Best AR magazine compatible calibers?

[ramairthree]

I am putting this in GD because because you may be an accuracy shooter, a hunter, a tactical seal clubbing, face shooting warrior, a nerdy mechanical engineer with a shooting fetish, a competitor, etc.
and a specific sub technical forum may miss some of this.

My question is,
In terms of range, trajectory, effectiveness, reliability, etc. what is the best AR15 magazine caliber you have direct experience with?

I only have AR15 platform experience with 5.56 and 300BO.
With 300 BO I have found issues with actual 100% mag reliability.
I am not impressed with the hypersonic to be honest, and with the subs I often find myself asking why the hell I did not just get a 9mm AR.

Anyways,
I look forward to some interesting stuff. My dream would be a six to seven mm 85 to 100 or so grain bullet pushing 3000 FPS.

[soulezoo]

Your post seems to ask a couple of questions. Is it magazine reliability? Or is it cartridge effectiveness? Or both?

[soulezoo]

The question is important as you list criteria of 6-7 mm projectile at 3000 fps. That leaves you with almost exclusively with AR-10 pattern rifles.

For instance, 6x47 Swiss pushes 105gr. Bergers to about 3150 fps. It uses a .308 boltface. Going up in size and weight will slow the projectile given the same brass. 6.5 Creedmore with 107gr. bullet is about 2950 fps. Creedmores will push the lighter 85 grain bullets well over 3000, but you lose the ballistic advantage of the heavier projectiles.

[grizzman]

By what do you mean effectiveness?

Does terminal performance on the impacted object matter, or will the bullet only impact paper or steel?

Does cartridge availability matter, or will it be strictly handloaded, opening the option to wildcats?

[ramairthree]

Your post seems to ask a couple of questions. Is it magazine reliability? Or is it cartridge effectiveness? Or both?

Yes.
Yes.
Yes.

[ramairthree]

Yes.
For killing shit.
For accuracy.
For trajectory.

Granted, pound for pound humans seem to be easier to kill than most other animals.

What round has the same reliability in an AR platform and magazines as 5.56.
What round has potentially more accuracy?
What round offers improved terminal ballistics.
What ro7nd gives up the least in terms of trajectory and velocity to achieve this.

I find the 300 AAC/BO\Whisper to be a round that gives up too much in velocity and trajectory, with crappier longer range accuracy, and the added insult of not being truly reliable in many AR magazines. I also don’t believe it offers a significant increase in penetration and lethality.

I think/feel a bullet heavier than 5.56 typical loads, but lighter than .308 typical weights, with an intermediate diameter, approaching or meeting 3000 FPS might offer an increase in lethality, comparable accuracy, reasonable trajectory, and reliability on par with 5.56 in the same platform with only the appropriate barrel.

I lack the ability to come up with the RA3 6.66. A 6.66 mm, 6.66 gram, 2666.666 FPS round to prove this.
Nor am I familiar enough and experienced enough with rounds in this range / area of performance to know.

So, basically,
If you are familiar and experienced with a round with more diameter than 5.56 but less than 7.62,
Using the 5.56 case,
Reliable in the AR platform and magazines,
Offering better velocity and trajectory, than, say, 7.62 x 39,
With some concept of its accuracy and ability to kill stuff,
I would like to know about it.

Off hand, I can think of a half dozen or more rounds meeting some of these criteria, but I am not experienced with them.

I always wanted to look into them more, but never got around to it,
But the recent M4 replacement thread made me wonder what would be a viable, potential replacement for 5.56 that did not require new bolts, magazines, platforms, etc.
Hell, maybe the case diameter of 5.56 could be increased by 1mm and still work fine,
And get a 15% increase in case volume.
The bullet would scale up close to .243.
Maybe 90 grain or so.
With a case volume deficit about 20 grains compared to 243 Winchester.
Maybe hit 2800 FPS or so.

Anyways,
I never walked into a meeting where the Army was deciding a new M4 cartridge RFN.
I not infrequently walked into or got called into crisis action planning at the pro level and, despite other relative weakness and shortcomings, with a processing speed and savant like pattern recognition often came up with a plan that was used.

Ten minutes of so on a 5.56 replacement with goal of only changing barrels leads me to the above.

I am simply interested in the results and experiences of others.
I might be totally jacked up, or in the area.

Others have more applicable expertise.

[ABNAK]

What round has the same reliability in an AR platform and magazines as 5.56.
What round has potentially more accuracy?
What round offers improved terminal ballistics.
What round gives up the least in terms of trajectory and velocity to achieve this.

