6mm ARC too slow for PRS?

[sigstroker]

Looking at the "new" 6mm ARC the factory ammo looks pretty good. Low SD's and apparently accurate. They claim 108's at 2800 out of a 24 inch barrel. A 26 would add 50-75 fps? 6 Dasher seems to generally go 75-100 fps faster. I like the idea of reasonably priced factory ammo. Would the velocity deficit be enough to make someone that's not competitive anyway but would like to avoid being embarrassed, look so bad that they should throw the idea of this cartridge out the window?

[ar15barrels]

That seems pretty slow but it's still a little flatter shooting and lighter recoiling than a 6.5 Creedmoor.
I run 6mm 107's at about 3030fps from a 243 but 243 is not as sexy as all the newfangled short cartridges.

[longrange1]

so this is basically a fat rat...my smith shoots a fat rat and 108s at 2850 are pretty easy out of a 24" barrel...is it to slow? no not at all...look at a lot of the top PRS guys...i believe jake verbert was running a 110 A tip at 2700ish last year as well as a few others running slow 6s.

[ShaunBrady]

I have a 6 fat rat AR and am glad to see Hornady standardize the cartridge. I also have a Howa 7.62x39 barreled action Brownells clearanced at $130 for a donor rifle. It was going to be a Rat, and now I'm glad I've put off ordering the reamer.

The problems for PRS would be Grendel magazines and bolt faces. They're certainly not insurmountable problems, but there isn't much incentive.

I think the cartridge minimalist trend in PRS bottomed with the Dasher and is heading back up with the GT. Not sure where the ARC brass will land, but my 6.5 Grendel based 6 Rat cases wind up at 37 grains of water for capacity. A Dasher is 41 in QL, not sure what current PRS brass is. 6x47 is 47 grains. I shoot a lot of 6xc, mostly because it's yesterday's news and I have piles of cheap clearance brass. It's 49.7 grains of water. 243 is around 54 grains. There is a great interview with George Gardner from the early days of PRS that's entombed in the internet. He questions why anybody would bother with anything other than 243.

[sigstroker]

Ha! I was thinking of that Howa mini-action too.

I'm hoping that with the military adoption that this cartridge gets a lot of traction so that a lot of companies, like Lapua, get on board. So that people that like expensive brass, and cheap people like me that like LC brass, can use what they want.

Thanks for the input everyone!

[sigstroker]

I think the cartridge minimalist trend in PRS bottomed with the Dasher and is heading back up with the GT. Not sure where the ARC brass will land, but my 6.5 Grendel based 6 Rat cases wind up at 37 grains of water for capacity. A Dasher is 41 in QL, not sure what current PRS brass is. 6x47 is 47 grains. I shoot a lot of 6xc, mostly because it's yesterday's news and I have piles of cheap clearance brass. It's 49.7 grains of water. 243 is around 54 grains. There is a great interview with George Gardner from the early days of PRS that's entombed in the internet. He questions why anybody would bother with anything other than 243.

One other point - because the Grendel sized bolt face is a bit dicey in AR bolts, they say they loaded the factory ammo to only 52,000 psi. That implies there could be some room to push velocities a bit in bolt guns. So maybe picking up a hundred fps with handloads is a reasonable possibility.

[NorCalTodd]

Would the velocity deficit be enough to make someone that's not competitive anyway but would like to avoid being embarrassed, look so bad that they should throw the idea of this cartridge out the window?

What is there to be embarrassed by? Everyone knows that it takes a while for new shooters to the sport to spool up both in equipment and skillset. No one expects a Top 20 finish out of a newer shooter or thinks less of someone if they don't. Just show up with whatever you decide to run, be safe, have fun, and don't take it too seriously. The rest comes with time, and practice.

[jimmykan]

One other point - because the Grendel sized bolt face is a bit dicey in AR bolts, they say they loaded the factory ammo to only 52,000 psi. That implies there could be some room to push velocities a bit in bolt guns. So maybe picking up a hundred fps with handloads is a reasonable possibility.

Well, yes and no.

While Hornady may only be loading to 52,000 PSI, and handloaders can load hotter at their own risk, Hornady has access to non-canister propellants that handloaders can't get. e.g. Hodgdon LEVERevolution and Superformance powders before they released it to the public.

