Conversion of 6mmARC brass back to 6.5 Grendel - Lazy's test results

[lazyengineer]

I've also posted on this over in another forum, but this is really where it should be. Here's my brass haul from the gong range last week.

It's pretty much always like this, where for 800 yard level shooting, pretty much nobody is running .308 or .223. 6.5 Creedmoor is king (at least for brass left behind), 6.5 Grendel is basically gone now, replaced by 6mmARC (which people I guess consider to be a better long-range gong-pinger, than 6.5 Grendel - which might be true but I still think 6.5 Grendel is the better general purpose do-all round - or I wouldn't be bothering with this).

Note I found a fair amount of 6mm Creedmoor brass as well. Relevance being I always just run 6mm Creedmoor brass through the Mandrel, and tadaa, it's 6.5 Creedmoor again.

So can you do the same with 6mm ARC? The answer directly is no - because they made the headspace shorter. To fit some particular bullet or something, I don't know, but it's super annoying, because the headspace of 6mmARC isn't right for Grendel, and 6.5 Grendel shooters have to basically just throw away all the 6mmARC brass they see, which is the only brass we see anymore - rather than Grendel.

Or do they? Hmm... I have an idea..

Let's anneal it and soften that up


Let's open it up the target diameter of 6mm to the do-all general purpose capable 6.5mm diameter, in two steps, with a small and then a larger 6.5mm Mandrel, in the progressive. And on station 3, I have a 6.8SPC die screwed down only half way, so as to only engage the top 1/2 of the casing. By squeezing it down, will that extrude the shoulder forward a bit, and thus move headspace forward?


Here's what they look like. I'm not too happy on how the 6.8 compression isn't even all the way around the case, but seems to prefer doing just one side. Not sure what's up with that, but it shouldn't actually be a problem, since the 6.5Grendel die resize step should even that back out well enough, and if doesn't - the fireforming sure will.


That brass got worked pretty hard with those steps. What the heck, it's cold outside so the heat's always good to have anyway - so anneal it again.

[lazyengineer]

Let's do some testing. 30.3 gr Benchmark pushing a 6.5mm TNT 90 bullet at 2.2" OAL

(right)
A - five x Conventional Grendel brass (5x fired)

(center)
B - five x 7.62X39 converted to 6.5Grendel brass, that had already been fireformed (3x fired)

(left)
C - five x 6mmARC brass NOT fireformed, but converted as described above (Squeezed down using 6.8SPC dies at half-way or so, and then mandrel'd to 6.5 dia)

(not shown)
D - only one - 6mmARC brass used as is (no headspace adjustment), with a 6.5Grendel 90 gr TNT bullet.



And head to the range, firing out of a 20" Alexander Arms.

[lazyengineer]

And here's how they did:

The 6.5 Grendel did like 6.5 Grendel and worked OK. Accuracy was... OK. Maybe I was still settling in.

2856.7 ± 16.7 FPS

The 7.62X39 brass did in Grendel what it often does - it shots just way WAY better than factory 6.5Grendel brass. I see this again and again. I think it's due to the thicker neck you get running .30 cal brass swaged down to 6.5mm. 6.5Grendel chambers are sloppy in the necks, and I suspect 7.62x39 sourced brass converted to 6.5 Grendel, compensates for that. Don't know, but it's typical for me to see this performance improvement by using 7.62x39 brass. Recall again, this is the fireformed 7.62x39brass on it's 3rd or 4th run, but even during fireforming I'll get my best groups with my Grendel.

2877.3 ± 23.1 fps

And the main test item - converted 6mmARC brass that was swagged from extended headspace. Success! In that no headspace separation or cracks. Accuracy was pretty Meh, but then for this fireforming that's not a big surprise since concentricity from that wasn't so uniform. (one of the primers out of the five was surprisingly flattened - not sure what that's about)

2881.6 ± 12.0 fps

And the wildcard suprrise! 6mmARC raw brass (no headspace adjustment) CRACKED right away. That's a FAIL

2844.9 fps

Here's all of them


So, I can conclude the swage technique did help mitigate head separation with this short-headspace brass for 6.5 Grendel.

