Optimized Caliber

[txgunner00]

I would think it could be done without too much modification. Maybe a different feed tray (or just the stop) to fit the profile of the Grendel case and different pawls, possibly just the front pawl can grab the neck instead of the case body.

[stanc]

On the old forum, somebody else suggested having the front feed pawl push against the case neck. I don't know whether or not that's a viable option, but I've never heard of any machine gun that does it.

Back in 2006, a gunsmith friend took some measurements on an M249. The front and rear feed pawls are 0.241" in width, with spacing as shown on the attached picture.

The feed tray would require a new cartridge stop, reconfigured as shown. The left side of the stop would need to be shifted ~0.03" to the right to allow for correct alignment of the fatter cartridge with the feed slot. And note that even changing the lower left from a 45-degree bevel to a 90-degree corner, contact with the case shoulder looks rather iffy.

[stanc]

I was browsing Tony Williams' forum tonight. They, too, have had endless discussions/debates about designing a cartridge that could replace 7.62 NATO. Apparently they've settled on what they call a 6.5/8/800 (6.5mm 8-gram bullet @ 800 m/sec).

One member suggests using an AR-10 in 6.5 Creedmore as a proof-of-concept demonstrator.

...to quickly evaluate the 6.5/8/800 concept from an assault rifle, then an AR10 clone with a 16" bbl like the L129 (with select fire option) chambered in 6.5mm Creedmoor might be a good start IMO.

http://forums.delphiforums.com/autog...ges?msg=5041.3

...a quick and cheap capability demonstrator one could use to help prove and sell the concept of 6.5/8/800 from a performance standpoint. It would demonstrate the superior exterior ballistics, demonstrate terminal ballistics, and demonstrate controllability of such a cartridge.

It would also be easy to convert a Mk48 LMG over to 6.5mm Creedmoor or 6.5x47mm Lapua to demonstrate the capability of the 6.5/8/800 concept to replace 7.62mm NATO in the machinegun role.

I may be wrong but I think the military brass may need to be sold on the need for, and the capabilities of, a 6.5/8/800 type cartridge before they can be sold an actual idealized 6.5/8/800 cartridge.

http://forums.delphiforums.com/autog...ges?msg=5041.5

Unfortunately, nobody seems interested in getting a gun and actually shooting it to see if reality matches their theory.



Another thread of possible interest:

I have been involved in a fairly lengthy industry discussion about medium caliber military cartridges. Now that everyone has access to basic ballistic calculators, there is some doubt about whether our ideal 6.5 mm to 7 mm cartridge would offer a significant enough weight saving versus 7.62 mm to justify adoption. In order to achieve a flat trajectory and to match 7.62 mm at 1,000 metres, it is argued that we would need a cartridge that's quite close to the M80 in size, volume and weight.

Cris Murray's 6.86 mm UIAC is an example of the problem: clearly a very good cartridge. But the weight saving isn't even 10%.

http://forums.delphiforums.com/autog...ges?msg=5023.1

[LRRPF52]

They have correctly identified one of the problems with the 7.62 NATO-based cartridges: Weight/size

I want to get rid of the whole T65/M80 case because of the weight/size penalty it brings with it. It also is not a good case for a high-volume military weapon for a 6.5mm because of over-bore and throat erosion, although some of that can be dealt with using modern metallurgy.

This further steers us to the shorter .445" case head diameter, made of strong brass capable of higher pressures, with a shorter overall length. We don't need a case volume of 44-50grs to beat the 7.62 NATO when pushing 6.5mm bullets either.

I really think a 130gr Grendel is the way to go.

[stanc]

I'm not sure how significant this is, but note the calibers:

SWEAT Course (Soldier Weapon, Equipment, and Test)
Sponsored By JSSAP

Page 15

Determine the Ballistics Protection/Bullet Traps for Each Station
•
Ballistic protection plan designed to preserve overall lifetime of course and keep a low total of cost ownership
•
Design Considerations
•
Create safe and realistic environment
•
Ammunition
•
9mm
•
5.56 (M855A1 in Phase II)
•
5.7mm
•
6.5mm
•
6.8mm
•
7.62
•
Maintenance life-cycle cost
•
Durability

http://www.dtic.mil/ndia/2011smallar...2791Wilson.pdf

[Tony Williams]

I've been photoshopping again

Some thoughts on the options for a GPC (General Purpose Cartridge) which could replace the 7.62x51 in portable small arms carried by dismounted infantry with substantial reductions in weight and recoil (and could also replace the 5.56mm in the infantry squad, although that would stick around for a long time in non-infantry applications).

I start with the assumption that the ideal GPC cannot be accommodated in the AR-15 platform. The Grendel is by far the best attempt to do so but, as is being discussed on another thread on this forum, the case width limits the maximum safe pressure to around 50,000 psi, which I suspect isn't quite enough to extract the performance required to replace the 7.62mm. Particularly when the US Army's preference for lead-free bullets is taken into account (longer bullets = less room for propellant). So all of the proposals below exceed the overall length of the 5.56x45.

