gatnerd said:

       EmericD said: at least 2 competitors (True Velocity and PCP tactical) used outsourced 135 - 136 gr solid copper bullets for their in-house development phase

We also have Soldier Systems initial reporting on the SIG's specs, which quoted 135gr @ 2850fps 13", 3000fps 16". So thats our 3rd indication of 135gr being relevant.

I agree that it is reasonable to conclude that the 135gr weight of the surrogate bullets is relevant.  However, that information does not necessarily indicate that the steel-tip GP projectile weighs ~135 grains.

Considering the expected cost of the tungsten-tip SP armor-piercing bullet, I think it's far more likely that all of those 135gr surrogate bullets duplicate the weight of the SP round, not that of the GP projectile.

The certainty in this forum that the pictured GP bullet weighs only circa 120 grains would seem to support that idea.

RovingPedant said:

Given that the TV round works out of anything that can run 7.62x51mm, FN or Knights Armament could have a go?

Perhaps they would be willing. 

If TV paid them enough $$$$$.

GD/TV may not have switched to conventional layout, but merely demonstrating acceptable velocity from a shorter barrel would make a conventional layout an option.  SIG demonstrated a conventional layout designed for higher pressure.  SIG also demonstrated a LMG.  If the contract had been written allow a mix and match of cartridges and weapons, TV ammo in SIG weapons would have been an option.  As I stated, seldom does one competitor have all the best ideas.  Often needless complexity is added just to circumvent someone else's better idea.  Look at the mess with new rifle selection in Germany for an example.

Depending upon the design, it is often not terribly difficult to modify an existing design to handle the higher pressure. To give you an idea, Olympic arms and Dtech both sold modified AR15's that shot the WSSM cartridges that had 0.535" diameter case heads and 65,000 psi chamber pressures.   Those cartridges produce slightly more backforce than the 6.8 SIG Hybrid at 80,000psi.  The change mainly involved increasing the diameter of the bolt and barrel extension.  From interviews with SIG reps, it appears that SIG kept the same bolt diameter as their model 716 7.62x51 AR)  and just changed to a stronger steel.

Although I am certainly not an expert of modern machine guns I know that many of them use use a rotating bolt that locks at the front of the receiver.  This means that receiver stretching is less of an issue than with lever or flapper locking designs that require a significant length of the receiver to withstand higher forces.

I would also be stunned if General Dynamics had not proposed a variant of their .338 LWMG, which was designed for comparable bolt forces.

Part of the reason that I suggested a two-stage competition is that, in my experience, the second iteration of a complicated device is almost always better.  The competitors had to guess which compromises the Army would prefer because the Army wanted the impossible.   After the first phase of competition, the vendors would know what the evaluators liked and disliked and go from there.  It appears that they did not want bullpups, but had no issue with an unusually high chamber pressure.  It did not matter at this stage that the Marines would prefer bullpups and magazine-fed automatic rifles.  The decision was up to the Army and the preference appeared to be short barrels. 

At this point GD/TV/Berreta could decide if it was feasible to bump up pressure to gain velocity in shorter barreled weapons.  In reality, GD backed out during the competition, leaving an TV, an ammo maker in charge of their bid.   If TV's neckless polymer case could in fact tolerate 80,000psi, I suggest that they would have put a maximum effort into: 

1) Demonstrating that their ammo could do anything that SIG's hybrid could do, but at less weight and lower cost, and

2) Demonstrating that a mere barrel/gas-system would allow other weapons, EVEN THE SIGs to use their new and wondrous ammo.

I am certain that TV et al would also evaluate if it would be possible to submit non-bullpup designs and it is conceivable that they would.  Even if that were the case, I suggest that they would want to make certain to show that their ammo in the best light and suggest it could be used in SIG's weapons.    It is possible that issues with the TV ammo would have surfaced and eliminated it from contention, but we will never know.  The weight savings and potential cost savings might have been dramatic, but we won't know.

