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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.

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NGSW Phase 2 Consolidation and info   Small Arms <20mm

Started 30/8/19 by gatnerd; 554025 views.
Mr. T (MrT4)

From: Mr. T (MrT4)

3/6/22

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.

EmericD

From: EmericD

3/6/22

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.

EmericD

From: EmericD

3/6/22

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..."

BruhMomento

From: BruhMomento

3/6/22

alr boys jesus dropped the vid on the xm5

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

(forgotten weapons)
 

nincomp

From: nincomp

3/6/22

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?

stancrist

From: stancrist

3/6/22

Mr. T (MrT4) said:

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.

Why fool around trying to make reliable steel 6 ARC magazines sized for the 5.56 mag well instead of developing new polymer mags specifically for 6 ARC?

Opting for the enlarged mag well and bigger magazine would also make it impossible for Private Snuffy to load 5.56 mags into a 6 ARC rifle, and vice versa.

Mr. T (MrT4) said:

There is realy no reason for the 6 ARC not to work from either magazines or belt

I think there is good reason to question if 6 ARC can be made to work reliably in belt-feds.

Note how 5.56 NATO fits into M27 links, how much case shoulder extends above the link.

That exposed part of the case is what the front feed pawl pushes against to advance the belt, and also what contacts the cartridge stop to align with the chamber.

Note that with 6.5 GREN, there is almost no case shoulder (extending above the link) for contact with either the front feed pawl or the cartridge stop.

This situation would be even worse with 6 ARC because the case head-to-shoulder dimension is shorter than on 6.5 GREN.

stancrist

From: stancrist

3/6/22

EmericD said:

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..."

stuck_out_tongue

nincomp

From: nincomp

3/6/22

Possibly an underwear change as well.flushed

EmericD

From: EmericD

4/6/22

BruhMomento said:

alr boys jesus dropped the vid on the xm5

So, the "practice round" is a 135 gr / 0.475 G1 at 2650 fps (my own guess) from the XM5.

8.75 g at 808 m/s equals to ~2850 J and ~7.1 N.s of impulse with a suppressor, that's:

  • 40% less impulse than the 7.62x51 mm without a suppressor,
  • 20% more impulse than the 5.56x45 mm without a suppressor.

The 0.475 G1 is not very high, but that's around 0.242 G7, so in the same league as the .308" / 175 gr SMK used in the M118LR, which is advertised as 792 m/s from a 24" barrel.

This "low recoil & practice" 6.8x51 mm fired from the M5 is delivering as much impact energy above 500 m as the 7.62 mm M80 fired from a M14, with 40% less impulse.

So, that could explain why people shooting the gun are impressed by it, but could also explain why SOCOM resumed it's 6.5 mm CM program.

poliorcetes

From: poliorcetes

4/6/22

I wonder if part of the hardware responsible for the funcionalities of a smart sight could be migrated outside of the rifle (mainly on the body or helmet of the user)

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