The first three on the left are based on the 6.8mm Rem only with the case extended slightly to 45mm and (more importantly) the overall length allowed to grow a lot more to use higher-BC bullets. The three versions show 6.35mm, 6.5mm and 6.8mm calibres. These were chosen because the ARDEC calibre survey tested cartridges in 6.35mm and 6.8mm (but not 6.5mm) and concluded that these two were both better than 5.56mm, 6mm or 7.62mm.

The next three (in the same calibres) are based on the Grendel, only with the case extended to 42mm. The longer OAL would ensure that they could not be chambered in modified AR-15 actions, so could be loaded to higher pressures. Maybe this could be designated something like 6.5GM (Grendel Maximum)? With the increasing availability of commercial rifles based on the AR-10 action, this could find plenty of good homes as a big brother to the Grendel (and being a bullpup supporter, I'd love to see one of these in a Kel-Tec RFB!). There is obviously room to extend the OAL to accommodate lead-free bullets. The aim would be to match the performance of the 6.5 Grendel in 24 inch barrels (in metric round figures, 8g at 800m/s, or 123 grains at 2,620 fps) only with longer, lead-free bullets in shorter barrels (20 inch, with decent performance maintained from carbine barrels).

The final cartridge is the 7.62x51 for comparison.

The cartridges shown in the pic are purely indicative and the designs would doubtless be amended by people who know more about the technicalities than I do.

Comments?

[JASmith]

...the case width limits the maximum safe pressure to around 50,000 psi, which I suspect isn't quite enough to extract the performance required to replace the 7.62mm. Particularly when the US Army's preference for lead-free bullets is taken into account (longer bullets = less room for propellant). So all of the proposals below exceed the overall length of the 5.56x45...

I like the photos!

There seems to be continuing confusion, however, about the pressure limitations of the Grendel cartridge.

Per Bill Alexander, the base width indeed places more thrust on the AR15 bolt, which results in reduced average bolt life when run at pressures significantly above 50,000 psi. This is a reliability issue unique to the AR15 bolt.

Also, Bill indicates that Grendel case itself can readily handle upwards of 58000 psi.

These pressures are safe in appropriate actions. When coupled with a longer COAL as suggested, then the desired velocities and bullet weights for supplanting the 7.62X51 M80 cartridge are likely attainable.

[Tony Williams]

I think that to have one cartridge in two versions, differing only in the length of the bullet protrusion and the chamber pressure, would be confusing. We don't want to get into a 6.8mm Rem situation, with two different chambers!

If we're no longer limited to 5.56mm action lengths, then IMO it's better to produce a slightly different case, extending it a bit to make sure that it can still deliver the required performance from 20 inch barrels when loaded with lead-free bullets (especially the even longer tracer versions). That might also mean that it needn't be loaded to maximum pressure to start with, which is no bad thing.

Edit to add: a little extra capacity would also make it easier in due course to produce a polymer cased (with metal base) version, since these tend to have thicker walls. These cases are improving all the time, and offer big weight savings - important for the military.

[stanc]

Some thoughts on the options for a GPC (General Purpose Cartridge) which could replace the 7.62x51 in portable small arms carried by dismounted infantry with substantial reductions in weight and recoil (and could also replace the 5.56mm in the infantry squad, although that would stick around for a long time in non-infantry applications).

I start with the assumption that the ideal GPC cannot be accommodated in the AR-15 platform.

The three versions show 6.35mm, 6.5mm and 6.8mm calibres. These were chosen because the ARDEC calibre survey tested cartridges in 6.35mm and 6.8mm (but not 6.5mm) and concluded that these two were both better than 5.56mm, 6mm or 7.62mm.

There is obviously room to extend the OAL to accommodate lead-free bullets.

Comments?

Looks good. Ought to work.

The 0.42" case offers a slight advantage to machine gun belt length (for 100 linked rounds).

The 0.44" case gives a small advantage to rifle magazine ergonomics (front-to-rear dimension).

No significant weight difference between the two case options. Probably no significant difference in muzzle velocities, either.

Major drawback to both is the non-availability of platforms optimized for intermediate-length, intermediate-diameter cartridges, which means it'd be necessary to use the bulkier, heavier weapons developed for 7.62x51 (such as the AR-10, Mk17, etc).

[Tony Williams]

Major drawback to both is the non-availability of platforms optimized for intermediate-length, intermediate-diameter cartridges, which means it'd be necessary to use the bulkier, heavier weapons developed for 7.62x51 (such as the AR-10, Mk17, etc).

True, development and initial use would be in AR-10 sized actions (although these could maybe be lightened a bit). But if such a round were ever adopted by the military, intermediate sized actions would appear...