Thoughts on necking down big bore ARs (.450 BM, .458 SOCOM, .50 Beowulf)

[BAC]

A thought came to mind about possible necking down a .450 Bushmaster, .458 SOCOM, or .50 Beowulf to something in the 6.5mm to 7mm range. Right now this is purely a mental exercise to find creative ways of getting more power out of the AR-15 platform, but is there any reason why this couldn't work? Are bolts available and durable? Would recoil be a problem? Should I expect a possible barrel burner like another fat/short cartridge necked down (6mm AR)? Since I can't find dimensions anywhere, would I get something about the same size as a necked up Grendel case or would I actually gain capacity (which is the goal)?

If it's a crazy-fool idea, I'm cool with that too. Just curious. ;D

 

[effendude]

No idea is crazy. DPMS produced AR-10's in 300 Rem Short Ultra Mag for many years and now 338 Federal (essentially .338-308). I'm not sure how much more power you are looking for? The 450 Bush is similar to a 460 SW handgun performance wise, and no one should feel undergunned with one. I sure don't.

Being a gas operated system, location of the gas port and pressures become very important at times. I saw a shooter have some .240NMC (essentially a 6xc) blow primers badly at a match. The cause was most likely running some "warm" bolt gun ammo in his AR-10.

Let's hear what other have to say.
Scott

 

[BAC]

I have no idea what to expect as far as power, to be perfectly honest. I like the idea of really pushing 6.8 and 7mm bullets hard; I figure if the .458 moves a 300 gr projectile at 2000 fps from a 16" barrel I can probably move 130-150 gr A-Max's, SST's, and TSX's pretty freakin' fast. I'm betting it'll burn through barrels like nobody's business but that's why for the moment it's just a mental exercise.

Pressure is something I hadn't thought off; if this is run hot the gas port will probably need to be opened up a little. Of course, running an adjustable gas block might be an option too.

 

[CJ6]

Wasn't that the intent of Remington going to the 30AR. I believe Rem used a shortend 284 (modified)vs 50 AE(modified) I would think that the case capacities you are looking at would be similar to the WSSM cases necked up to 6.5-7-30 etc. Also contact Bill Shehane D&B Supply he has various SHORT versions of his 284 based wild cats.

 

[Rust]

Do it the easy way, get an AR-10 in .260.

 

[357Mag]

necking down big bore ARs (.450 BM, .458 SOCOM, .50 Beowulf)

Bac -

Howdy !

You might just want to bend-up some sample cases, and see if total case cap & ease-of-forming are everything you hope for.

You might be able to use FL size dies for the larger CorBon
cases, and start out by necking down from .50 Beowulf to
.440 COrBon... for instance. Use of " stock " ( non-custom ) dies saves you $$$, when just experimenting.

Bolt thrust might be some question, but the .50 Beowulf users don't seem to have a problem...that I am aware of.

As regards an AR application..... you may want to check- out bolts and "uppers" available from Virginia Arms.

I'd have to re-check .50 Beowulf specs against .50 S & W,
to compare things like rbated rim diam; and case oal.

Best of luck in your endeavors.

Regards,
357Mag

 

[BAC]

Thanks for the input guys.

Rust, while I have in my head a project that will use the AR-10, for this one I wanted to stick with the AR-15 chassis. I like light and maneuverable and I already have a lot invested in the system. ;D

CJ6, it looks like you're right about the 30 Rem AR; Wiki says it's based off the .450 Bushmaster, which is based off the .284 Win, so the idea has precedent and at least one successful example. In fact, basically I just want a more efficient cousin for the 30 Rem AR, hence, my draw to the 6.8mm-7mm range of bullets. Depending on the availability of brass, it might be easier to neck down a 30 Rem AR than try to create something completely unique.

357Mag, I appreciate the advice picking non-custom dies for estimation/experimenting purposes; I hadn't thought of that either, and any way to save money makes me happy. Good call in case I do decide on a different case than the 30 Rem AR.

 

[placek59]

I am no expert on this subject, but have built a few ar's. Anyway, the .30 rar intrigued me, and I thought that might be my next build, as I was reading an article, they stated the pretty much the entire upper reciever was developed specifically for this cartridge to deal with the higher chamber pressures that were needed to compete with the .308 win. (it needed thicker side walls and beefier this and that.. etc...) - Just a thought as that kind of sounds like what you are trying to do. I may be wrong but should be something that is looked at for safety's sake. I am also not sure if those uppers are available to build on. I like the idea though! Keep us posted on what you find out. ;D

 

[BAC]

Speaking frankly, a .50 BMG has nothing that appeals to me. If I wanted a bolt action rifle then I would buy a bolt action rifle, and there are better (not to mention less costly) options for 1000 yard competitions. I think it's cool that it can be done, but it's not what I'm interested in.

placek, that is something that concerned me. It seems like the only things required for the big bore AR-15s are a bolt, barrel, and sometimes magazine change; the .30 Rem AR lists the same max operating pressure as the .223 Remington (55,000 psi). I know one person who's owned a big bore AR, a .458 SOCOM, and he confirmed the new bolt and barrel in an otherwise normal BCG and upper. My concern lies with what happens to these big, long-necked cases after we neck them down to a smaller bullet; logically we would have to shorten the neck, either by trimming the case (to maintain similar capacity) or by pushing the shoulder further forward (increasing capacity). I have no idea how either of these would impact pressure.

Thanks for the input so far folks. Keep it comin!

 

[dan06]

I have an AR-15 in 6.5-.243 wssm Detech custom made it....... I love it. About 260 ballistics. I think it would be about ballisticly the same as what your talking about. Olympic arms makes Ar's in 22-243-25 wssm as a factory offering.They are a little picky about loading for but i think necking down a .450 might cause some problems too.