single cartridge for multiple "assault rifles" your choice?

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jason41987

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hey everyone.. im looking to convert all my and future military semi automatics if intermediate caliber to a single cartridge... and im trying to decide what that is... ive been wanting 6.8mm SPC, but... some comments ive received leads me to question what would be the absolute best choice... what would offer me the best performance in these rifles as far as reliability, power, and availability?...

as for availability, i dont necessarily need to be able to find factory ammo in this cartridge, but atleast be able to reshape common brass for loading ammo... so i guess even wildcats are up for debate, so long as i can get dies to reshape the brass with, and someone to make the barrels

so. restrictions... i would say nothing larger than a .270 caliber, because the BC in AK/AR and similar size actions wouldnt be enough to give better performance outside of close range.. anything smaller than .270 caliber should have suitable ballistics coefficiencies...

overall length, with modification i believe the longest an AR15 is capable of handling is about 2.300"

so, those are the limits.. thats the absolute maximum as to what id try fitting into a magazine, and the largest bullet diameter im willing to go

----

other criteria is it would need to have a good bullet selection for reloads, and something i can use common twist and groove barrels for.. so something that doesnt require specialty grooves or twist rates to perform well... having a rim diameter that fits a pre-existing bolt is a plus, but anything a .223/5.56 bolt can typically be machined to is ok

so... of all cartridges out there, pre-existing wildcats included as well as ideas for new wildcats that may offer a better for for this criteria is good as well... if you were going to chamber all of your "assault rifles" to one cartridge, what would you pick?
 
cost and availability of ammo aside... sure 5.56mm is common, but is it really going to offer the best performance?.. id like to step outside of 5.56mm if i can
 
5.56X45, it is not just the ammo. 5.56 parts are everywhere. Wildcat, order it and wait 2-6 weeks or longer. Find a mag on the shelf for your 6.5 SuperDuper at your LGS. Customs and wildcats are cool until you remember parts are available from only one manufacturer and then only as a custom order.

Want more "knock down power" step up to the AR-10.
 
The only thing that really fits your criteria and isn't .223/5.56 is the 6x45. Everything other than the barrel is the same on the rifle. Velocity out of same weight bullets is higher with the 6mm. You can run up to 87gn bullets from the mag. 6mm bullets are almost as inexpensive as .223 bullets as long as you ignore the availability of bulk FMJ and pulled surplus. It will be a reloading only proposition at this point.

My opinion, not worth it. I have ARs in 6x45, 6.8, .300BLK .204 and 5.56. The various ones (4) in 5.56 gets shot the most.

The 6.8 has the most power but magazines are an issue and it uses a proprietary bolt that, while available, still isn't common. Bullets and ammo for it are not cheap.

The 6x45 is my newest addition and was built specifically as a coyote rifle with light to medium weight bullets (up to 70gn) in mind. I like the cartridge but wouldn't have it as my only round.

The .300BLK was built almost on a whim. It gets shot from time to time but is a lot more expensive to shoot than most of the others, even with the pulled bullets that I'm using. It doesn't have much at all to offer past about 150yds until you get into really expensive ammo with very high BC bullets and an optic that will allow you to compensate for the rainbow trajectory of said ammo. I'll keep it around to put a can on it, but until then it's more a curiosity than a useful rifle. It works best with USGI mags because of the diameter of the bullet interfering with the alignment ribs in the various poly mags.

The .204 is another purpose built rifle, this time for small varmints like ground squirrels and prairie dogs shot from a fixed location. It's heavy with a 24" bull barrel and a large optic and set up for heavy for caliber bullets that will not feed from the mag. It shoots the 32gn pills great but really shines with the 50gn. It is picky with magazines because of the location of the shoulder on the case. It won't work reliably from a PMag but is OK from USGI mags. Brass is not common (often out of stock) but not expensive; however unlike the 6x45 and .300BLK you can't just make it from .223. Quality bullets are the cheapest of all but they have a poor BC and really fall off in effectiveness outside of 300yds when the wind starts to really push them around. They are also no bullets other than varmint style available.