Funny you mention this, as since the weather is shitty here I spent most of the day looking into something I swore I wouldn't do---another caliber AR platform. Specifically 6.5 Grendel. Yeah, different bolt/barrel/mag, but still doable in a non-.308 AR lower. Now, the Grendel isn't going to give you the 3000fps juice you were looking for, but the bullet weights are certainly within the parameters you mentioned. I think you'd be looking at like ~2500fps from a 16" barrel with a 120gr bullet. Supposed to still be supersonic well past 1,000 yds. Obviously the ballistic coefficient must be pretty awesome to pull that off.

From perusing the internet today a Sabre Defence chrome lined upper (purchased before they went under) will be about $450. Then you've got to have a bolt, and the JP Enterprises Enhanced one seems well made at $132 (I can cough up a carrier, but I did see Toolcraft 6.5 nitrided BCG's for $99). Alexander Arms E-Lander 24rd mags are gonna run around $25 each. "Shootin' ammo" is going to set you back about $300+ for 500rds. Yeah, I need to step away from the computer.....

As far as 300BLK is concerned, in a PDW-sized platform the supers edge out 5.56 in the same barrel length (say maybe a 9" barrel). No, you ain't gonna be popping Muj at 600m on some distant Afghan hillside but for CQB out to maybe 200 you'll be good to go. And that PDW is really small and compact. I have no use for a 16" 300BLK because 5.56 blows it out of the water in carbine-length tubes. So for me 300BLK is a VERY niche round but with supers in my wannabe PDW I'll take it.

[Todd.K]

Not going to happen in 5.56 mags. An increase of caliber will mess up the stack, as the cartridge neck runs on one of the ribs of the mag.

300 BLK suffers from not having a magazine with that rib designed for it. Being a shorter case means that you can put the bullet ogive on that rib at the same diameter as a 5.56 neck. But it limits bullet design and many rounds are not loaded like that. Rounds that are loaded like that have been reliable for me.

[gunnerblue]

Funny you mention this, as since the weather is shitty here I spent most of the day looking into something I swore I wouldn't do---another caliber AR platform. Specifically 6.5 Grendel. Yeah, different bolt/barrel/mag, but still doable in a non-.308 AR lower. Now, the Grendel isn't going to give you the 3000fps juice you were looking for, but the bullet weights are certainly within the parameters you mentioned. I think you'd be looking at like ~2500fps from a 16" barrel with a 120gr bullet. Supposed to still be supersonic well past 1,000 yds. Obviously the ballistic coefficient must be pretty awesome to pull that off.

From perusing the internet today a Sabre Defence chrome lined upper (purchased before they went under) will be about $450. Then you've got to have a bolt, and the JP Enterprises Enhanced one seems well made at $132 (I can cough up a carrier, but I did see Toolcraft 6.5 nitrided BCG's for $99). Alexander Arms E-Lander 24rd mags are gonna run around $25 each. "Shootin' ammo" is going to set you back about $300+ for 500rds. Yeah, I need to step away from the computer.....

As far as 300BLK is concerned, in a PDW-sized platform the supers edge out 5.56 in the same barrel length (say maybe a 9" barrel). No, you ain't gonna be popping Muj at 600m on some distant Afghan hillside but for CQB out to maybe 200 you'll be good to go. And that PDW is really small and compact. I have no use for a 16" 300BLK because 5.56 blows it out of the water in carbine-length tubes. So for me 300BLK is a VERY niche round but with supers in my wannabe PDW I'll take it.

I was going to suggest 6.5 Grendel but I thought the OP was interested in staying with the 5.56 case. If that’s the case, 6x45 maybe?

6.5 Grendel and 6mmAR were both designed as performance upgrades to the 5.56 in an AR15. I’ve had no problems with Grendel mags from C-products and Elander and Alexander Arms has done a good deal of fully automatic testing to demonstrate reliability. With IMR 8208 XBR, 123 gr Nosler CC’s and an 18” barrel, I’ve taken my Grendel out to 860 yards with repeatable hits. I’ve no doubt it can go further, just haven’t bothered yet.

I’ve just purchased a Toolcraft Grendel BCG for a budget SBR build. Seems well made, properly staked, etc.

[grizzman]

A search for 223 Remington based wildcats should give the available cartridges, and it's a short list. The 6x45 is likely among the best options.....apparently not.

My guess is that most members of M4C aren't that interested in running wildcats since the 6.5 Grendel and 6.8 SPC are proven performers.