6mm AR15 cartridge options

[ipreferpie]

Hi everyone, I was wondering if any of you can provide advice/experience on a good 6mm AR15 cartridge. I plan on building a competition upper with a 18"-20" barrel for my lightweight AR15 which I use for 3 gunning. In terms of usage requirements in order of priority:

1) can be reliably loaded in normal AR15 magazines up to 10 rounds,
2) ability to reach 600-800+ yards,
3) hopefully 3000+ round barrel life,
4) manageable recoil and follow ups for PRS and F-class competition,
5) easy to reload as a beginner,
6) greater resilience to wind drift &
7) perhaps opportunity to take down medium game.

In terms of the cartridges I've been considering, I'm looking at:

1) 6mm Mongoose,
2) .24 GPC,
3) 6x6.8 SPC,
4) 6mm FatRat &
5) 6mm Grendel.

Since these are all wildcats, it's hard to find direct comparisons on the forums. Heard lots of great things about all these, but would love to hear everyone's thought. Many thanks!

 

[TonyTheTiger]

Your criteria #5 almost makes a straight 6 Grendel or 6 Valkyrie the best answer. Just remembered that Nosler is making 24 Nosler brass now too, that may be the really easy button.

 

[RampedRaptor]

Any 6mm with a Grendel case behind it is a win win. Achieves all your goals and will surprise you at 1000yds when the wind isn’t too crazy.

 

[Blutroop]

I’d like to hear some differences too. OP, someone’s selling a 6fatrat upper in the px right now.

 

[lennyo3034]

Interested in this too. My foray with .224 Valkyrie has been a colossal disaster so now I have a top shelf precision AR just needing a barrel and bolt.

 

[Jmccracken1214]

I’ve wanted to try 6mm grendel, but can’t ever find dies for sale?

 

[TonyTheTiger]

I’ve wanted to try 6mm grendel, but can’t ever find dies for sale?

You just need a 6.5 Grendel bushing die and the correct bushing.

 

[Jmccracken1214]

You just need a 6.5 Grendel bushing die and the correct bushing.

No sh*t...... any idea what bushing it is?

Curious of the barrel life difference from 6.5 to 6mm. I’m sure there’s a loss but betting it’s still 4000+ rounds with the 6mm

 

[TonyTheTiger]

No sh*t...... any idea what bushing it is?

I've got a Redding .269 bushing I've made a few 6G cases with. Neck tension feels fine using Hornady brass.

 

[sam4886]

I've got a 6mm predator that is basically the same as a 6 grendel and 243 LBC. I load 5 rounds just fine but I like the shorter mags for prone.

It shoots a little flatter out to about 700 with 90 gr game changers than my 6.5cm with 143 eldx.

Longer mags hold 10 or 15 fine. I got custom dies with the barrel. The black hole weaponry forum has lots of info on the 243 LBC.

 

[Jmccracken1214]

I've got a 6mm predator that is basically the same as a 6 grendel and 243 LBC. I load 5 rounds just fine but I like the shorter mags for prone.

It shoots a little flatter out to about 700 with 90 gr game changers than my 6.5cm with 143 eldx.

Longer mags hold 10 or 15 fine. I got custom dies with the barrel. The black hole weaponry forum has lots of info on the 243 LBC.

This has me wanting to get the ruger predator Grendel and rebarrel to a 6mm

 

[TonyTheTiger]

Cheap way to try the cartridge out:

https://www.snipershide.com/shooting/threads/6mm-grendel-barrel.6962441/

 

[sam4886]

This has me wanting to get the ruger predator Grendel and rebarrel to a 6mm

I've had the same thoughts about a Howa mini.

 

[bjay]

I got a 6mmAR turbo 40° with gain twist bartlein its very accurate at 600y
4 shots stack up on that 600y rock

 

[Jmccracken1214]

I got a 6mmAR turbo 40° with gain twist bartlein its very accurate at 600y
4 shots stack up on that 600y rockView attachment 7151524
View attachment 7151529

Well, here goes some more money out of my pockets!

