New AR build - caliber selection stepping out of comfort zone

I am planning to put together a new AR build, and am going to go a direction I have not gone before and wanted to get some input.

To start, since I am a big proponent of keeping the goal in mind, I am looking at a hunting build that my vertically challenged oldest son can handle with ease (~11, but he has celiac disease and is a good bit shorter than he should be thanks to a couple years awaiting the diagnosis). He can handle an AR pistol with a 10.5 bbl with ease, so the thought was an AR pistol for him to use hunting. He manages to carry a 20 ga pump with a 20" bbl, but I am thinking AR pistol will be a little less unwieldy for his small size (and more accurate). We hunt rifle counties, so no concern there.

All of that said, I was thinking to go with a .300AAC bbl in 10.5, and already have a barrel lying around, as well as a couple spare stripped uppers. But, I got to thinking about it, and am really interested in using one of the big boys - .450 bushmaster, .458 socom, or .50 Beowulf. Of those, I like the .458 socom the best because it has a tapered case and I understand may have some inherent accuracy benefits over the other two straight cases (and also a wider range of loading options).

But, I don't know much about moving to these calibers and what modifications are required compared to moving to say, .300 or 6.8. I know the bolt will need to be changed, but are standard AR uppers compatible without modification? What about magazines? I believe the .458 will work in standard mags, but I'm not sure about the other two.

My son isn't taking shots beyond 200, goal is less than 150, and hope is to stay under 100. From what I have read, the .450 bushmaster came first, and was designed to be effective to 250 yards, and the other two have similar goals.

So, opinions on using one of the three in a pistol length set up? What to expect in accuracy potential (MoA or Mo-barn)? Potential problems with compatibility beyond bolt?

I have a slew of .30 cal rifles, so the more I think about this, the more I want to try out something "less traditional." I'll still put together the .300 AAC, but want to do this too. Suggestions - and warnings for potential pitfalls - appreciated!

If you don't have a .300 BLK yet, I would suggest that. Very easy, as all you need is a different barrel. Everything else is the same. Very versatile cartridge. Fantastic on deer in supersonic, a ton of fun subsonic and suppressed.

If you're looking for something non-traditional, take a look at 9x39. Still in its infancy in the US and totally dependent on Russian ammo imports, but it's a heavier bullet than 300 BO and seems to offer better performance.

Brownl345 said:

If you're looking for something non-traditional, take a look at 9x39. Still in it's infancy in the US and totally dependent on Russian ammo imports, but it's a heavier bullet than 300 BO and seems to offer better performance.

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That's a cartridge I'm excited about.....

Almost finished my .50 Beo build. That may be next......integrally suppressed, of course....

Brownl345 said:

If you're looking for something non-traditional, take a look at 9x39. Still in it's infancy in the US and totally dependent on Russian ammo imports, but it's a heavier bullet than 300 BO and seems to offer better performance.

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If I were building another AR this is what I would be doing

teratos said:

That's a cartridge I'm excited about.....

Almost finished my .50 Beo build. That may be next......integrally suppressed, of course....

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VSS Clone. I can dig that.

It seems you would almost double the knock down power by moving to the larger calibers you mention so thinking this may also be a goal, given the amount of change I am thinking ask to have a range day with some friends to let "him" decide.


My Answer: one of each

I'm finishing out the .300 - that is not going to be abandoned, but I really want to put one of these big boys together. 9x39 looks... interesting, intriguing even, but not quite the direction I'm looking at this point.... Any issues to watch out for or keep in mind when selecting between the three? Compatibility issues? Extra costs one versus another (apart from ammo differences)? Twist rate suggestions?

I am not sure how recoil sensitive your son is. While none of these cartridges have excessive amounts of recoil it is much more than a typical AR. The 300 blackout has the least amount of recoil of the cartridges listed. Pistols don't allow you to absorb recoil the same way rifles do.

jcutonilli said:

I am not sure how recoil sensitive your son is. While none of these cartridges have excessive amounts of recoil it is much more than a typical AR. The 300 blackout has the least amount of recoil of the cartridges listed. Pistols don't allow you to absorb recoil the same way rifles do.

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I thought about that, but he will shoot slugs out of his 20 ga for hours on end if I let him - and I figure if my M1 Garand doesn’t have him run for cover he will be alright.

I built a 50 Beowulf pistol. I then built it into a SBR. It makes for a good flamethrower too. All kidding aside, I like mine. Always have. After building the SBR, I sold my carbine. Should have kept it but someone had to have it. Now it sits in his safe with not 1 round through it since he bought it from me.