6mm ARC - "Optimum 6mm" & Powders & Lapua Brass

[Lemonaid]

Old Bob that was great post! That may be the answer to many unexplained case crushing incidents.

[Old Bob]

Old Bob that was great post! That may be the answer to many unexplained case crushing incidents.

Another potential problem from brass with too thick neck walls would be chambering loaded ammo. How tight of a fit will those loads be in your rifle's chamber? The necks of the resized Lapua brass might benefit from a little neck turning for both seating bullets & functioning in your rifle.

I didn't neck turn my resized Lapua brass because I didn't need to using the Wilson seater. I haven't yet been able to shoot the 6mm ARC loads I made from that brass so I don't know if they're gonna be a tight fit in either of my 6 ARC rifles. I'll eventually get around to it & let y'all know how it worked out.

[r.tenorio671]

Converted 200 Starline 6.5 Grendel brass with no issues using my Hornady 6mm ARC Custom dies (2 die set).  My process as follows:

1. Anneal brass straight from bag.
2. Size on Lee Loadmaster using concocted alcohol/lanolin mix lube. 
3. Wash using Dawn & pinch of Lemishine to remove lube, rinse, fan dry.
4. Trim to 1.486-1.488" on my Forster Classic trimmer w/24 pilot.
4. Prime, charge, seat bullets.

NOTE:  During the sizing I left the decapper & expanding button in the die, had no issues.  Neck tension is very good, calipers measured the openings at average just under .243"  Seated bullets CAN NOT be pressed in nor did they change during feeding tests hand cycling or via bolt release.  Hoping to get out for 1st testing soon, loads are Nosler 70VG w/8208 & 105 RDF w/748.

I also tested resizing some un-annealed cases, took a little more pressure on handle  when doing the bump and shoulder springback was evident, more variation.  Pre-annealed cases shoulder bump was MUCH MORE consistent and WAY LESS force required on the press handle, almost effortless, virtually no springback occurred.

Just a quick note, I had to pull some of the RDF's because I overcharged them (scale battery was dying out) and using a kinetic buller just wasn't cutting it, ended up using a collet puller, but had to use a 22 cal collet until my 6mm collet comes in   Not worried about the marks left on bullets, only going to be used for pressure testing.

[Crowriverjim]

Just stating up front, I am the developer of the 6mmAR, the 6mmAR Turbo and the 6mmAR Turbo 40, all based off the 6.5 Grendel case.  It was no surprise that someone (Hornady in this case) ultimately commercialized a 6mm cartridge similar to the 6mmAR.  What I did not expect was to see the name of the cartridge I named as a part of the name of the commercial cartridge (i.e. 6mm ARC vs my 6mmAR).  I guess I should be flattered that someone thought enough about what I created to copy it, but in another sense what Hornady has done by making the name of their cartridge so similar to mine is confuse a lot of people as to the differences between the two.

So here's the rub - the 6mm ARC is essentially very close to a 6mmAR, but the 6mm ARC gives up 2 gr of water capacity to the 6mmAR (6mm ARC has a 34 gr water capacity vs 6mmAR that is 36 gr of water capacity) and this occurs because the body of the 6mm ARC case is about .027" shorter than the 6mmAR, and the overall case length is .030" shorter.

I have heard statements that the 6mm ARC is the "optimum 6mm" for an AR-15 cartridge, but I do not agree with that because the AR-15 platform can handle a longer and larger case with that case head, with better capacity, better velocity, and give up nothing anywhere else in terms of accuracy, reliability and functionality (i.e. our 6mmAR Turbo 40 has shown that and the T40 has a proven record in the mid-range and long-range match circuits of 600 and 1000 yards respectively).  When you can push the 105 - 108 gr 6mm bullets out at 2800 fps with .5 MOA accuracy - - now you have a real long range contender.

Regardless of the above, I won't take anything away from the 6mm ARC - - like it's big brother the 6mmAR, the 6mmARC is inherently accurate as all get out, it can feed with super reliability out of an AR-15, and it has the capacity to shoot the 105 - 108 gr bullets with excellent velocities out of an AR-15 while maintaining superb accuracy.  When you top it all off with the plethora of great 6mm bullets in the marketplace, as well as the availability of good magazines, great brass for it, and now factory ammo for the 6mm ARC, how can you lose?

From extensive testing, the powders that I have found to be best with the 6mmAR and the 6mm ARC, with bullets in the 90 - 108 gr range, especially for magazine length loadings of 2.265" OAL are:  H4895, RL15, AR Comp, VVN140 and VVN540 (with my personal preferences being H4895 and N140).  The reason I say these powders is that they are the slowest burning (stick) powders that will allow you to get to a max pressure load (highest velocity) before you transition into a compressed loading that starts to sabotage consistent accuracy.   There are some ball powders that will give superb velocities but if you looking for true consistent accuracy, they typically just never make the grade over the wide ranges of temperature, humidity and atmospheric pressures one might be shooting in.  I also never found more compressed loads to shoot with consistent accuracy either, and that's one reason why Varget is not on my preferred list of powers (i.e. mostly too bulky for magazine fed loads with big bullets - when you get to the point where the velocity starts to get in a decent spot, you're typically compressing the load too much - and the 6mm ARC is worse than the 6mmAR there because of its smaller capacity).  When I say consistent accuracy I am talking about consistency tight 10 shot groups.  If you're ok with a 1.5 MOA gun then you may be ok with ball powders and heavier compressed loads.  When you hear a stick powder crunching as you load your bullets, it's too compressed and Varget is notorious for that in the 6mmAR and the 6mmARC.  Powders like Benchmark, 8208XBR, and some others in that burn range work ok but they typically pressure up too fast and too high with the big bullets, especially since their burn rate is faster - - ok for varmint loads with lighter bullets, but not optimum with the heavies.