If you want the maximum amount of flexibility and effectiveness, the 5.56 is where it's at. You can bulk cheap bulk ammo for screwing around. You can get surplus bullets and powder and roll your own for cheaper than any other cartridge. The AR magazine was designed around the cartridge, so mag availability and reliability are not an issue. Bullets of every type of construction are available from the super lightweight varmint bullets to heavyweight long range match bullets to middle weight bullet suitable for medium game (Barnes TSX and TTSX, Winchester Powerpoint, Federal Fusion, Nosler Paritition and several others) that can all be loaded from the magazine. No other round is nearly that flexible or convenient.
 
I'm really not familiar with what all you can convert to. Can you convert to just about anything? If that's the case, something a step or two up from the 5.56, such as the .243 win or 6mm Rem would be pretty cool. You got twice the power with relatively high availability, and I'm assuming they'd be just as reliable... Their trajectories are fantastic, too, and recoil would still be very manageable during rapid fire.

The 6.8 rem SPC performs similarly to those two, but not quite the same; though, it's still a step up from the 5.56. Although, it is a .277 caliber, higher than what the OP wanted.
 
The .243 doesn't fit in the AR15 and the 6mmRem certainly doesn't.

.277 IS .270 caliber and is exactly what the OP was talking about. The 6.8SPC is not in the same league at all as the .243 though. Performance isn't even close.
 
jason, why would you want to do what you are proposing? The availability of parts and ammunition alone make a strong argument for having at least one rifle chambered in 5.56.

Although I can the benefits of "standardizing" on one caliber, there are also drawbacks, especially when you're talking about going with a caliber that isn't mainstream.
 
Any of the myriad AR forums are a good start, but in short, cartridge overall length must be 2.26" or less to fit in most of the magazines out there. There are some mags that will let you take it longer than that, but they are the exception. Also the case head can't be any larger than that of the 7.62x39 (and those bolts have had problems because too much metal was removed) but anything other than .373" (like the .223/5.56) is going to vary in availability between uncommon and rare.
 
well, to try to answer a few questions from different people i cant really quote just one person...

the reason i was asking is the 5.56mm does seem like an obvious choice, but is it truly the best, and most well rounded cartridge out there?... i would take a 5.56x45mm over a 7.62x39 or 5.45x39 any day, but there seems to be a trend of people stuffing new cartridges into AR and AK length actions, as well as a number if wildcats out there so i was looking for something that would offer an increase in ballistics, and offer more power

6.5 grendel and 6.8SPC viable choices as well.. i like the greater versatility of 6.8mm in different barrel lengths vs the 6.5, but the 6.5 uses a bolt face thats fairly common and if im not mistaken the brass could be made from 7.62x39 brass

i really do like the performance of the .243 and believe something like a .243 or 6.5 creedmor would be amazing ballistics, however, i dont actually own any military type .308 caliber rifles (yet) and this is a different class of military rifles and at some point when i do start adding these to my collection, ill be looking for a single cartridge to cover this class as well.. but for now, im looking for something thatll fit into rifles like AKs, AR15s, SKS, VZ58s, sig rifles, as well as some of the other similar-size rifles out there i plan to add to my collection

problem with going above a .270/6.8 caliber is then the ballistics coefficiencies start to get too low to be more than a close range cartridge (i wouldnt trust 7.62x39 at anything beyond 200-300 yards max) however, i would have more faith in 5.56, 6.5 grendel, or 6.8mm far beyond that

one reason i dont want 5.56mm specifically is because although im confident in being able to hit targets at ranges out to 500 and possibly beyond, it doesnt carry much energy with it, as some of these rifles i like to use for longer range target practice so i was looking for something with the versatility of more power close range, and for hunting, but could reach out a bit further for recreation

so i guess i could say im looking for something that uses either a 6mm, 6.5mm, or 6.8mm projectile thats small enough for a good ballistic coefficiency, but still large enough to carry more momentum and energy
 
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I do own more 5.56mm rifles than anything else.

But I also like 6.8 a lot. Works good on the deer around here. Which should mean it would work well on bipeds as well.

