Best AR Calibers

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giggitygiggity

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What is your favorite AR caliber that can be fed in a standard AR15 lower?

I am interested in building three ARs and there are plenty of caliber using regular (AKA non-.308) lower receivers. Of the calibers that can be accommodated by your standard AR lower, what are the fan favorites? I am definitely building one in .223Rem and one in .200BLK. I have plenty of 7.62x39 rifles so I am not interested in that caliber. However, there are others such as 6.5 Grendel, 6.8SPC, and I'm sure a handful of others that I'm sure some THR members would love to try to sway me to build.

I'd like to stay away from any bizarre, super expensive calibers as I'll most likely be buying an upper, buying a lower, and slapping them together. So let's hear it... what are your favorite AR calibers and why? Convince me to build the third AR in your favorite caliber. Any feedback on uppers that work particularly well in a certain caliber is also welcomed. Thanks.
 
I don't think this thread would be complete without a big bore upper in your arsenal. The market is pretty evenly spilt between three calibers. 50 Beowulf, 450 bushmaster and 458 socom. Each has its minor differences and advantages. A search of "big bore AR" or "large bore AR" will give you hours of reading on the subject.


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I have a WSSM in .358. It's neither common, inexpensive, or easy. And my upper took about a year to get. But it is excellent. Probably the best combination of distance versus whack in an AR.

I'd probably look at the .458, simply because it takes rifles bullets. I had one. Hard hitter, but not cheap. Definitely a reloaders round IMO. Filling a ten round mag with factory ammo takes $30. Ouch.
 
I am a huge 6.8 fan I wouldn't own an AR until they became available in a better caliber.
Had AR's been available in 6.8 when I bought my AK, I wouldn't have bought the AK.
My second AR was a 5.56 DPMS and I bought ammo for it, but quickly converted it to 6.8 also and sold the ammo.
Just looking at the 6.8 ballistics vs. 5.56 it completely blows it away. It has loads ranging from 85gr. to 120gr. and only loses 5 rounds in a standard magazine to a 5.56 magazine.
Consider the 5.56 is .22 caliber and the 6.8 is .270, I would not hunt deer with a .22 even though people do. But I would have no reservations about hunting deer with a .270 caliber.
I honestly don't see what the attraction is regarding the 5.56, maybe cheaper plinking ammo. Start shortening the barrel length and it becomes even less effective on soft targets let alone any barrier penetration qualities. This is the reason for the creation of the 6.8spc in the first place as the 6.8 excels in short barrel lengths. Other than that the 5.56 cartridges are ever so slightly smaller and lighter and the recoil difference is negligible.
I'd rather have the barrier penetration qualities of the 6.8 then the "soft" target only ability of the 5.56.
Both my 6.8 AR's have 16" AR Performance barrels and they are deadly accurate. I happen to like the 16" barrel lengths, seems just right to me so I don't have to SBR them.
 
Haven't shot it but 25-45 Sharps looks interesting...ammo is hard to come by, but it's a neat idea.
 
I love my 6.8

Great hunting round for almost everything in North America

1k ft lbs out to 275 yards with Hornady SST 120g
 
My favorite AR-15 caliber is by far the 7mm Valkyrie...it is the best AR-15 cartridge when medium to large game is considered. It is a true wildcat round but "packages" are available online that include everything needed its just assemble and reload.

A legitimate elk catridge within reasonable distances and no white tail will ever be safe.

A close second would be the .30 Rem AR brand new rifles are available from several places online. Factory ammo is available and it's a legitimate elk catridge also.
 
I have them in 5.56, .300 B/O, 6.5 Grendal and .50 Beowulf. Each is my favorite in their own nitch.
 
I ran the #'s of the 7mm Valkyrie with my personal pet load for the catridge. I'm getting 2782fps with a 120gr NBT in a semi auto with a 20" barrel it's a mild load too. It has 1272ft lbs of energy at 300 yards.

The 30AR is a semi auto 22" barrel. I shoot a 150gr SST @ 2526 it has 1236ft lbs at 300 yards.
 
6x45 is arguably the most generally useful of the AR15 calibers. It has about the same reach as 77 gr. 5.56, but with a much heavier and somewhat larger diameter bullet giving superior terminal performance.

6.5 Grendel is the worst AR caliber to ever get to the point of commercial manufacture. It breaks bolts or extractors (depending on which of the two deficient designs you choose), and offers zero performance advantage over 77gr 5.56 or 6x45. It's an example of what happens when idiots who don't understand ballistics design cartridges.
 
So apparently my Grendel didn't get the memo about breaking bolts and extractors...

I guess 5.56 can fall into the worst category too because I've broken extractors on my 5.56
 
The abject failure of the 6.5 Grendel bolt design is well documented. The case head is simply too large for the platform, and the pressures too high.
 
So I'm guessing my .308 bolt head in a AR-15 @ 60k psi isn't possible the and it's a time bomb waiting on a broken bolt... it uses standard bolt carrier and barrel extnsion.
 
CarJunkieLS1 said:
So I'm guessing my .308 bolt head in a AR-15 @ 60k psi isn't possible the and it's a time bomb waiting on a broken bolt... it uses standard bolt carrier and barrel extnsion.
Click to expand...

You'll have to explain yourself here. The .458 SOCOM has the .308 bolt face and a 35K PSI limit because of bolt issues.
 
7mm Valkyrie has a max PSI of 60k...it uses Lapua 6.5x47 or can use any .308 casing. The brass is converted to 7mm Valkyrie by cutting the case, using a forming die, and then turning the neck for its match grade chamber for better accuracy.

It works I promise you and myself and 30ish others have ZERO bolt issues.
 
That frankly looks like a horrible idea. While the bolt thrust per PSI will be lower than .458 SOCOM due to a narrower case, there's no way that's within the safe limits of an AR15 bolt.

Just because you haven't blown anything up YET doesn't mean it's correctly engineered with an appropriate margin of safety.

Has anyone tried to shoot a ~85KPSI proof load yet? That's what is required to be safe in normal operation at ~60KPSI.
 
Well a lot of people smarter than I designed the round they did pressure testing to back it up. I can't say for certain what the bolt has been tested to, but I know the bolt is more than capable to withstand the pressure associated. It's a bolt made by AR performance
 
Llama Bob said:
That frankly looks like a horrible idea. While the bolt thrust per PSI will be lower than .458 SOCOM due to a narrower case, there's no way that's within the safe limits of an AR15 bolt.

Just because you haven't blown anything up YET doesn't mean it's correctly engineered with an appropriate margin of safety.

Has anyone tried to shoot a ~85KPSI proof load yet? That's what is required to be safe in normal operation at ~60KPSI.
Click to expand...

It uses a totally different bolt and bolt extension designed from the ground up to handle those pressures and a .473 case head. It is nothing like a 5.56 bolt and extension. Well documented safe performance across a number of different chamberings in various calibers.
 
Do you mean "barrel extension" not "bolt extension"?

CarJunkieLS1 previously said the barrel extension was unchanged.
 
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