As for brass, I love Lapua brass, and that brass is very consistent and has the hardest case head and body, yet with a very well annealed neck and shoulder.  In the 6mmAR, the 6mmAR Turbo and the 6mmAR Turbo 40 I have never found better brass.  Hornady is decent brass but I have not found it can handle the pressures Lapua brass can.  The truth is you can rather easily make 6mmARC brass from 6.5 Grendel brass.  Take your 6.5 Grendel brass and lay out the cases on newspaper in a line side by side with the mouths pointing away from you, then use Hornady OneShot spray lube to spray the neck and shoulder area of the cases, let them sit a few minutes and roll them a half a turn and spray the neck and shoulder area again, let them sith a few minutes, then run them through your Hornady 6mm ARC sizing die.  What you will then have is a 6mm ARC brass case that's .030" too long in the neck area - so you need to trim it to around 1.490" OAL.  I hate trimming but I do it when I need to - so I do it - and you can make yourself some truly great brass for your 6mm ARC.  You can do that with Lapua 6.5 Grendel brass, with Hornady 6.5 Grendel brass, or Starline 6.5 Grendel brass - - I've done it with all of them, but they will all need to be trimmed when all is said and done.

Today's testing using an 6mm ARC upper I built up with a 16" barrel (4 groove cut rifled barrel 1:7.5" twist) with a SAAMI 6mm ARC chamber:

26 gr of H4895, CCI BR4 primer, Berger 105 gr BT, Lapua reformed and trimmed 6.5 Grendel brass, 2.265" OAL magazine fed = 2453 fps - .505" 5 shot group - accuracy excellent.

27 gr H4895, CCI BR4 primer, Berger 105 gr BT, Lapua reformed and trimmed 6.5 Grendel brass, 2.265" OAL = 2565 fps = .457" 5 shot group - max load - can go higher but looking at the brass and flatness of primer, that's about as high as I would want to go - and accuracy was great.

27 gr H4895, CCI BR4 primer, Berger 105 gr BT, Hornady reformed and trimmed 6.5 Grendel brass, 2.265" OAL = 2572 fps = .637" 5 shot group - absolute max load - can go higher but looking at brass and flatness of primer, that's about as high as one might consider going.  Hornady brass showing more pressure signs than Lapua brass (e.g. some signs of "flowback" starting of brass into extractor and ejector recesses in bolt face - not bad, but visible upon close inspection - brass would not be good for multiple reloads with a load this hot).

Going to put together 20" and 24" barreled uppers and I am optimistic the 24" upper will yield the best velocities (likely 2700-2750 fps with the 27 gr H4895 loading, with the 20" being likely about 75 fps slower).

Picture shows two 6mm ARC cartridges, with Hornady reformed and trimmed 6.5 Grendel brass on left and with Lapua reformed and trimmed 6.5 Grendel brass on right.  Other pic shows .457" 5 shot group from 27 gr. load of H4895 with Lapua brass noted above.  Other pic shows upper with heavy walled upper receive and side charging handle, etc.

Hoping this is helpful to some.

Best regards - Robert

[Crowriverjim]

Hello, Just a bit of info on the 6mm Arc. You just about cant find brass. I have been reloading for about 60 years, yup I am old. I like my 6mm Arc, I had a bunch of new .762x39 brass and some once fired, It no big deal to use it in the 6mm arc. I take the decapping pin out of my 6mm Arc sizing die, lube the case good and you can make Arc brass with one pull. As for the big deal made about the large primer, No difference. Lots of cal.s use both large and or small primers.. I also use my favorite powder in My ARC with a .39 group, H4895.  Hope this helps . Good hunting..

[Jurive]

Second the h4895 superb accuracy, velocity, and reliability. Never tried this unicorn varget, although it getting easier to find

[popgun]

But what of Varget?
It's right there near the same burn rate, albeit ever so slightly slower. GRT prediction with a 105gr, well over 90% case fill with my measured cases, mid 90's burn rate, efficiency 29% with a more room to go before peak pressure (gas gun).

Varget works great in my gun. A little slower velocities than Lever but better accuracy. The new StaBall match seems to be close to Varget also but a ball powder. The few people I've seen try it shows promise.

[StoneHendge]

But what of Varget?
It's right there near the same burn rate, albeit ever so slightly slower. GRT prediction with a 105gr, well over 90% case fill with my measured cases, mid 90's burn rate, efficiency 29% with a more room to go before peak pressure (gas gun).

Varget also works great for me. Phenomenal accuracy and low SD. I only do about 2450 out of a 16" Lilja* with 105 gr RDF's, but my zero and dope has never been affected in practical matches out to 700 yards where the temps have been as low as the high 40s up to over 100. I've run the same charge over 4 seasons in the Phoenix climate.

* I've never heard anyone call their Lilja barrel "fast".

[GRH]

But what of Varget?
It's right there near the same burn rate, albeit ever so slightly slower. GRT prediction with a 105gr, well over 90% case fill with my measured cases, mid 90's burn rate, efficiency 29% with a more room to go before peak pressure (gas gun).

Varget works great in my gun. A little slower velocities than Lever but better accuracy. The new StaBall match seems to be close to Varget also but a ball powder. The few people I've seen try it shows promise.

I was excited at the prospect of a stable ball powder and as chance would have it, Charlie over on GRT Discord has worked up a GRT model for 6ARC bolt gun.

Unfortunately when I plugged in my 20" barrel length and max PSI while leaving a margin of safety, the result is pretty anemic. A longer barrel and the higher pressure of a bolt gun seems to make all the difference and I would be all over it in that case.