 

[Big Timber]

what about the 6x45? It is just a straight necked up 223/5.56. Not as much capacity as the others mentioned, but it is stupid easy to reload and who cares if you lose a few pieces of brass.

I have 3 6x45's and 1 6x6.8, and I hardly ever shoot the 6x6.8.

 

[TonyTheTiger]

what about the 6x45?

Definitely lags behind the other options listed for criteria 2, 6 and 7.

 

[ipreferpie]

Thanks everyone for the recommendations so far. It looks like there's been a lot of improvements in the 6mm AR options over the past years. It looks like I can sort of lump them into several categories by parent casing:

1) 5.56mm (6mm Mongoose & 6x45)
-love how I can use cheap brass
-never tried fire forming for the Mongoose, but doesn't look too daunting
-many have used the 6x45 since it's been a proven wildcat, but on the AR15 platform, I heard some feeding issues when maximizing mag capacity

2) 6.5mm Grendel (6mm FatRat, 6mm Grendel, 6mmAR Turbo 40)
-more velocity
-lots of development/evolution from 6mmAR
-easy reloading, especially long bullets (?)
-good for hunting
-but are there issues using Grendel bolts? I heard there's long term durability issues vs 5.56 & 6.8 bolts. Maybe it's been fixed?
-I'm wondering how many rounds I can stuff into a 10rd 7.62x39 mag?

3) 6.8mm SPC (.24 GPC & 6x6.8)
-most velocity
-best for longer range target rounds, but have to be more picky with bullets (?)

I'm very curious if anyone has experience with the 6mm Mongoose. It seems to only have slight downsides, but not much considering it uses 5.56 brass. Also, any thoughts on 6mm FatRat vs 6mmAR Turbo 40?

Lastly, any chances that these will get SAAMI certified?

 

[sam4886]

My guess is that the 6mm grendel derivatives will have the best chance at SAAMI because they are simple neck downs, same goes for the 6x6.8 I suppose.

From my research, the grendel cased stuff will give you slightly more velocity than the SPC case. Like 50 fps with like bullets. The grendel is a shorter case so you can use longer bullets than with the SPC, so theoretically that is better for the long range stuff. In practice, probably not much difference either way.

I haven't had a bolt break in anything, but I think of you buy a quality bolt you shouldn't have issues. The JP high pressure bolt session to get the best praise and is what I have on mine.

Turbo 40 is about as hot rod as you can get in a 6mm small frame ar I think. If you want to mess with all the forming steps its a great round. I chose simplicity as the 6mm predator is my first wildcat.

 

[LeadZeke]

1.) For your uses the 6mm Grendel or any derivative thereof makes the most sense. The more you improve it (fatrat, turbo 40, etc), the more performance you get but the more prep you need to do to reload. Which doesn't really make it harder, just longer process.

2.) The 6mm Grendel derivatives actually have more velocity and are better with longer, higher BC bullets than any of the 6.8spc parent ones. The 6.8spc parents will (usually) give a touch more velocity with smaller lighter bullets but it doesn't sound like that is what you want to do with the rifle (and there are better ways to do that anyways).

3.) The 6.5G bolts situation has been solved for a long time. Buy the JP bolt--->don't worry.

4.) Why not just use 6.5G mags?

 

[Constructor]

Hi everyone, I was wondering if any of you can provide advice/experience on a good 6mm AR15 cartridge. I plan on building a competition upper with a 18"-20" barrel for my lightweight AR15 which I use for 3 gunning. In terms of usage requirements in order of priority:

1) can be reliably loaded in normal AR15 magazines up to 10 rounds,
2) ability to reach 600-800+ yards,
3) hopefully 3000+ round barrel life,
4) manageable recoil and follow ups for PRS and F-class competition,
5) easy to reload as a beginner,
6) greater resilience to wind drift &
7) perhaps opportunity to take down medium game.

