"This is my best guess also.  I think that an important detail is that at least one report states that the Sig entries could be easily converted to 7.62x51 with barrel swaps.  My cynical self thinks that Sig is betting that this project will mostly fail and that the weapons will spend most of their lives shooting standard or reduced weight 7.62x51 or 6.5 Creedmoor cartridges.   They will provide the OPTION  to shoot the 6.8 Wondercartridge when needed, though.   Sig also touts major felt-recoil reduction.  Now the military would have 7.62x51 (or 6.5mm) weapons as controllable as their 5.56x45 predecessors.  They might believe that this will be enough of an improvement over the existing weapons to tilt the scales their direction.  Let someone else provide reduced weight ammo for the existing cartridges."

Yes, the versatility of the SIG LMG - being able to fire 7.62 and 6.5C in addition to 6.8 - is a huge advantage. SOCOM for example is looking right now for a 6.5C LMG under 14lbs, making SIG a prime contender, and would also allow them to ramp up to 6.8 in the future.

It also provides an option for NATO countries who currently use 7.62 to get a much improved 7.62 LMG, and then have a future 6.8 capability. 

And of course, should say True Velocity or Shell Shock start selling lightweight 7.62 cases, paired with the 12lb SIG LMG, it offers a greatly enhanced, LSAT like weight savings while still remaining conventional and being able to use old stocks of brass 7.62 in an emergency / for training purposes. 

"The comment that "the carbine appears to have a deep receiver with what may be a battery pack" is uninformed speculation by the author.

The "tumor" covers part of the rifle's operating mechanism."

I had taken it to mean that the reason the 6.8 has a larger 'tumor' then that seen on the previous 6.5CT carbine is that in addition to room for the operating mechanism, they also added a battery pack on top of that. 

This would actually make a certain amount of sense; if the 'tumor' will remain, that part of the handguard is effectively useless for holding, so why not embrace that and use it for a battery location as well. Placing the battery in the handguard is much more convenient for a powered rail system then trying to integrate it into a collapsible stock from a design standpoint . 

Come to think of it, the True Velocity cartridge also has a normal-looking rim.  It is entirely possible that General Dynamics' weapons would be able to use existing 7.62x51 with a barrel and possibly a bolt change.

I think that you guys are wrong about the CT carbine.  That's not a tumor.  It looks to me like that carbine is pregnant.  All we have to do is wait a few more months and Ta-Da, a CT PDW! ;)

gatnerd said...

"The comment that "the carbine appears to have a deep receiver with what may be a battery pack" is uninformed speculation by the author.

The "tumor" covers part of the rifle's operating mechanism."

I had taken it to mean that the reason the 6.8 has a larger 'tumor' then that seen on the previous 6.5CT carbine is that in addition to room for the operating mechanism, they also added a battery pack on top of that.

Note that he says, "what MAY be a battery pack," not "what IS a battery pack."  He is speculating, not stating established fact.

gatnerd said...

...if the 'tumor' will remain, that part of the handguard is effectively useless for holding...

I can't imagine why you would say that.  Its shape is very similar to the M60 handguard, which is quite usable for holding.

nincomp said...

I think that you guys are wrong about the CT carbine.  That's not a tumor.  It looks to me like that carbine is pregnant.  All we have to do is wait a few more months and Ta-Da, a CT PDW! ;)

"Come to think of it, the True Velocity cartridge also has a normal-looking rim.  It is entirely possible that General Dynamics' weapons would be able to use existing 7.62x51 with a barrel and possibly a bolt change."

Yes, thats entirely possible.

If the TV 6.8 maintains the 7.62 COL/Width, then GD's weapons would be able to fire 7.62 and 6.5 - a smart move for GD, as it would allow them to 'recycle' these weapons for other military contracts should they not win NGSW. 

Likewise, if the TV 6.8 can work in 7.62 weapons, theres also the possibility that SIG's 6.8 LMG could be converted to the presumably much lighter TV 6.8 cartridge. This would be smart as it would give TV '2 bites at the apple' for getting into the NGSW contract. 

If it is indeed 7.62 size, that also opens the interesting possibility of a NGSW 'split decision' - The Army liking SIG's LMG more then GD's, but liking GD's rifle and ammo.

gatnerd said...

