I've been working on an AR-15 project. I'm sure that those that know me can imagine the number crunching I have been up to. One of the fundamental decisions that has to be be made early on is what cartridge to use. I'm struggling a bit with this.

Applications will be primarily subsonic, but I'd like to be able to 'go supersonic' if the occasion arose. Subsonic applications (1050 f/s muzzle velocity), will be constrained to 100 yd and less. Supersonic applications, infrequent as they might be, will likely be kept at 200-ish and below.

Once one makes the decision to set a muzzle velocity 'ceiling', the only way to increase energy to the target is to up the weight of the projectile. One can only up the weight of the projectile by: 1) Making it of denser material - like going from solid copper to solid lead - or, 2) Increasing the diameter  - getting bigger around. There are no materials readily available that are denser than lead, so that leaves 'getting bigger around'.

This cartridge has to fit in the AR-15 "system" of magazine, bolt face, and barrel breech. (The breech of an AR-15 is "special" in that it has 8 locking lugs. This limits to some degree what can be done with the bolt head/face.) It also has to operate the gas-driven recoil system within a reasonable range of operating pressures. The pressure-time curve for the cartridge has to be sufficiently robust that it will provide a relatively wide window of appropriate pressure, but also not be so 'vigorous' that the advantages of subsonic MVs aren't over-powered by large volumes of gas hitting the air in front of the muzzle. You can probably appreciate the number-crunching I have been doing.

I have 'narrowed' the selection down to 3 cases (.223 Rem, 7.92x33 Kurz, and 7.62x39), and 5 calibers (8mm, .338, .358, .375, and .416). The .223 case can only handle up to the .338 caliber even when blown straight out, so for this particular thread, I am not going to include it. That leaves the Kurz and 7.62x39 cases. While one might assume that the longer 7.62x39 case would have more capacity and therefore internal ballistic 'flexibility', that assumption would be in error. The x33 Kurz case, by virtue of it's "wide body", has more capacity than the longer x39 case.

All of the calibers fit the AR-15 magazine max length constraint of 2.26". All of the cartridges fit the magazine, and should feed correctly with minor modification of the feed ramp. So...

Which would you choose AND WHY? The reasons don't have to be "rational". They can be completely "emotional" - "I LIKE/dislike this or that cartridge case" for example. Here is a picture that might help with your choice.



They are in caliber pairs from left-to-right: 8mm, .338, .358, .375, and .416 with the Kurz case on the left in each pair. You'll note that the .416x39 is looking a little 'stressed'. That's because it would have to be fire-formed to completely blow the case out appropriately for the .416 bullet. It would be a true 'straight-walled' cartridge at that point.

Paul

I voted for the 8X33. It looks the coolest but the .416X39 will be the best for subsonic with heavy bullets and supersonic with lighter bullets. I would think it should also have enough "pressure" to operate the gas-driven recoil system.
 
That big bullet is going to have to look mighty hard to find somthing to stop it even at slow as 1050 fps.
 
RJ

I like the .358x33, would like it better if it had a 40 or 45` shoulder.
.358 has a good selection of bullets available, can be quite heavy and has a reputation as a good, clean, killer at most speeds.

Thanks for the comments, fellas.

I agree with the 8mm choice, RJ. Mostly just because I like the caliber. It is also true that the 8x33 is not a wildcat, meaning no "special" reamers, no special reloading dies (already have them as I am thinking of putting this cartridge on the rolling block project), and I have an 'armload' of 8mm take-off barrels 'laying around'. Another convenient aspect of the 8mm caliber is that the .223 case 'works' with it.

On the other side of that coin is the "thump" factor. I'm a little surprised neither of you mentioned the .375. I think I like that one over the .358, but it's a close call. Heck, they're all "a close call" or I wouldn't be asking for comments!

The x39 case is a good choice, but I have just never 'liked' the 7.62x39 cartridge. Can't really put my finger on 'why', but there's just no 'spark' for me with that case. However, being as objective as I can, it's a 'nice' little case, and there are AR-15s made to shoot 7.62x39 cartridges, so magazines are no problem. (Magazines aren't that big a problem for any of these.)

There are bigger "factory" cartridges made for the AR-15 than these. One is the 450 Bushmaster,

and the other is the .458 SOCOM.

The Bushmaster uses a case that has a rebated .473" rim (ala .308 Win) and .500" head (ala .284 Win). The SOCOM uses a case with the .473 (nominal) rim, but a "magnum" .545" head. I mention them to illustrate that I am not way off base with this project.

There are of course "factory" parts - magazines, cases, AR-15-breeched barrels, etc. -  for those two .450-ish caliber cartridges. I could therefore use one or both of them. But really, now, surely no one thinks I would even consider something that simple. There would be little 'fun' and almost no learning if I went down that path.

The truth is, since I have the means available to make barrels, and all of the other 'pieces' for an AR-15 are available "over-the-counter", I could simply make a barrel up in each of these calibers. Were I to consider that, I would definitely have to get my reamer-making skills honed. I remain frustrated on that front. Not on the shaping, but the hardening. I have been completely stymied with that. VERY frustrating.

Paul

If yer gonna go big . . . . . . . .
 
RJ

I just checked your vote, RJ. You said in your first post that you chose the 8x33, but the poll shows you chose the 8x39.

Paul

Go big and slow, ya wanna go super sonic inside of 200 use a 22LR with a milk jug suppressor to quiet it down.

Yup...

Paul

Quote from: gitano;119660but the poll shows you chose the 8x39.
 
Paul

I was thinking 33 :undecided:
 
RJ

Oh, my, that sort of problems and only 52, what on earth is it going to be at 62 and 72 and,,,,,,;)

I had to vote in order to see the results without opening a new page. I'm still 'thinking'.

Paul

Quote from: drinksgin;119664Oh, my, that sort of problems and only 52, what on earth is it going to be at 62 and 72 and,,,,,,;)

He will greatly resemble many of us I fear.

Since you didn't include the X45 5.56/.223 case, I voted for the .358.  I wouldn't care which case really.
My reasoning has a lot to do with the opportunity to try lightweight (90gr?, 110gr?) .357 pistol bullets for additional velocity, thereby satisfying your (and mine) affinity for an LFC combination.

Be sure you get an adjustable gas block though.

While the 250gr .358 rifle bullet should be adequate for a subsonic thumper through the can.

I think you know my thoughts concerning the 8x.223 Paul.  But I'm afraid it would rapidly deplete your stock of 125gr 8MM bullets!

Glad you chimed in Jamie.270.

Excellent point with the LFC pistol bullets. I had completely forgotten about those in the .358.

One of the 'logistical' problems with the 8x223 is that I have to cut the complete 8x57 chamber off of the take-off barrel. That in turn removed the chamber "swell". I'm not sure whether there is enough 'meat' ahead of a take-off's chamber swell to provide the pressure shoulder for the barrel extension. I suppose that technically/theoretically one doesn't ABSOLUTELY need that shoulder, but I strongly suspect that without it precision could be difficult to come by.

Paul

That would certainly make for less economical option.
Starting with a full diameter barrel blank, and duplicating the OE '98 Mauser profile could be done though, if you wanted to make the investment in time and dollars.
I am pretty sure you have one you could use for a pattern .

I'm not so sure that with a barrel as short as 16.25", that the chamber swell would contribute as much to precision.
A barrel that short doesn't have the rigidity problems that the 23.6", or the earlier 29" Mauser barrel do.