What is the the maximum CASE diameter for a round to single stack in and still feed out of an AR15 magazine? The reason I ask is this, why doesn't someone do a "short mag" type thing based on such a case (largest diameter that will single stack in and still feed from an AR15 mag), preferably using a 7.62x39mm bolt, neck it down to .308 and have themseles a round that duplicates or exceeds .308 ballistics out of an AR15?

Tsh77769

Quoted: What is the the maximum CASE diameter for a round to single stack in and still feed out of an AR15 magazine? The reason I ask is this, why doesn't someone do a "short mag" type thing based on such a case (largest diameter that will single stack in and still feed from an AR15 mag), preferably using a 7.62x39mm bolt, neck it down to .308 and have themseles a round that duplicates or exceeds .308 ballistics out of an AR15? Tsh77769

Ah, Grass Hopper....

Well, the trick is that using a standard barrel extension, it sets the max diameter of the case, and that would be just about the Beowulf/SOCOM/LW size.  So that limits you there.  If you take the Oly Arms system, you can use the WSSM case which is actually a hair wider than any of the other three.

Now, when you neck it down to 30 cal, you start getting some atrocious bolt thrust.  Using STANDARD AR bolt will not give you the desired effect, you will soon start collecting a heap of failed bolt (lugs sheared off).

What you are looking for is the .30 Olympic, (or what I have dubbed the 308 WSSM because that makes more sense), which is the 30 cal version of the WSSM case.  In order to use it, you need to Oly Arms upper, which employs a custom bolt, carrier, upper receiver and barrel extension.  It will, however, give you performance just shy of .308 Winchester.

However, only with 10 rounds in the mag, and with custom ammo.  If you want this type of performance, I really have to recommend something like a (B)AR-10 ....

Marty
PS I am not a RKI, I just like to play one of ARFCOM

OK, so we are on the same page, cool. Regarding the .308 WSSM, HOW "just shy" of .308 are we talking? I was hoping to evenly mathc it. When you say just shy, please elaborate. For example, is it that the range of projectile selection would be reduced or is it that the ballistics just aren;t quite there regardless of projectile selection?

The .357 Sig was an attempt to replicate .357 Mag ballistics "with certain bullett weights", not a 100% direct emulation. I would be happy with merely replicating "certain .308 loadings" or is taht too much to hope for already.

Specifically, I was looking to replicate the US military match round (I think it is somewhere right around 168 gr, hey it is 3:30 AM here).

I understand what you say about recommending a step up to a .308 sized AR platform, but to be honest that is precisely what I do NOT want to do. The goal here is to try to "shoehorn" a .308 into a standard AR, even if it isn;t really a .308 anymore, just so long as it performs like one.

BTW, what about using the 7.62x39mm bolt, with the attendent rebated rim, possible or no?

Is the performance close enough that something small like switching to a specialized from of rifling could make up the difference?

Tsh77769

With 150 gr the 308 WSSM is right there with the 308 however with higher pressure.  Going up in bullet weight and you will start falling behind the .308 somewhat.

Rebating the rim to x39 will mimic my 308 Sabre, which is .284 Win cut to 1.575, necked to .308 and rebated to x39.  With 150 gr it is about 100-200 fps slower than the .308.

Going to Mike Rock's 5R will get you 50 FPS maybe more

Marty

What about some form of polygonal or gain twist rifling. There is a company that makes an exotic barrell for an AR upper with a modified operating system that claims, if the claims are true, some significant velocity gains. Is there any truth to this?

Also, even if I could find someway to make up the 100-200 fps difference, that would only get me matching speed for that load right, the benefit might not carry over to allow heavier projectiles to also match the .308 loads, correct?

Tsh77769