In terms of the cartridges I've been considering, I'm looking at:

1) 6mm Mongoose,
2) .24 GPC,
3) 6x6.8 SPC,
4) 6mm FatRat &
5) 6mm Grendel.

Since these are all wildcats, it's hard to find direct comparisons on the forums. Heard lots of great things about all these, but would love to hear everyone's thought. Many thanks!

I've been shooting a Grendel and 6.8 since 2005 then several wildcats based off both. If you push anything based on Grendel brass it will swell to look like a belted magnum and the cases will not last long. IMO Norma makes the toughest brass. If you shoot a ton you will not like fireforming anything, stick with straight neck downs. Any cartridge based on the 6.8 will handle 58000psi easy. The Grendel 52-54000 without issue. 3-5000psi is worth another 100fps.

 

[steve123]

Been happy with my 6mmFatRat for the most part. I found the 95SMK to be the best compromise for the cartridge. Basically a .5 BC at 2870 fps in a 22" barrel. I'd go for a premium barrel vs getting the Wilson.

I have a melonited 6mmAR barrel that I'd sell if interested.

 

[RampedRaptor]

I've been shooting a Grendel and 6.8 since 2005 then several wildcats based off both. If you push anything based on Grendel brass it will swell to look like a belted magnum and the cases will not last long. IMO Norma makes the toughest brass. If you shoot a ton you will not like fireforming anything, stick with straight neck downs. Any cartridge based on the 6.8 will handle 58000psi easy. The Grendel 52-54000 without issue. 3-5000psi is worth another 100fps.

Not as much experience as you but I can’t say I’ve ever seen my lapua Grendel brass do that.

 

[sandwarrior]

Thanks everyone for the recommendations so far. It looks like there's been a lot of improvements in the 6mm AR options over the past years. It looks like I can sort of lump them into several categories by parent casing:

1) 5.56mm (6mm Mongoose & 6x45)
-love how I can use cheap brass
-never tried fire forming for the Mongoose, but doesn't look too daunting
-many have used the 6x45 since it's been a proven wildcat, but on the AR15 platform, I heard some feeding issues when maximizing mag capacity

2) 6.5mm Grendel (6mm FatRat, 6mm Grendel, 6mmAR Turbo 40)
-more velocity
-lots of development/evolution from 6mmAR
-easy reloading, especially long bullets (?)
-good for hunting
-but are there issues using Grendel bolts? I heard there's long term durability issues vs 5.56 & 6.8 bolts. Maybe it's been fixed?
-I'm wondering how many rounds I can stuff into a 10rd 7.62x39 mag?

3) 6.8mm SPC (.24 GPC & 6x6.8)
-most velocity
-best for longer range target rounds, but have to be more picky with bullets (?)

I'm very curious if anyone has experience with the 6mm Mongoose. It seems to only have slight downsides, but not much considering it uses 5.56 brass. Also, any thoughts on 6mm FatRat vs 6mmAR Turbo 40?

Lastly, any chances that these will get SAAMI certified?

Okay, let's sort out what you think you've heard.

1. 6-whatever. off .a .223..doesn't have the poop behind it to get you competitive. It WILL get you there, just not as easy as something faster.

2. 6.5G...if I were to do it again, I'd go with the 6mm version. And the least expensive to boot. We're talkin' 6mm AR, .243 LBC, 6mm Grendel, etc. The better velocity of good 6mm's beats the slower, but good wind drift of 6.5's. The big thing with these is when you push them hard velocity-wise, bolts have been known to break. Newer 9130 steel is supposed to mitigate that. But, if you were to take a standard 7.62x39 bolt, you run the risk of breaking it. A physical fact of less material on material.

3. Again, a compromise, as a full length 6.8 case won't neck down and hold the best 6mm's (too long) that will get you the speed you wish. So, the .shortened.224 Val case necked up to 6mm is a nice compromise. Enough increase in powder, not as much of a decrease in lug to lug material from bolt to barrel extension. Gives you enough length to load 6mm's out.

