stancrist said:

PAAC study?

"Platoon Arms and Ammunition Configuration"

https://www.armytimes.com/news/your-army/2020/11/06/the-army-is-looking-for-a-new-machine-gun-and-the-marines-might-get-a-new-caliber-too/

Thanks!

Guardsman26 said:

I think the US Army's new PAAC study will end-up trying to replace 5.56x45 mm NATO with something like 6x39 mm ARC.

Okay, I just read the article linked to by Emeric, as well as a few other articles.

They reported the PAAC study as looking for a replacement of the 7.62 M240.

None said anything about trying to replace 5.56x45 with something like 6 ARC.

ETA:  Unless NGSW is cancelled, why would they adopt a new round like 6 ARC?

After all, 6.8x51 was developed to replace 5.56x45 in the Close Combat Force.

nincomp said:

       Guardsman26 said: Everything I've read here so far convinces me that 6.8x51 mm can replace 7.62x51 mm NATO, but not 5.56x45 mm. 

I tend to agree.

The Army seems to disagree.

"The Army recently announced that the Next Generation Squad Weapon, the XM5 rifle and XM250 light machine gun will replace the M4/M16 rifle and the 249 light machine gun...  The new weapon system will use the 6.8 mm family of ammunition instead of the 5.56 mm ammunition the M4/M16 utilized."  

NGSW signifies an evolution in Soldier lethality | Article | The United States Army

nincomp said:

The big question to me is to what extent the 5.56x45 replacement will be hobbled by the existing AR15/M4 platform.  The barrel extension and bolt strengths pose pressure limitations and the magazine well dimensions pose cartridge size limitations.  At this point in time,  I would not bet on the reliability of 6mm ARC/65 Grendel-sized cases feeding through AR15/M4 magazine wells.   FWIW,  just this month the 65 Grendel forum had yet another thread concerning magazines and feed issues.

Yes, the reliability of 6.5 GREN/6 ARC stainless steel magazines has been an ongoing issue for many years. 

But, in the unlikely event that 5.56 NATO were to be replaced by 6 ARC, I doubt that it would be a problem.

I expect that 6 ARC variants of the M4 carbine would follow the lead of the LWRC Six8 and have enlarged mag wells, with caliber-specific, polymer 30-rd mags.

However, another issue is that no one has yet demonstrated that 6 ARC will work in a belt-fed LMG, which would be a critical factor in adopting this caliber.

EmericD said:

UK SF adopting the 6.8 mm SPC

Any more details, Emeric? Which weapons, what ammo and is it general SF issue or just available if needed?

6 ARC magazine issues stem from them not being dedicated mags but rather 6.8SPC mag bodies occasionally with new followers. I have been sourcing mags for my rifles from Israel's E-lander while they work ok , they are not dedicated 6ARC mags , that would need slightly modified stamping. But if it were to be used militarily i am certain tooling for proper mags would be made. Similar problems exist with .300BLK mags as most are just marketed btu realy .223rem , mags , so when you load up the subsonic rounds with heavy 200grain projectiles things fall apart. The ones that work best for my rifles are Lancer systems. There is realy no reason for the 6 ARC not to work from either magazines or belt

Sig actually made this .300blk bullet as a workaround for lazy ass magazine manufacturers .

As you can imagine the magazine contour from 6.8SPC is not realy well fitting to the 6.5Crendel or 6ARC case.

autogun said:

Any more details, Emeric?

Unfortunately no.

I've seen that LMT could deliver 6.8 mm SPC barrels for the MARS rifle, but it's just an hypothesis.

Mr. T (MrT4) said:

Sig actually made this .300blk bullet as a workaround for lazy ass magazine manufacturers .

Should be also perfectly fit into a 5.56 x 45 mm chamber!

Of course, one should bump into accuracy (if the bullet exit the barrel) or safety issues (if it doesn't), but, as Stan would point out, "nothing that can't be solved by a barrel change..."

alr boys jesus dropped the vid on the xm5

https://www.youtube.com/watch?v=MTZRCEh1Czg

(forgotten weapons)
 

FWIW, one of the interesting points I discovered in the long and convoluted history of the 6.8 SPC is that virtually all of the commercial barrels made for it are internally undersized based the SAAMI specification.  The SAAMI specification for most cartridges includes the internal area, but many US barrel makers seem to ignore it.

For the 6.8 SPC, the SAAMI specification includes:  "min. bore and groove area 0.0596 sq in (38.451mm^2)", and even a suggested rifling profile (4 grooves, 0.160" wide).  This results in roughly a 25/75 land-to-groove ratio.   This compares to the 50/50 barrels that unsurprisingly typically had the worst pressure issues (although some of them had undersized bores and flawed chambers as well).

As it turns out, the SAAMI specifications of most of the 6.8mm or 270cal cartridges specify a similarly large bore cross-sectional area, so I wonder if overpressure issues will plague some of the new cartridges like the .277 Fury.  Only the new 6.8 Western specifies breaks with tradition and specifies something more like a 33/66 land-to-groove ratio.

Has this been an issue with other cartridges?