Using the 6mm AR as the closest matching cartridge to the 6mm ARC (.441 base diameter, 30 grains H2O case capacity) in QuickLOAD, when limited to 52,000 PSI, the highest 108 grain ELD-M muzzle velocity from a 24" barrel is from 29.6 grains Hodgdon CFE223, yielding 2680 fps (it makes 2820 fps from 31.2 grains at 62,000 PSI). The 6mm ARC cartridge body is slightly shorter than the 6mm AR, so I don't see how the 6mm ARC would top the 6mm AR in velocity without some special propellant.

Who knows, Hornady/Hodgdon may very well release whatever powder they're using to make 2800 fps with 108s from a 24" barrel at 52,000 PSI.

But even if they do, a drawback to these types of high-energy progressive burning powders is that they are almost always double-base formulations with high nitroglycerin content. These types of powders have a reputation for being more temperature sensitive than conventional single base powders, and much more temperature sensitive compared to Hodgdon "Extreme" series powders.

Then the long range dope would also be temperature sensitive: at 600 yards, hitting high on hot days, hitting low on cold days. Not an insurmountable issue, but there is a reason PRS shooters load with H4350 and Varget, it just eliminates one more variable they have to consider.

In my opinion, the most temperature stable powder that's appropriate for the 6mm ARC is IMR 8208 XBR, a powder formulated for 22 and 6mm caliber small benchrest cartridges, but it would only make 2600 fps if limited to 52,000 PSI and 2730 fps if limited to 62,000 PSI.

[ar15barrels]

One other point - because the Grendel sized bolt face is a bit dicey in AR bolts, they say they loaded the factory ammo to only 52,000 psi.
That implies there could be some room to push velocities a bit in bolt guns.
So maybe picking up a hundred fps with handloads is a reasonable possibility.

That's correct.
AR bolts in 0.440" and 0.420" size are notably weaker than in 0.380" size because the bolt root diameter is 0.531".
0.440" bolts are generally limited to 51k psi and 0.420 cases to 55k psi to improve bolt lug life while 0.380" bolts are rated for 62k psi.
Even at reduced pressure levels, oversized face AR bolts do not live long happy lives.

[ShaunBrady]

The SAAMI velocity for the 6arc is 2700 fps with a 108 grain bullet from a 24" barrel.

You may or may not be able to get 2800 fps from a 26" with better brass. I'm sure many will try and have no pressure signs at all all day long. Then the threads on cheap brass will start.....

Back in the real world, it out performs AR mag length 77smk 223 loads, but looses a lot of luster it comes out of an AR15 magazine. Even at 2800 fps, it'll only do a little better in the wind than the 223 75 ELDm loaded to 2.5" that I shoot from a 1:7 26" barrel. It certainly can't be any cheaper and I already have a bunch of rifles that do better in the wind.

Hornady has standardized a useful AR15 cartridge that's been around awhile. It's nothing earth shattering. A few more AR15 bolts will die, but the kittens will be ok. I run the 105 Amax at 2740 fps from a 24" barrel with the Rat. When I drop the 108 ELDm into that QL model, it gives 2700 fps at the same pressure. Not really that amazing.

There may be a contract from the US military for samples to test, but they will not be converting to it any time soon.

The 6 Rat is easily made from 6.5 Grendel brass, so good brass is already here.

[kevins750]

That seems pretty slow but it's still a little flatter shooting and lighter recoiling than a 6.5 Creedmoor.
I run 6mm 107's at about 3030fps from a 243 but 243 is not as sexy as all the newfangled short cartridges.

[longrange1]

its not that they are crappy or useless its that they are not as efficient...my BRA will run real close with your 243 the difference is im running about 10 grains less powder.

[LynnJr]

The 6 Dasher runs 3025 fps with 103-109 weight bullets in a 28 inch barrel for comparison.

[fish45]

If I wanted to sell lots of bullets and make lots of money I’d come out with a new cartridge that’s “better” than everything else every couple years. I wonder if they’re following iPhone releases.

[sigstroker]

No. They're following newly adopted military cartridges, which is what the 6mm ARC is.