The common thing I see is the .224 Val length case, 6mm gives you enough room to seat heavies out. You also have more material to material engaging surface ( and volume) of lug to lug that it won't break. Note: I do not own or have tested that length version of a 6mm.

 

[Constructor]

3. Again, a compromise, as a full length 6.8 case won't neck down and hold the best 6mm's (too long) that will get you the speed you wish. So, the .shortened.224 Val case necked up to 6mm is a nice compromise. Enough increase in powder, not as much of a decrease in lug to lug material from bolt to barrel extension. Gives you enough length to load 6mm's out.
.

TAC 6- apx 2gr more than a 224 Valkyrie necked up to 6mm, case shorter than full length 6.8.

 

[Constructor]

Not as much experience as you but I can’t say I’ve ever seen my lapua Grendel brass do that.

That photo actually came from the Grendel forum but I've had plenty that look like that and 1 that Ruptured in 6mmAR...don't try RL10 behind a 90gr Scenar.

 

[sandwarrior]

TAC 6- apx 2gr more than a 224 Valkyrie necked up to 6mm, case shorter than full length 6.8.

...i.e. OPTIMIZED! ?

 

[lte82]

6 Fat Rat. Run 6.5 Grendel though the sizing die, load, shoot. It’s the easy button for 2750-2850 FPS with a 105 class bullet and they are incredibly accurate.

 

[Herstal 12]

I had a 6mm fatrat around 2013-2014, loved it, no recoil, tack driver and easy to load for. Made the mistake of letting a good friend shoot it, after that he became insufferable until I agreed to sell it to him. You have been warned
Cheers

 

[Jmccracken1214]

This thread has made me decide to just finish my 6.5 grendel ar and I’m going to buy the ruger American grendel and rebarrel it to a 6mmgrendel so I can keep using AR mags. Bed it in the factory stock and have a badass cheap rifle

 

[sandwarrior]

6 Fat Rat. Run 6.5 Grendel though the sizing die, load, shoot. It’s the easy button for 2750-2850 FPS with a 105 class bullet and they are incredibly accurate.

So, what magazine do you run those through?

 

[lte82]

So, what magazine do you run those through?

I have four 10 Rd Alexander Arms Grendel mags, but I also have used some of the GI 20 Rd 223 mags (only 10 rounds loaded) with good luck.

 

[10000yards]

the 6mm ar turbo based on the grendel case or a 6mm-6.8 spc (valkyrie brass would likely be the easier to form route these days dependent on quality) are going to offer some of the best performance from the platform aside from going to a 243 wssm.
Nosler is to be releasing a 24 Nosler one of these years (my guess is the next shot show) according to the sammi spec's it looks to be essentially a 6mm valkyrie with the rebated rimmmed case. that being said however the .224 valkyrie seems to be gathering some steam and with the bc of the 90-95gr smk it may still have an edge on a 6mm variant. (as a target cartridge, game hunting different story)

 

[steve123]

That photo actually came from the Grendel forum but I've had plenty that look like that and 1 that Ruptured in 6mmAR...don't try RL10 behind a 90gr Scenar.View attachment 7152485View attachment 7152488

Unless this brass was cooked when annealing, or defective to begin with, I can't wrap my head around why someone would push this case so hard, I'm saying there was something wrong - like using a powder not appropriate for the case ??? Looks to me like this AR was unlocking early causing the web to swell for whatever reason??? Adjustable gas block needed maybe and use of a normal pressure load.

Using common sense is all it takes when reloading, if there are pressure signs then it's time to back down, right??!!

I can honestly say this has never happened to me with 6mmART40 or FatRat.

 

[Constructor]

Unless this brass was cooked when annealing, or defective to begin with, I can't wrap my head around why someone would push this case so hard, I'm saying there was something wrong - like using a powder not appropriate for the case ??? Looks to me like this AR was unlocking early causing the web to swell for whatever reason??? Adjustable gas block needed maybe and use of a normal pressure load.

