats possible. Here are the some of the more common and more interesting uppers that are (or have been) produced for the standard AR-15 lower (may not be up-to-date):
6 x 45
7 x 40
357 Mag Rimless
300 Whisper / .300Fireball / 300-.221
6.5 x 40
.30 Coyote (30 x 45)
.17 Remington
.17 Mach IV
.20 Tactical
.19 Calhoun
5.7 FN (developed by Rhinland Arms - no word yet on who is going to mfg it)
.204 Ruger
6.8 SPC
30 HRT
338 Spectre
10 mm Mag
219 WASP-C
7.62 x 40
7.62 x 39
.338 x 39
6.5 Grendel / 6.5 AR / 6.5 PPC Improved
22 PPC
7 sCAR ....
30 Carbine
5.45x39 / .21 Ghengis
7 BR
6.5 BR
300 Sabre
8mm (belt fed upper)
.308 (again a belt fed upper - no longer available)
44 Mag
440 CorBon
44 AMP
.475 Tremor
50 Beowulf
.50 BMG (single shot)
.50 Blackpowder (1 made)
22 LR
17 HV
224 BoZ
.357 Sig
.357 Auto
223 WSSM
243 WSSM
300 WSSM
.30 RAR
Link Posted: 8/29/2011 8:11:09 AM EST

6.5 and 6.8 are going to have better long range ballistics.
Largest calibers will have more nockdown power at close range.
Rem .30 AR supposedly mimics .308 but when I looked it seemed to have light bullet and 2800 fps range velocity.
You can buy Rem ammo off shelf easier in most places I would think.
Link Posted: 8/29/2011 9:39:24 AM EST
[Last Edit: 8/29/2011 9:44:58 AM EST by Circuits]
I run a 50BMG upper on a standard lower. Any caliber too big to fit in the magwell, however, will be a side or top mag feed design, or a break-open or bolt action single shot design.

.50 Beowulf and .458 SOCOM are the biggest I know of that will fit and feed through the standard magwell.

For hunting, a 7.62x39 or .300 whisper are probably going to be the easiest, least expensive .30 cal to get into with a stock lower. 7.62x39 has long been a factory caliber variant, and .300 whisper is pretty widely available, uppers and ammo both.

458 SOCOM is a hard-hitting bastard if you can find an upper and ammo for it. It's basically 45-70 ballistics or better from a modern self loading platform.
Link Posted: 8/29/2011 11:53:24 AM EST
One guy said that a 7.62 will not mate with a standard 5.56 lower, but they another guy said 7.62 x 39 will mate up with a 5.56 lower... Which is correct? I am hoping that the 7.62 x 39 will work because of the cheap and availability of the rounds.
Link Posted: 8/29/2011 12:35:18 PM EST
7.62x39 will fit, howevever, 7.62x51 will not.
Link Posted: 8/29/2011 12:58:42 PM EST
Originally Posted By warpig8654:
7.62x39 will fit, howevever, 7.62x51 will not.

Exactly. 762x51 (aka .308) needs another rifle size.

The 3 big calibers are .50 Beowulf, .458 SOCOM, and .450 Bushmaster.

Of those 3 I've only shot .450 BM. Thumps like a heavy hitter, but without a ton of recoil. (Tons of fun on F/A)

Also, some people may have read hunting without reading "Elk" hunting. I'm not a hunter but I believe 7.62x39, .300BLK or Whisper might be on the anemic side for such a large animal.

.450 BM may be the easiest and least expensive upper to get ($550). .50 Beo may be the hardest to get support for, as I know Alexander Arms had been having issue delivering ammo and sales have been tapering off.
Link Posted: 8/29/2011 1:09:43 PM EST
Donvian, I found this in the archives with someone essentially asking the exact same questions.