But - standby...

[lazyengineer]

So I took the cracked 6mmARC casing (that did not get my headspace adjustments before firing it)


There's the crack and you can see the stretching inside. Conclusion: Don't do that. Don't just run 6mmARC as is through a 6.5 Grendel die and run it, or the headspace is going to crack, if not fully separate. (Which I kind of expected).


And I took the HEADSPACE ADJUSTED 6mmARC casing, the one with the flattened primer, and sectioned it.
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well... OK it didn't crack, but that's definitely still stretched. I was curious if there would be any deformation from where the 6.8die stopped, which had left kind of a shelf on the shell body - but nothing I can see at that area. That part looks fine. But the Head looks like it's headed towards incipient head separation.

I can conclude the adjustments did prevent cracking or head separation on the first shot. But they didn't prevent the head-stretch (or whatever you call it), that will lead head-separation in the future. Which, darn. Yes, this sort of worked. And I can probably get another firing if I want - but that's probably about it.

Can I adjust the practice to do better? I think so. I think running it further up the 6.8 die might help. Or I might see if there's another die I can use now that I've used the 6.8 Die - like maybe 9mm. I may tinker some more.

[A5Blaster Number 2]

Good on you to mess with this.

To much trouble for me, I will just buy grendel brass or loaded grendel ammo to shoot for the brass.

Oddly enough, I have never found a single piece of arc or 6mm creedmoor brass at any of the ranges I go to.

But I'm in a pine tree ocean filled with neck beards who think you need 300 win mag to kill a 130lb deer under 100 yards.

[grayfox]

I'm not surprised at the results at all, but I too say thanks for going through this.
My take: a 6 Arc is not a candidate for conversion to grendel, primarily because it's a small case to start with and the shorter shoulder (0.030) means not enough brass to go around as far as the case or the neck goes. The Arc was made so it wouldn't seat and fire in the grendel. Think of a grendel who, when fired, the case stretches 0.030 in OAL... mighty thin case wall somewhere!!

The 6 Creed, on the other hand, is exactly the same dimensions as to shoulder and case body, only the neck is somewhat narrower, so bumping out to 6.5 neck diameter will cause a slightly shorter neck length but the body is untouched - I expect the neck brass to stretch out and top go slightly down from the neck/shoulder interface, up; body relatively untouched. They both have the OAL dimensions of 1.920 max as well. Some mfrs, or a slightly over-aggressive trimmer, can wind up w/ a short neck on the 6.5 Creed, and as stated they will still work.

But LZ, come on now - you want to shoot 800 yds accurately but want to rely on range brass pick-ups? I don't see those 2 ideas co-existing... but this is a nice case (lol) study none the less!

[lazyengineer]

...
But LZ, come on now - you want to shoot 800 yds accurately but want to rely on range brass pick-ups? I don't see those 2 ideas co-existing... but this is a nice case (lol) study none the less!

It was really more for fun and experimentation - I'm not actually poor. And as I note, you can be surprised. My most accurate brass in my Grendels is actually converted 7.62x39 brass. Or at least my best groups seem to correlate to when I use that. So who knows what I might discover with 6mmARC. If you look close at the data, the ARC brass had the fastest velocity, which is interesting. But in the same vein as recovering 7.62x39brass, the motivation had more to do with sustaining operations during shortages and reusing "free" materials; if they'll work. Also, a portion of my shooting is done in settings where brass isn't so easily recovered. And typically aren't really super-precision in those settings. I like to reserve and set aside random brass for those runs, and stuff like this would be used for such.

As to effort, once the experimentation is done, the effort actually isn't that much. It's 3 dies in a progressive press. I anneal anyway. So the only added work is basically one ram stroke on the progressive, and out comes the casing. That's not that much effort, really.