Using common sense is all it takes when reloading, if there are pressure signs then it's time to back down, right??!!

I can honestly say this has never happened to me with 6mmART40 or FatRat.

The bolts have a .136" recess which pulls the case out of the chamber .012" further than a normal bolt then there is a chamfer at the breech. All the flow you see is where the brass swelled into that chamfer.
I've been a wildcatter since 1982. In the beginning someone must test all powders and bullets and develop loads for others to follow. I was testing in 2006 and 2007 before most info was around.
In the end of all of that testing 29gr H4895 will push a 90gr Scenar to 2960fps out of a 22" Mike Rock 5R barrel.

 

[steve123]

The bolts have a .136" recess which pulls the case out of the chamber .012" further than a normal bolt then there is a chamfer at the breech. All the flow you see is where the brass swelled into that chamfer.
I've been a wildcatter since 1982. In the beginning someone must test all powders and bullets and develop loads for others to follow. I was testing in 2006 and 2007 before most info was around.
In the end of all of that testing 29gr H4895 will push a 90gr Scenar to 2960fps out of a 22" Mike Rock 5R barrel.

You stated had, the bolts, were these a prototype bolt made in the beginning that caused the swelling, are bolts different now? The old chambers must be different now, right??, not the amount of chamfer there was to begin with?

That load seems like it's acceptable in pressure .

 

[Constructor]

You stated had, the bolts, were these a prototype bolt made in the beginning that caused the swelling, are bolts different now? The old chambers must be different now, right??, not the amount of chamfer there was to begin with?

That load seems like it's acceptable in pressure .

The bolts are standard Grendel bolts with a .136" recess. 5.56, 6.8 and 308 bolts have a .124" recess. Grendel bolts pull the cartridge .012" further out of the chamber than a normal bolt. That means .012" more of the case is unsupported by the chamber walls.
When Bill Alexander got involved with the Grendel(called the 6.5CSS then) he needed to open the bolt face to accept a .442 diameter rim. The rim on Lapua cases were thicker than a 5.56 rim and the extractor would not snap over. His fix was while opening up the bolt face was to also go deeper during the same operation to allow the extractor to snap over the rim. Back then it was not as easy to get someone to make custom bolts and he didn't have CNCs to machine them.
All barrels have a chamfer to help guide the bullet tip into the chamber.

 

[260284]

I've been shooting a Grendel and 6.8 since 2005 then several wildcats based off both. If you push anything based on Grendel brass it will swell to look like a belted magnum and the cases will not last long. IMO Norma makes the toughest brass. If you shoot a ton you will not like fireforming anything, stick with straight neck downs. Any cartridge based on the 6.8 will handle 58000psi easy. The Grendel 52-54000 without issue. 3-5000psi is worth another 100fps.

An adjustable gas block and a heavier buffer solved that issue with my Grendel. You can only push the Grendel so hard, especially if it is over gassed.

 

[Strykervet]

Been happy with my 6mmFatRat for the most part. I found the 95SMK to be the best compromise for the cartridge. Basically a .5 BC at 2870 fps in a 22" barrel. I'd go for a premium barrel vs getting the Wilson.

I have a melonited 6mmAR barrel that I'd sell if interested.

My neighbor has one of these, apparently it's one bad motherfucker. But I don't think it brings much to offer over a 20" 6.5G slinging 120 Nosler BT's. Satern made my barrel and it's .33MOA consistent with the right ammo (those 120BT handloads). And it better fucking be given the price.

I'm still happy with 6.5G though... Wildcats just ain't my thing anymore ever since the ones I did use went mainstream. But that .375 Reaper or .358 Yehti is calling my name I fear.

