if so what were the results...

Seems like with 20% more case capacity than the 6.8 spc that you would be able to improve on the anemic velocities of the 6.8 spc when fired from 16 inch bbls.

I'd be surprised if it hadn't already been done for most calibers. Further down you will see a 7mm-30 Rem AR wildcat.

I do not consider the 6.8 SPC anemic. It is no magnum and it isn't as powerful as a .270 Win but it was never designed to be. The performance is close to what .250 Savage would be if fired out of a 16" barrel.

There are a lot of cartridges people are stuffing into ARs that are much bigger than the platform was ever designed for. Some are simply too big, in my opinion.

Chambering an AR15 for the larger cases results in reduced chamber wall thickness. Consider the 25 WSSM. It is .555" diameter and is a SAAMI 65,000 psi cartridge. Chamber wall thickness is seriously reduced. There are many very happy users of ARs in 25 WSSM but there have also been a number of people who have experienced blowups. Even if all the blowups are attributed to handloads it still suggests reducing the chamber walls so much has considerably reduced the margin of safety for reloading. In m opinion, the WSSM is just too much to stuff in an AR.


The 30 AR is based on the .450 Bushmaster/.284 Win case. The .450 Bushmaster is a SAAMI 40,000 psi cartridge while the 30 AR is a SAAMI 55,000 psi cartridge. Both are .500" diameter cartridges. Cutting a .450 Bushmaster chamber for an AR15 results in chamber walls significantly thinner than for the original 5.56mm cartridge. To compensate, the .450 Bushmaster is loaded to lower pressures. But while 30 AR chamber walls are just as thin as Bushmaster chamber walls, the 30 AR is loaded to the full pressure of the original 5.56mm cartridge.

Left - 30 AR, Right - 5.56mm



Left to right - 5.56 NATO, 6.8 Remington SPC, 7mm-30 RG wildcat, 30 Remington AR



Remington must have determined that there is ample chamber wall in an AR15 chambered for their 30 AR. One thing that helps out is increasing barrel diameter at the threads but that means decreasing the thickness of the barrel extension. So the choice is weaker chamber walls, weaker barrel extension, or a little of both. Any of those result in reducing the margin of safety compared to the same rifle chambered in 5.56mm, or for that matter chambered in 6.8 SPC.

I think that is worth taking note of and respecting. If I had a carbine in 30 AR or a wildcat based on that case I would be VERY careful and cautious with any handloads I fired in it.

However, it's purely academic. I will never own an AR15 in 30 AR. I have fired well over a hundred thousand rounds through various M16/M4 and AR15 rifles in 5.56mm and never had a blow-up, a blown case, or anything suggesting I was close to that, even with smoking barrels, cherry red gas tubes, and automatic fire. The platform is very strong when used as designed. I want it to stay that way.

I'm comfortable shooting 450 Bushmaster because it compensates for greatly reduced chamber walls by reducing pressure. I am very comfortable with the 6.8 SPC because that case, at .422". is only slightly larger in diameter than the 5.56mm case and the operating pressure has not been increased. The same goes for the 7.62x39 and 6.5 Grendel rounds. Those have diameters of .439" and, again, are only a little fatter than the 5.56mm.

But for me, 25 WSSM and even 30 AR are pushing the envelope and just begging for trouble. Blowups may be very rare and, of course, not everyone using those will have a blowup. I'm just going to make sure it will never be me

If you really need to make a huge leap from the 5.56mm platform then I suggest you switch to an AR10.

Here is more info on the 7mm version:

http://www.realguns.com/articles/234.htm

I think it's interesting, but I've never actually seen 30AR brass in person. The wildcats I've made were to start with something that's available and make it do what I want. Not start with something rare and expensive and make it even more so. If I had a pile of 30AR brass though...


BTW, his article mentions:

"The 30 Remington AR is most often noted as a .284 Winchester derivative, however, the 30 Remington AR has a larger rebated rim; 0.492" compared to 0.473" for the .284 Winchester rim."

Nobody ever said I was practical...

It looks like he actually built it and tested it, though not in an AR:

http://www.realguns.com/articles/263.htm

http://www.realguns.com/loads/7mm30RG.htm

The bolt action mini-Mauser is a much better platform for those rounds than an AR15.

Also, notice the velocities realguns achieved were with a 21" barrel. Cut the barrel back 5" to make it 16" long and......

There is a fellow posting on 68forums as Float Pilot who did a comparison with 16" and 24" barrels in his AR. His AR was originally had a 16" barrel and he ran various handloads and a couple of factory loads over the chronograph. Then he rebarreled it with a 24" barrel and chronographed the exact same loads he had fired in the 16"barrel. He had some very big increases in velocity with the 24" barrel, especially with handloads.

Here are the differences he saw with the factory loads:

110 grain Hornady Factory Ammo.

110 grain BTHP
16 inch = 2,455 fps
24 inch = 2,699 fps 1.5 inch group

110 grain Ballistic tip type load
16 inch = 2,506 fps
24 inch = 2,790 fps 1.0 inch group

If you are familiar with the .250 Savage you will see that the 6.8 SPC with a long barrel easily matches it, even bests it in factory ammo. This just emphasizes how much velocity you are giving up with the 16" barrel. That should always be taken into consideration, especially, before judging one of the AR15 rounds as "anemic".

I am curious what the velocity would be if pressures were reduced to 55,000 psi. Assuming it's linear for the portion between 55K and 65K you might be able to achieve about 2550 fps with a 140 grn 7mm...that is reasonably interesting performance and is better than 6.8 spc given the 115/.277 @ 2575 but I can't say it would be worth all the effort and money.

With respect to the 21 inch bbl, I think the issue of 16 vs 21 is not relevant. The 16 inch bbl has its purpose for CQB rifle but the purpose of doing any of this would not be a CQB rifle.

Understand, Mike. Regardless of barrel length, if you really want any significant improvement in ballistics you should look to a larger platform like the AR10. You can only whip so much out of the AR15 and once you start pushing the envelope the costs (not just monetary) outweigh the benefits.

In my opinion the 6.5 Grendel is probably the best reasonable AR15 chambering improvement for gaining velocity, distance, and energy down range.

quote:

Then he rebarreled it with a 24" barrel and chronographed the exact same loads he had fired in the 16"barrel. He had some very big increases in velocity with the 24" barrel, especially with handloads.

I'm sure it is obvious to everyone that this is a backward way to do this and the test results are completely anecdotal.
We all know that any barrel over another can be slower or faster.
The most accurate way to have done this is of course to develop the load in a 24" barrel then cut it to 16", those would be accurate results...

True, but does it matter anecdotally? If cutting the barrel back would have given him 180 fps difference instead of 280 fps difference the principle is still the same and the point made.

The purpose of sticking with the AR-15 is the AR-10 is a pig; i.e., quite heavy. I owned one and didn't like it.