And it was fun, and you know darned well I'm not the only guy wondering about it. Yes, it's doable. Yes, the concept works - but the execution isn't quite there - I need to push that headspace forward just a bit more. But the concept of "does pushing the headspace forward by overly sizing the brass with a smaller die, mitigate the casing from stretching forward to fill the chamber, vs radially expanding now that headspaced, and so mitigate case separation?" the answer is a hard yes based on comparison to what happened to the raw 6mmARC brass alone. I just don't think I pushed the headspace forward quite enough. In theory, I suppose the casings, now that are correctly headspaced after the fireform, can likely be used a fair number of times yet, MIGHT be true, but right now I'm not satisfied and it's It's still not quite there.

I'll see what I can do. Tinkering is fun

[biodsl]

You're a better man than I. Seems like a lot of work, though I have done the reverse: converted some well used Lapua Grendel to 6mm ARC.
BTW, I had never heard (or don't remember) your theory on 7.62x39 brass. Does Lapua still make 7.62x39 brass?

[lazyengineer]

You're a better man than I. Seems like a lot of work, though I have done the reverse: converted some well used Lapua Grendel to 6mm ARC.
BTW, I had never heard (or don't remember) your theory on 7.62x39 brass. Does Lapua still make 7.62x39 brass?

Interestingly my best results are with PMC 7.62x39 brass. But that 5 shot (four into 1-hole) group with 7.62x39 consisted of 2 PMC, 2 S&B and 1 Lapua 7.62x39 casing. I find Lapua 7.62x39 tough to process. the rim is just a bit thick for some shell-holders, and it's just hard to fully resize it so it'll fit in a Grendel for me, for some reason. PMC is actually my favorite 7.62x39 brass for Grendel, and has given me some incredible results.

Just me - I've never seen anyone else ever say any of that. So maybe I'm completely wrong. But this isn't the first or second time I've seen 7.62x39 brass outshine factory Grendel brass for me on paper.

[37L1]

Just thinking outside the box but do you think it might be possible that you get better results with the 7.62 x 39 brass because it uses a large rifle primer?

[Constitutionalist]

Not sure if the LRP makes a difference but I have also seen better than average accuracy fire forming (and shooting fireformed) 7.62x39mm to 6.5 Grendel

[lazyengineer]

Not sure if the LRP makes a difference but I have also seen better than average accuracy fire forming (and shooting fireformed) 7.62x39mm to 6.5 Grendel

Thanks for posting that. Maybe it's not just me afterall. I have my own theory on why it does better with 7.62x39 sourced brass - curious if maybe you have any other theories on it?

[vaguru]

lazy.

To correct the head space, try expanding the neck up to 7mm, then back down to 6.5 in the FL die. This should form a false shoulder for the case to seat on. I have done this in the past with other calibers and it works very well to control head space and eliminate case head stretch.

Then of course one could also order a special hydraulic die to fully form the shoulder where it needs to be.

[vaguru]

lazy,

To correct the head space, try expanding the neck up to 7mm, then back down to 6.5 in the FL die. This should form a false shoulder for the case to seat on. I have done this in the past with other calibers and it works very well to control head space and eliminate case head stretch.

Then of course one could also order a special hydraulic die to fully form the shoulder where it needs to be.

[lazyengineer]

lazy,

To correct the head space, try expanding the neck up to 7mm, then back down to 6.5 in the FL die. This should form a false shoulder for the case to seat on. I have done this in the past with other calibers and it works very well to control head space and eliminate case head stretch.

Then of course one could also order a special hydraulic die to fully form the shoulder where it needs to be.

I thought about that. the appeal being easier and likely more concentric for the fireforming. Mt next step up though form 6.5mm is 7.62mm, and that's just a really big jump from 6mm. I was worried about trashing the brass. I also wasn't sure if that would be enough shoulder contact to prevent case-stretch during fire, to fill the headspace.

it might work though, it's not a bad idea. being fresh annealed, it might work, but for now i slightly prefer just swaging down the case body..