I did my analysis on the high-BC EPR-style bullet before I saw the SIG rendering and had originally planned to use the photo that Stan posted.  The SIG rendering is just a worse example. 

 I wonder if the contestants had many of the actual Army projectiles with which to perform experiments before their designs were finalized.  Depending upon the exact composition and shape of the official General Purpose Projectiles, there are any number of pertinent characteristics that might be difficult to duplicate.   It would not surprise me if the developers were surprised when they saw the results of the trial.

There must be some explanation behind the odd chamber used by GD, and I suspect that it may have something to do with the neckless polymer round.  The relatively large "forcing cone" would imply some issue having to do with alignment of the bullet as it is being chambered.  The forces on the cartridge as it is stripped from a magazine and fed into a the chamber may come into play here.  I would expect some flexing.  After all, TV's cartridge is a plastic tube with metal weights at both ends.

EmericD said:

The ogive of the 6.8 mm GP is much longer than the SMK (in both actual dimensions, and proportionally to the bullet length)...

The boat-tail is also significantly longer.

Thanks, Emeric.  Can you post ogive, shank, and boattail lengths of both bullets?  It seems like if those dimensions are "much" and "significantly" longer, it would be quite noticeable.  But, in comparing the photos, I do not see a discernible difference in lengths of those features.  They look pretty much the same to my eye.

I don't have a problem with the infantry having organic small arms for fights at 600+ meters.  IFV support may not always be available.

However, I agree with the author that there is no need to penetrate body armor at 600 meters, that 100-200 meters would be enough.

But, I do not get the fascination in this forum with neckless ammo.  The appeal of polymer cases is understandable, but why neckless?

Increases ogive space substantially for a given OAL, and with seemingly (until now) no downsides

And yet, both of these have 51mm case lengths, loaded with the same bullet.

The first discussion on this forum (that I recall, at least) about neckless cartridges was on a GPC thread over a decade ago.  It was about using bullets with a much longer ogive and BC in a (possibly) modified 7.62x51 weapon.  The goal was to retain the energy of the M80 cartridge at long range and reducing recoil GPC levels.  Energy at 800 and 1000m was a big deal on this forum at the time.  The quest for a neckless 5.56 cartridge is mainly about utilizing lower-drag bullets.

For the 6.8mm cartridges submitted for the NGSW trials, case design was not specified so the competitors were allow to optimize as they saw fit.  Presumably, the neckless case submitted by True Velocity is optimized for the properties of the polymer.  The very fact that GD/TV did not use a conventional case and neck design implies that there was something to gain. 

We do not know the thought process that led to TV's submission.  Maintaining current chamber pressure levels seems to have been a priority.  That is one reason why I want to see TV demonstrate a polymer cartridge at higher pressures.  It is a "put your money where your mouth is" moment for them.

nincomp said:

The first discussion on this forum (that I recall, at least) about neckless cartridges was on a GPC thread over a decade ago.  It was about using bullets with a much longer ogive and BC in a (possibly) modified 7.62x51 weapon.  The goal was to retain the energy of the M80 cartridge at long range and reducing recoil GPC levels.  Energy at 800 and 1000m was a big deal on this forum at the time.  The quest for a neckless 5.56 cartridge is mainly about utilizing lower-drag bullets.

Yes, I remember those discussions.  Sorry, I was imprecise. 

I meant that I do not understand the fascination with this particular neckless ammo -- which has no advantage over a conventional necked case in regard to bullet ogive length -- and the insistence that it should be used in the SIG guns.

nincomp said:

For the 6.8mm cartridges submitted for the NGSW trials, case design was not specified so the competitors were allow to optimize as they saw fit.  Presumably, the neckless case submitted by True Velocity is optimized for the properties of the polymer.  The very fact that GD/TV did not use a conventional case and neck design implies that there was something to gain. 

We do not know the thought process that led to TV's submission.  Maintaining current chamber pressure levels seems to have been a priority.

I can't imagine why conventional configuration polymer-cased 6.8x51 would be infeasible.

TV apparently can maintain current chamber pressures with a variety of different calibers.

stancrist said:

Thanks, Emeric.  Can you post ogive, shank, and boattail lengths of both bullets? 

I will do that tomorrow.

The 135 gr SMK is 1.25" - 1.26" long (L/D ~4.5), while (according to the picture) the 6.8 mm GP is around 1.40" - 1.42" (L/D >5), so there should be a difference somewhere!

stancrist said:

I can't imagine why conventional configuration polymer-cased 6.8x51 would be infeasible. TV apparently can maintain current chamber pressures with a variety of different calibers.

I did not state that they could not make conventional ammo.  I stated that the design of TV's neckless composite appears to be an attempt to optimize the performance of their submission using a polymer material and maintaining "normal" pressures.  The properties of their polymer are different from those of brass so it is not surprising that cartridge designed to exploit those different properties would look different from one made of other materials.

I am curious to see if TV can make a polymer version of a cartridge that can match the performance of SIG's Hybrid.  That means, however, that it will be running at higher than typical pressures.  

Returning to the properties of HPBT bullets.  The process used to make these bullets allows the lead core to be pushed down into the jacket.  Without a sectioned 139gr SMK, we don't know for certain, but a 139 grain .277" bullet would scale down to that a 120-123 grain 6.5mm one.  Here is a picture of a 123 grain Lapua Scenar in a 6.5mm Grendel case that you posted a few years ago.  It should be more stable than a solid version with the same shape.  The gyroscopic stability depends upon the distribution of mass, with stability becoming worse as a larger proportion of the mass is moved toward the axis of spin and moved farther along that axis away from the center of mass.  That is one of the reasons that lightweight polymer tips are becoming increasingly popular with high BC match bullets

stancrist said:

I meant that I do not understand the fascination with this particular neckless ammo, and the insistence that it should be used in the SIG guns.

As far as I'm concerned, the answer is "because every conventional geometry plastic case I shot, lost necks in the gun chamber".

This part is very thin and prone to case separation during extraction, and once you lose a cartridge neck in a hot chamber, you screwed...