270 WSM (29g)= 130gr @ 3000fps from 16"

6.8x51 (24g) = 130gr @ 3000fps from 16"

-->6.8 is 20% lighter for same performance 

Hopefully I'm wrong.

Could be, but I don't think that the people writing the requirements would buy that "demonstration". At a minimum, I would say that the 6.8x51 mm should produce a weight reduction of 20% compared with the M80, so the round should be around 19-20 g per round.

With a 8.75 g bullet and a 3.3 g powder load, that leaves 7-8 g for the case which seems a possibility since an all-steel 7.62 mm case made with 1950s technology weight slightly less than 10 g.

The weight of a conventional case is not "requirement driven" but "process driven", that means that the final weight of the case is not a function of "what you need", but a function of "the way you manufacture it", so maybe changing the manufacturing process allows to achieve the right base strength and the right body strength.

"Could be, but I don't think that the people writing the requirements would buy that "demonstration". 

They might though, considering what some of the other candidates are offering.

-Textron = weird tumor tuna rifle ergos 

-GD/TV = completely new and untested cartridge design 

-PCP = absurdly fat steel/polymer case:

-Cobalt-Mars = Absurdly fat brass case:

With that lineup, I can see them letting SIG stick around for Phase II of testing even with a 24g cartridge.

"With a 8.75 g bullet and a 3.3 g powder load, that leaves 7-8 g for the case which seems a possibility since an all-steel 7.62 mm case made with 1950s technology weight slightly less than 10 g.

The weight of a conventional case is not "requirement driven" but "process driven", that means that the final weight of the case is not a function of "what you need", but a function of "the way you manufacture it", so maybe changing the manufacturing process allows to achieve the right base strength and the right body strength."

Its possible; I imagine that SIG, like Shell Shock, will have a thin wall body, where the material is a constant thickness from neck to base. Whereas a conventional brass cartridge is gets progressively thicker walled from neck to base. That would save some weight. 

That said, to 20% weight savings over M80 is tough. 24g = 370g; 20% weight savings = 296gr

6.8:

Projectile = 135gr

Powder = 51gr

Primer = 3gr

--> 189gr 

----->107gr weight budget for case

Current LC 7.62 brass = 179gr 

So they need their hybrid brass/steel case to be 40% lighter. And Shell Shock aluminum / steel cases are 50% lighter.... it seems improbable.

Here is the patent for SIG's Hybrid case:

http://www.freepatentsonline.com/20190226817.pdf

-Steel Base / Brass body described as being 'sight reduction in weight' 

-->The SIG 6.8 Steel/Brass casings are likely 23-24g

-Aluminum base / Titanium body described as being 'very light weight with a tensile strength far exceeding 62kpsi'

-->Wildcard - SIG is only displaying steel base / brass body cases to demonstrate its 3 piece case design, but is actually submitting an Aluminum/Titanium case?

It's the only combination they list thats both 'very light weight' and suitable for pressure 'far exceeding 62kpsi.'

On the other hand, using titanium in a disposable cartridge seems insane / extremely expensive and wasteful. Current value is $26 per pound vs $1.55lb for stainless steel, and $2.70 per lb for copper. 

gatnerd said...

 

 

-->Wildcard - SIG is only displaying steel base / brass body cases to demonstrate its 3 piece case design, but is actually submitting an Aluminum/Titanium case?

It's the only combination they list thats both 'very light weight' and suitable for pressure 'far exceeding 62kpsi.'

On the other hand, using titanium in a disposable cartridge seems insane / extremely expensive and wasteful. Current value is $26 per pound vs $1.55lb for stainless steel, and $2.70 per lb for copper. 

 

Interview on The Firearm Blog with SIG CEO Ron Cohen seems to answer the question explicitly. 

"SIG’s answer, the 6.8x51mm, uses a hybrid case design with a brass upper and a lighter steel base. “Our ammunition enables everything else, I think the ammo is the key to everything.” SIG state that the hybrid round is 20% lighter and Cohen feels that with the hybrid metal case SIG are “achieving the goal of more velocity while reducing the round’s weight” 

.....

SIG also confirmed that they are continuing to refine the cartridge and there is scope for a further reduction in weight, as much as 30% total. This, however, would require the use of more expensive, exotic metals like titanium in the place of the steel base."