Oh, and for you guys needing dies and shit, because wildcat = custom dies... Yeah, a bit of advice: stay the fuck away from CH4D --they were fine until they weren't and then holy fuck nothing could prize $50 from their clutches nor honor a warranty (quality is only on par with Lee too FWIW). Whidden, that's who to use now, and if you're getting rifle dies or parts for rifle dies or reloading dies or anything with "die" in it, call the die manager directly, he knows his shit and will square you away and you won't be disappointed. No fuck-fuck games like CH4D. They custom made me a Grendel trim die for the Dillon 650 and I couldn't be happier with it.

 

[steve123]

When shooting long range side by side with a friend and his 6.5G vs my 6mmART40 there was a noticeable difference in wind drift past 600Y or so.

That same old story like with a 308 with 168's, just left and right of the 15" steel at 1000Y and occasionally connecting for him while I hit it most of the time. His load was factory 123's at 2545 fps. Mine was 105 Amax at 2880 fps. I can connect often on our 19" steel at 1122Y with my FatRat.

Not a whole lot of difference inside 500Y between the two.

I had a 243 wssm, it had great ballistics but the brass sucked and it wasn't accurate or reliable.

I like the lower recoil of the 95's vs the 105's, it's surprising the difference.

 

[RampedRaptor]

Correct: the 6mm wildcats perform substantially better in terms of elevation and wind defection when compared to a 6.5g.

 

[Peterpan]

I just had a custom 6br built in an ar15 platform. 26” bartlien barrel 8 twist. I’ve only got a couple hundred rounds on it and I’m going to have to sell it since my club stopped shooting the long range gas gun steel challenge matches, but the 6br is a viable ar platform gun as well.

 

[Tx_Aggie]

I just had a custom 6br built in an ar15 platform. 26” bartlien barrel 8 twist. I’ve only got a couple hundred rounds on it and I’m going to have to sell it since my club stopped shooting the long range gas gun steel challenge matches, but the 6br is a viable ar platform gun as well.

Tell me more.....

 

[Peterpan]

Tell me more.....

Billet vltor upper. Failsafe bcg. Woa free float handguard. Muzzle threaded 5/8x24. It seems to cycle very reliably but I havent put more than 4 cartridges in a mag since I only needed that for load development. Seems to like the 105 Bergers

 

[Tx_Aggie]

Billet vltor upper. Failsafe bcg. Woa free float handguard. Muzzle threaded 5/8x24. It seems to cycle very reliably but I havent put more than 4 cartridges in a mag since I only needed that for load development. Seems to like the 105 Bergers

Interesting. Who built the barrel and bolt for you, and what mags are you using?

 

[Peterpan]

I don’t remember his name. It was Randy something. Out of Gillette Wyoming. I’d be happy to get you his phone # if u want it.
So far I have just used elander Grendel mags. I looked into buying wssm mags but never did. The elander mags have been reliable for what I need them for so far. Also should note that I dremeled out the front of my mag for a little less jump.

 

[Tx_Aggie]

I don’t remember his name. It was Randy something. Out of Gillette Wyoming. I’d be happy to get you his phone # if u want it.
So far I have just used elander Grendel mags. I looked into buying wssm mags but never did. The elander mags have been reliable for what I need them for so far. Also should note that I dremeled out the front of my mag for a little less jump.

Ah, no need. Thanks for the info.

 

[Blutroop]

Has anyone thought of necking up 22 nosler? I know they had problems with cases early on but are they resolved? More case capacity then Valkyrie 224 and 223 bolt face.

 

[TonyTheTiger]

Has anyone thought of necking up 22 nosler? I know they had problems with cases early on but are they resolved? More case capacity then Valkyrie 224 and 223 bolt face.

Just remembered that Nosler is making 24 Nosler brass now too, that may be the really easy button.

No need.

 

[PNW_Steve]

Has anyone thought of necking up 22 nosler? I know they had problems with cases early on but are they resolved? More case capacity then Valkyrie 224 and 223 bolt face.

I think that someone has thought of that..... They call it the .24 Nosler.