New Upper: 6mm ARC or 6.5 Grendel?

Which cartridge for an upgraded AR?

  • 6mm ARC

    Votes: 23 65.7%

  • 6.5 Grendel

    Votes: 12 34.3%
  • Total voters
    35
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I chose the 6.5 Grendel as my second AR caliber after 223. I am not a reloader so the option of economical wolf ammo for practice was very appealing. As is the relatively economical Hornady Black offerings. My purposes were different than yours as I was looking for something more suited for deer in the AR platform. I went with a complete upper from Sanders Armory and have been very pleased with it. My son, who is pretty recoil and blast sensitive, has used it the past two seasons on deer in South Dakota and I have been very impressed with the results.
What made me lean towards the Grendel isn't as much of an issue for you being a reloader and that you are looking primarily for target uses. I think those both lean more towards the 6 ARC. Either one is a win in my book because in the end it's another gun in the safe.
 
Thanks, all. @Varminterror, those are a couple of nice builds! I've built a couple of AR lowers in the past, never an upper. Not being an AR guy, I'd rather purchase a complete upper and learn to change parts out of it (new barrel) when it becomes necessary.

I'm liking the look of the Aero Precision/Ballistic Advantage uppers available through Brownells. Open to other suggestions. Am I nuts to consider a 24-inch barrel?

Oh, and what magazines are y'all running with your 6mm ARC?
 
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Legionnaire said:
Am I nuts to consider a 24-inch barrel?
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Not nuts to me. If you’re shooting long and small, 24” makes sense to me, in either cartridge. The 6.5 Grendel can get away with a bit shorter barrel for “efficient burn,” and “the small case doesn’t gain as much per inch past 20,” but it does gain, and that length DOES add stability, so for me, there is no downside.

Long range rifles are like Funny Cars… you CAN build them with short instead of long, but it comes with consequences, not rewards.

Legionnaire said:
what magazines are y'all running with your 6mm ARC?
Click to expand...

I’m not as hip to the new poop as some guys just coming around to these cartridges, but I’ve been using E-Lander and ASC mags for Grendel case cartridges for a long time and have been very happy. I also don’t typically mess with the 24-27 round mags, so I can’t speak as well as some might to their performance. But 10’s from Elander and ASC keep me happy.
 
My 24" 6.5G just for posterity, posing with a couple other uppers.

index.php
 
Varminterror said:
243LBC - a straight 6 Grendel. I ordered an extra 243LBC barrel in the early days of the 6 ARC speculation, before reamers were available - it’ll be converted to 6 ARC when the barrel burns out, just to facilitate easier logistics.

@Legionnaire - I voted 6 ARC over 6.5 Grendel, for the simple fact it has a better ratio of bullet weight to case capacity, and is capable of using bullets which maximize the ballistic efficiency of the caliber.

6.5 Grendel is a close second to the ARC in terms of performance in the AR platform, but if I’m picking one, it’s an ARC.

Here’s a 20” 6.5 Grendel I did several years ago (actually a 264LBC marked barrel, from Black Hole Weaponry, using a Type I bolt), absolutely wonderful rifle. Sub-MOA, smooth, capable, and doesn’t look too bad for an AR.

View attachment 1045071

A few weeks ago, I finally put together a spare parts build I had laying around - I ordered an extra barrel and changed my mind last year, so it sat in a box on the shelf. For all intents, this one might as well be a 6 ARC, even though it is actually a 243LBC - and it’ll be an ARC soon enough when this barrel burns out. A little longer case and shoulder, forcing me into shorter ogive bullets but offering a bit more powder capacity… but basically an ARC on steroids (or rather, about an ARC bolt gun equivalent, but at gas gun pressures - and this also uses a Type I bolt). It also needs a weight kit to move the balance point forward where it should be for my purposes - 4lbs or so of M-lok weights coming soon. Smooth, fast, flatter shooting, better balance (extra weight and length), albeit a bit cliche in aesthetic and a bit bulkier and longer than the Grendel rifle above. Absolutely love the Geissele Mk18 handguard, even if it does cause some headaches for AGB selection.

View attachment 1045072

243LBC, 6.5 Grendel necked down to 6mm, without pushing back or shortening the case, such is the ARC.
View attachment 1045074
Click to expand...

My buddy has the 243 LBC (pre-dating the 6 ARC) and it is a hammer. Like 3 rapid shots, and 3 hits on the 800 steel. His is the reason I would consider a 6 ARC.
 