Going back to the earlier discussion of polymer versions of standard ammo.  Today I happened to come across the 6.5 USA cartridge while looking for something else.  It was suggested that its unusually gentle shoulder angle of less than 18 degrees per side (almost the same as for the 7.62x39) was to make it more amenable for polymer cases.  It indeed resembles the internal geometry of the TV cartridge in the photo you found.

nincomp said:

Part of the reason that I suggested a two-stage competition is that, in my experience, the second iteration of a complicated device is almost always better.  The competitors had to guess which compromises the Army would prefer because the Army wanted the impossible.   After the first phase of competition, the vendors would know what the evaluators liked and disliked and go from there.  It appears that they did not want bullpups, but had no issue with an unusually high chamber pressure.  It did not matter at this stage that the Marines would prefer bullpups and magazine-fed automatic rifles.  The decision was up to the Army and the preference appeared to be short barrels.

Actually, the competition was a two-stage affair, at least in regard to what became the XM250.

The first stage was NGSAR (Next Generation Squad Automatic Rifle), with a PON in early 2018.

Contracts were awarded in July 2018, and no preference was shown to barrel length, weapon configuration, or feed method.

When the downselect was made in mid-2019 for Phase 2, again no preferences were shown in regard to those characteristics.

So your premise that competitors would know after the first stage what the evaluators liked, is not really supported by history.

nincomp said:

At this point GD/TV/Berreta could decide if it was feasible to bump up pressure to gain velocity in shorter barreled weapons.  In reality, GD backed out during the competition, leaving an TV, an ammo maker in charge of their bid.   If TV's neckless polymer case could in fact tolerate 80,000psi, I suggest that they would have put a maximum effort into: 

1) Demonstrating that their ammo could do anything that SIG's hybrid could do, but at less weight and lower cost, and

2) Demonstrating that a mere barrel/gas-system would allow other weapons, EVEN THE SIGs to use their new and wondrous ammo.

I am certain that TV et al would also evaluate if it would be possible to submit non-bullpup designs and it is conceivable that they would.  Even if that were the case, I suggest that they would want to make certain to show that their ammo in the best light and suggest it could be used in SIG's weapons.

They could have done all of the above this time, but did not.  They also have not yet shown that TV ammo can safely handle the pressure of SIG's hybrid case.  This suggests that maybe it cannot.

nincomp said:

Going back to the earlier discussion of polymer versions of standard ammo.  Today I happened to come across the 6.5 USA cartridge while looking for something else.  It was suggested that its unusually gentle shoulder angle of less than 18 degrees per side was to make it more amenable for polymer cases.

I don't know.  I have no information on the matter.

In regard to the "unusually gentle" shoulder angle...

.264 USA

I did not think that I needed to add the qualifier "recent" or "designed in the last half-century".  I was aware, for example that the shoulder angle is similar, but not equal to, that of the 7.62x39 cartridge.   To be more precise then, upon reviewing SAAMI approved cartridges, the .264 USA has an unusually gentle shoulder angle for an American cartridge designed in the last 50 years,  with the exception of the 7-30 Waters which, for reasons known only to the designer himself,  uses a shoulder angle of 34°24'. 

BTW, this includes cartridges based on older case designs that utilize the shoulder angle of the parent,  like the 25-06 Remington which was approved in the 1960's.

I agree on this idea of funding development phases that merge the best of what we had presented into a single best of everything final product.

We probably also need better baseline funding for incremental improvements and r&d.

It seems like a criminal waste to not keep funding both sig and Lonestar/tv to improve what they have and even to give each team a copy of the other teams work so they can both make improvements.

I think if anything this particular competition has shown us that the way we're handling development and implementation is wildly suboptimal and that in order to really get the most out of what we have we need to do things differently.

nincomp said:

I did not think that I needed to add the qualifier "recent" or "designed in the last half-century". 

To be more precise then, upon reviewing SAAMI approved cartridges, the .264 USA has an unusually gentle shoulder angle for an American cartridge designed in the last 50 years...

Seeing as how this is not the Sporting Arms and Ammunition forum, I thought that you meant it's "unusually gentle" for US military cartridges.

The 17.5-degree shoulder of the .264 USA is the same as on the .30-06, and only slightly gentler than the 20-degree shoulder of the 7.62 NATO.

BTW, the only comment I found about .264 USA being "amenable" to use of polymer cases is this speculation on TFB:

"Not only has the .264 USA possibly been designed specifically for composite cased ammunition, or at least with it in mind..."

POTD: The Polymer Cased .264 USA -The Firearm Blog

Since that says nothing about .264 USA case shoulder angle, I'm curious as to the source where it was suggested.  Link?

I have to question how such a small difference in shoulder angle (relative to 7.62 NATO) is likely to be significantly better.