New AR10 upper build caliber?

Golddiggie

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Alright... I have a stripped AR10 upper (Aero, high profile DPMS pattern) that is feeling a little unloved. I'm trying to decide what caliber to use on it (that won't be ghey) for another option for the rifle setup. I already have a 308 Winchester (18" barrel) upper for the lower. Also trying to decide what barrel length to get it in. I'd prefer to be able to use the same BCG that I have now with the next upper. Making it a bit more economical.

I did some hunting last night and see some decent barrel options for 6.5 Creedo (up to 24" IIRC). Right now I'm not setup for longer distance shooting, but won't rule it out in the future. My club (PF&G) has a 600 meter/yard range on it, along with steps between 100 and 600. So I have a location I COULD stretch out on. Well, once I get all the knowledge needed to actually shoot at such distance(s).

So, post up your thoughts on what caliber, and barrel length, to go with for this build. Right now, I'm leaning towards a barrel no longer than 20". Mostly because of how much will stick out from the handguard at that point. Only a few go beyond 15" from what I've seen.

For reference, I have AR15's in a few calibers. Your standard 5.56/.223 (carbine and rifle setups as well as one pistol setup), 300 Blackout (one carbine, one pistol) and 6.5 Grendel (18" barrel build). I also have an AR10 pistol setup (12.5" barrel) because why the F not. ;)

After the caliber for the new AR10 rifle upper gets picked, I'll have to start thinking about glass options. I might just get the same for both rifle uppers to keep things easier (on me at least). Not looking forward to THAT spend. Considering what it costs for even mid tier glass these days.
 
I suppose I could just save the upper for another pistol build... Once I'm sure the current one works 100% of course. I think I had the wrong buffer in it before I did the Cerakote to the thing. Might need to get a different buffer spring too (not sure).

Just seems kind of a waste to have the stripped upper sitting in it's little box not getting any love.
 
6.5cm.

It can do everything the 308 can, but better.

Good ammo is not expensive (you can get sub MOA without using match ammo) and reloading is stupid easy - H4350 + 143g ELDX /ELDM will shoot sub MOA as long as you dont s*ck and develop a load.
 
Since you already have a .308 18 and 12.5(nice btw I have an itch for a 12.5 .308 right now)
Why not do an 8.6 blackout to go along with your 300 blackout.
Not really economical. But you would have the 2 quietest big calibers out there.
 
I might get something in 8.6 in the future. I don't have any cans at this time. Budget limitations lately have prevented doing that. Besides, whatever I end up building will [most likely] work with whatever can I get for my 300 Blackout as well.

BTW, if you want to check out the 12.5 308, let me know. Thinking about taking it to the range this weekend to function test it out. Got a decent supply of ammo I can feed it.
PXL_20241111_194311155.jpg
 
If you have a 308 and no other special need, go with a 6.5 CM. And no need to go for a long boy. A 16” has the same sort of external ballistics as a 20” 308.

But, if you do have specific needs, these other calibers could be interesting and useful:

8.6BLK or 375 Raptor: Heavies for subsonic hunting if that’s your thing. 375 Raptor is better, but you need to load your own ammo.

7mm-08: Great general purpose hunting round

6 CM: Good for competition or hunting, anything from varmint through deer

22 CM: Great varmint cartridge. Like a modern 22-250 capable of using longer/heavier bullets
 
That new military round? 6.8x51. I know almost nothing about it. It fits in an AR10 and supposedly operates at 80,000 PSI. I didn't think an AR could handle pressure that high.
 
That new military round? 6.8x51. I know almost nothing about it. It fits in an AR10 and supposedly operates at 80,000 PSI. I didn't think an AR could handle pressure that high.

They’ve ended up lowering it to 70k psi because they couldn’t get enough accuracy at 80k psi.

Kind of a bait and switch for the government who wanted a certain velocity in the program.

All that said, I haven’t seen a single AR10 barrel maker offer the 6.8x51/277 fury chambering and advertise full pressure. They all say their barrels are only for normal pressures. Now, it’s probably just liability. If you use a JP high pressure bolt, you’re probably fine, especially with 70k instead of 80. But the XM7/SPEAR is the only gas gun currently advertised as handling the higher pressures. I’m not sure if they did anything to their bolt that’s different.
 
They’ve ended up lowering it to 70k psi because they couldn’t get enough accuracy at 80k psi.

Kind of a bait and switch for the government who wanted a certain velocity in the program.

All that said, I haven’t seen a single AR10 barrel maker offer the 6.8x51/277 fury chambering and advertise full pressure. They all say their barrels are only for normal pressures. Now, it’s probably just liability. If you use a JP high pressure bolt, you’re probably fine, especially with 70k instead of 80. But the XM7/SPEAR is the only gas gun currently advertised as handling the higher pressures. I’m not sure if they did anything to their bolt that’s different.
Are they selling full pressure ammo (or even the three part case) to us plebes? If not, I can't see the barrel limitation mattering that much.
 
Are they selling full pressure ammo (or even the three part case) to us plebes? If not, I can't see the barrel limitation mattering that much.

They are. It’s just super expensive because it’s so low quantity. Sig is making the ammo for the government as part of the current contract and it’s not yet made by Lake City
 
20-22” in 6.5 Kardashian. Ammo is on par with 308, price wise. Thing is, you’ll have nothing anywhere near NH where you could stretch it to its limits. Or just go full battle rifle in 308 with irons. But then, I’ve seen your 👓. I doubt your eyes got any better in the last 10 years or so.
 
Yeah, iron sights for me are pistol range only. As in under 25 yards. My Rx changed enough that I couldn't use a set of frames I really liked since they couldn't handle the thickness of the lenses.
 


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