I have both, although the Grendel is a lightweight 16" hunting rifle, and the Arc is a heavy 21" range anchor. For your purposes I'd definitely get the ARC. I like both but I agree with LoonWulf and Varminterror that the 6mm bore diameter is really a better match for the case capacity, you can get pretty much any 6mm bullet up to a useable velocity, whereas the percentage of 6.5 bullets that really work well in the Grendel is much smaller.

That said, for those top velocities, you'll be stuck with relatively temp sensitive powders like LVR, CFE223, or AA2520. That's also kind of true for the Grendel, but some temp stable offerings like 8208 H4895 and AR-Comp will net reasonable velocities (although they won't match CFE or LVR in the Grendel either). I suppose those same powders might work in the ARC with lighter bullet loads.

As for barrel length, my next ARC will be a lightish 18" for more general purpose use. In my 21" AR, I've found decent loads at ~ 2,820 fps for 95gr SSTs, ~2,970 fps for 85gr SGKs, and my 105gr loads match 108gr factory at right around 2,700 fps out of my rifle. All of my loads use LVR, if you want much more velocity than that, you'll want a longer barrel.

Found this video a while ago, but its a good watch. I believe they are using factory 108gr loads, and they certainly seem to work well.
 
CapnMac said:
Well, we all start somewhere.
And, I have this bias. I'd likely get a 6ars in a 16 or 18 inch barrel, and the 6.5 in 20 or 22 inch. One as a handy (and meaty) carbine; the other as a varmint deathray. My bias.
Click to expand...

My christmas present is a bunch of parts for an M16A1/A2 based build in 6.5G with 20" barrel and boyds wood furniture. Should be a fair bit handier than my 24" Grendel.
 
CapnMac said:
That sound cool as all get out. Will be cool to see when you post the photos here [:)]
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I will definitely post when its finished!

gotboostvr said:
I was thinking about building an XM-177 clone in 300blk.

They'd go well together
Click to expand...

I am in the painting stage of an XM-177 build, more specifically a GAU-5A/A from the Son Tay raid, complete with a Singlepoint OEG. Though i stayed true to the 5.56 caliber
 
Well, after all the discussion and my continuing research into what components I have and can acquire, I've decided on a 20-inch PSA upper in 6.5 Grendel as a starting point. 6mm ARC remains an eventual option, but the 6.5's proven history and its greater potential as a hunting round, along with more currently-available options, tipped the scale toward the Grendel. If that goes well enough to get me more interested in the AR platform, I won't rule out a 24-inch 6mm at some point in the future.
 
What’s the mag situation with 6mm?
If no one makes reliable mags, don’t bother.
I shoot 6.5 Grendel, my favorites are CPD/ Duramag 20s and E-Lander 17s. Very reliable.
The 30 round CPDs work, but like 7.62x39 mags, they’re bulky.

I got a few Unimags last year, usually they’re $35-$40 a pop but they’re supposedly out of production and a few places have them on clearance for $20. I’ve run 5.56, 5.45, 7.62x39 and 6.5 Grendel without issue. A complicated follower setup but seems to work.
 
My 6.5G is real fond of the 90gr Speer Blitz over H335.
#8208 would not give me the speed w/o going over pressure.
(Bad lot#?)
120-123gr offer everything I need for larger game.
I save the heavier bullets for the .260Rem...
Still in early stages of playing with the 6.5G.
 
I would go with the ARC. I am more of a 6.8 SPC II shooter, because I value its advantages for hunting, and reliability.. But between the 6.5 and ARC I would go with the 24 cal.
 
Eagle103 said:
I have a couple 20" PSA uppers and I don't think you'll be disappointed. Grendel is a great deer round as long as you keep the range reasonable (under 250).
Click to expand...
Thanks for the affirmation. I'm actually more likely to try it some of these Texas hogs if the opportunity presents itself.
 
Legionnaire said:
Thanks for the affirmation. I'm actually more likely to try it some of these Texas hogs if the opportunity presents itself.
Click to expand...
I just got my dads AR rebuilt as 20" Grendel, its first live targets are probably gonna be some pigs next time I get the call to help clear some fields. Its a solid performer on game, been happy with the ones ive used for such.
 
Well, PSA got me. I was pleased to find their 20-inch upper on sale for $450. I'm still waiting on delivery due to weather delays. But when I got their "Shot Show Special" email today and saw the Magpul MOE complete lower for $150 with free shipping, I couldn't help myself. :)
 
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