Go spend a few hours reading on this site and all your questions should be answered:

http://68forums.com/forums/forum.php

Gregg
 
well thats another thing... is i would like to hunt with something with a bit more performance than 5.56 offers, despite it being illegal to hunt with in many, if not most states... and i do intend to do some out of state hunting for elk as well in the near future.. so id like to avoid 5.56mm
 
all that im aware of in the 6-6.8mm range is the 6.5mm grendel and 6.8mm SPC, but i havent done much looking around, and havent paid much attention to the market of like calibers in a while... so i was wondering if there was something newer, better, or perhaps any wildcats made from common pre-existing brass that would be fairly simple and cheap to make that would offer me equal or better performance to these... im not sure something based on .308 brass would fit in these rifles very well... but who knows, it might.. but mag capacity would be severely reduced

can 6.5mm grendel brass be made from 7.62x39mm brass?.. it seems it would be difficult as the 7.62 has a sharp taper and the grendel doesnt, might stretch the brass up around the shoulder too much.. but i dont know, never tried it..

i guess if im looking for compatibility in a variety of rifles.. a lesser taper is probably going to be best, so i guess 6.8mm SPC, 6.5mm grendel, and possible wildcats in this range are what id be more interested in
 
The 270 isn't popular as a target round because of poor bullet selections. Stick with .223 unless you want to step up to a large frame AR and shoot 308ish family rounds
 
The only one that meets your requirements that is not a 223/5.56 would be the 7.62x39. You can use it in multable platforms, AR, AK, SKS, CZ, Mini-30 and maybe others, not sure.

You would probable be better off with a AR-10 and buy multable different uppers for it.

Just my view on the matter.
Jim



IMG_1848.jpg
 
A 500 yard effective intermediate cartridge is almost an oxymoron. Your larger, heavier bullets are going to have rainbow like trajectories, and your superlight flat shooting bullets would best be limited to ground hogs and the like.

I would think the best bet for a long range intermediate cartridge (again, notice the contradiction in terms?) would be those specialty 5.56nato sniper cartridges in the 75+ grain weights. There has been significant testing of those cartridges, to the point of them being adopted by the military. Of course, you'll need to shoot them out of 1in7 twist 24" barreled ARs to get the most from them.

5.56nato does give you the most flexibility:

1) M193 is an excellent short range bulk ammo from a carbine length AR
2) Bump up to M855 for increased penetration power
3) The 62gr Winchester Power Point (?) (223 remington) is recognized as a good short range deer cartridge
4) There are many hollowpoint/frangible/controlled expansion cartridges to choos from for specific applications.
5) There are a few good medium range (500 yard) heavy cartridges (75+ grain) when fired from an appropriate AR.

Also, remember the extreme modularity of the AR platform. Given appropriate uppers, you can switch from a CQB carbine to a long range target rifle in a matter of seconds.
 
If you mean multiple weapon designs, 5.56mm is probably going to give you the best range of options and designs. I'd think that 7.62x39 gives you the next largest pool of designs, though not all of those are necessarily credible fighting guns (I'm thinking specifically of 7.62x39 ARs here, with their wonky magazine issues), with the boutique AR loadings and then 5.45x39 further down the list.

(And if by "assault rifle" you mean "military style rifle" there's also a good number of options with 7.62x51, though they're outside the realm of assault rifles as strictly defined.)
 
I don't think there is a best. Doesn't hurt to try maybe, but you can debate it to death and not get anywhere. This coming from a 6.8 fanboy.:D
 
Perhaps the best, least expensive general-purpose load for 5.56 is the Mk318 62gr OTM (about 75 cents/cartridge when purchased in bulk).

I'm frustrated by the lack of inexpensive barrier blind, general-purpose bullets as reloading components. I handload the Nosler 60gr Partition softpoint bullet (@2900 fps from a 16" bbl) for use as my general-purpose load (out to 300 yds), but just the bullets themselves are about 50 cents each. It would be nice if I could purchase similarly performing bullets (e.g., 55gr or 60gr Speer Gold Dot) for $20-$25 per hundred.

I handload 6.8 SPC (Hornady 110gr OTM W/C @ 2600 fps). I have a Colt AR15A2 that I rebarreled with a 20" barrel for use as my general-purpose hunting rifle. I buy brass in bulk from Silver State Armory.

I've looked at .300 BLK - I like the ability to form cases from .223/5.56 brass. With a 110gr bullet it provides 7.62x39mm-like performance. If I wasn't as heavily invested in 6.8 SPC then I'd probably buy a couple of uppers to swap out with my 5.56 uppers.

7.62x39 is a no-go in an AR as far as I'm concerned. The bolt lugs are too fragile. Same is true with 6.5 Grendel.
 
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