Originally Posted By RealTeacher:
Such rifles and cartridges would be "instruments of destructive surgery" on an animal as large as an elk. Keep in mind that your ballistics are not as good as a 41 magnum at the muzzle, but with the heavy bullets it would catch up and pass a 41 mag at over 75 yds.
Elk are big and can be tough. I hunt them every year, and I have for many years. I have been a guide for hunters for them for 30 years. I have killed one with a bow and arrow in the past, and several with rifles. I have seen the results of many many others. Hit with precision with a bullet that won't come apart, you can kill elk (or any animal) with a much smaller cartridge than most men use, but you had better hunt as if you were hunting with a bow. Just hitting them on the shoulder is NOT going to do a reliable job with such shells as you are discussing here. You might get away with it, but you might not too.
I have had to track an elk once for 6 hours that was hit on the shoulder blade with a 7MM REMINGTON MAG with the wrong bullet (140 grain, at close range) I have tracked more of them that I can easily count that were hit with 7mm and 300 mags (when they were hit a bit too far back) and it's not a fun job. An elk with a liver shot, gut shot, or hit in just one lung, can go a LOT farther than a white tail, and you measure those jobs in MILES and HOURS, not minutes and yards) Also keep in mind that most of these tracking jobs go into places you would not want to walk if your were not trying to track something, let alone doing it bent over. They will go into the steepest and thickest cover they can find in the Rocky Mountains, and trust me, there is a LOT of such country in the Rockys
IF YOU ARE WILLING TO GO HOME WITHOUT AN ELK rather than even risk a questionable shot, I would say go ahead.
But most men reading this don't live in elk country, and can't just "come back tomorrow" for free, or at least for very little money.
If you are wanting to hunt elk with an AR-15, and you don't have limitless time, please do your self and the elk a big favor, and get a 458 SOCOM or a 50 Beowulf.
Or, get an AR-10 (and it's copies) size rifle in a 260, 7-08, 308, 338 Federal, and never look back. And you should be shooting very good bullets in the 260 and 7-08. I recomend Barnes X for such hunting and Nosler Partitions in the heavier weights too.
In the 308, use 180 grain bullets of solid construction. In the 338 fed, go with a 210 gr Nosler Partition or a 225 grain Partition.
The suppressed sub-sonic weapons are very cool, and they really have a special function, but elk hunting is not that function. Run at full power without the suppressor, they are better, but it's still a light hitter compared to a 270 Winchester, 308, 30-06 and so on. Think of them like you would an archers arrow, and you can do well, but it's not the weapon I would choose unless you just want that extra challenge.

I hunted elk 3 years ago with my 6.8 SPC I didn't get one) and I think the shell is too small for general elk hunting, but I walked away from 3 small herds of them because I could not get the shot I wanted. If you can do that too, then I'd say you are an ethical hunter, and I would not ask you to leave the rifle home
But if time and money prevent you from walking away from the only shot you'll get on a hunt of a lifetime, (and at the expense of several thousand dollars,) GET A 458 SOCOM or a 50 Beowulf! Or an AR-10 sized platform in at least a 308 and a 338 Fed would be better.
Or just use more conventional rifle in 270 or larger caliber. And use tough bullets!

Another thing to think about, the price of a new AR15 upper might be more than a dedicated Remington 700 in the caliber you want.
Link Posted: 8/30/2011 3:57:48 AM EST
[Last Edit: 8/30/2011 3:59:17 AM EST by vick1000]
6.8 SPC and 6.5 Grendel are the most popular production ammo.
Link Posted: 9/1/2011 7:40:06 AM EST
Originally Posted By TCBA_Joe:

.450 BM may be the easiest and least expensive upper to get ($550). .50 Beo may be the hardest to get support for, as I know Alexander Arms had been having issue delivering ammo and sales have been tapering off.

Finally someone mentioned the .450BM!
The .50Beo and .458SOCOM are both more expensive to feed if shooting factory made ammo.

While it may not have the range of bullet weights the .458 has, the .450BM is also cheaper to shoot and has 2 factory produced ammo available (Hornady 250gr FTX and Rem 260gr Accutip).

If you handload (most big bore AR enthusiasts I know do) and have a swaging press, you can use some of the heavier .458 bullets after they've been resized to .452)

Five shot Sub MOA groups @ 100yds with Factory Hornady ammo is easy to achieve, even for novices. I love mine!
Link Posted: 9/2/2011 7:22:36 PM EST
Originally Posted By Donivan:
One guy said that a 7.62 will not mate with a standard 5.56 lower, but they another guy said 7.62 x 39 will mate up with a 5.56 lower... Which is correct? I am hoping that the 7.62 x 39 will work because of the cheap and availability of the rounds.

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