EmericD said:

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

Were any of those rounds True Velocity ammo?

The reason that the TV and SIG cases are virtually the same length shows that the companies submitting them both made the decision to maximize propellant capacity in a cartridge compatible with existing 7.62x51 weapons.  Note that one of the initial competitors, Bachstein Consulting submitted the 6.8 Sherwood, a cartridge with a larger diameter brass case and utilized normal chamber pressures but did not make it to the next level of competition.  Larger, heavier ammo was clearly not wanted.

The need for a greater thickness of material at the neck is apparent if you consider the mechanical properties of polyethylene (PE) based polymer versus cartridge brass.  Existing cartridges and their cases have been optimized for brass or steel.  Polyethylene (PE) based polymers have much lower tensile strength and require a greater thickness to come close to matching even thin-walled brass.   PE is much more elastic, so again, a greater thickness is needed to provide rigidity to the section that holds the bullet in place.    If you look at the cutaway section of the TV case, the shape of the interior wall is very much like that of a brass case and indeed has a "neck".   The exterior is a different shape because additional material is added to reinforce it.  As Emeric noted, the traditional problem with polymer cases has been with the necks.  Elsewhere in the case body, wall thickness can be adjusted as required by material properties.  For the neck however, the case's wall thickness is specified by the clearance between the bullet and the chamber.  It will be interesting to see a sectioned TV case for 7.62x51 to see if they thickened the the area inside the shoulder to provide more strength.  That eats into case capacity and reduces the amount of propellant that can be used.

On a brass case, a relatively long neck is needed to align the bullet and provide tension to hold it in place.  Brass cases are formed by progressive stamping, sometimes called cold forming.  In my experience in the automotive industry,  the walls of a cylinder formed by progressive stamping must stay the same thickness or become thinner at the open end.  The neck walls of a brass case are the open end and are the thinnest section.  They must, therefore be relatively long for normal wall tension to hold the bullet in place.  The surfaces of most PE polymers have a low friction coefficient on the other hand, so mechanical or chemical bonding must be used to hold the bullet in place.  The length and thickness of the "neck" section of a polymer case is more determined by the strength needed and by bonding parameters.   Looking at the cutaway, it appears that a relatively short length of bonding area was needed, so TV pushed its interior "neck" foreword to maximize the propellant compartment.  Even doing this, the TV submission requires a longer barrel than the SIG to reach the required velocity.

The performance of the TV cartridge is impressive when compared to cartridges of similar diameter and OAL.  Note that it uses a barrel 5.5" (140mm) shorter than the others in the comparison.   I have included the SAAMI spec numbers as well as data for Hornady Superperformance cartridges which are generally the fastest commercial load.

6.8 TVC (per article): 18.5" barrel,  3,001fps 

260 Remington:  24" barrel,  SAAMI spec 120gr = 2880fps; Hornady Superperformance 129gr SST (lead core) = 2930fps

6.5 Creedmoor: 24" barrel,  SAAMI spec 120gr = 2900fps; Hornady Superperformance 120gr CX (copper)= 3050fps; 

          129gr SST(lead core)= 2950fps

7mm-08: 24" barrel, SAAMI spec 140gr = 2845 fps;  Hornady Superperformance 139gr SST (lead core) = 2950fps;

          139gr CX (copper) = 2910 fps

The reason that I posted a comparison of the 6.8 TVC to existing cartridges of similar length and diameter is to show that the odd design of the neckless polymer case apparently provides a velocity advantage over a traditional one.   The 7mm-08 has a larger surface area and produces a larger swept volume than the 6.8 TVC in a given-length barrel, so if anything, it should produce higher velocities.  It does not.  The 270-08 cartridge exists as a wildcat cartridge but has no commercial ammo.  Even if TV made a polymer cased 270-08, it would almost certainly have a significantly lower velocity than the 6.8 TVC.  

nincomp said:

The reason that the TV and SIG cases are virtually the same length shows that the companies submitting them both made the decision to maximize propellant capacity in a cartridge compatible with existing 7.62x51 weapons.

That seems likely to be true.  It's also quite irrelevant.  It doesn't alter the fact that the TV neckless design has no advantage over SIG's necked round insofar as the touted ability of neckless cases to use bullets with longer ogives.

nincomp said:

The need for a greater thickness of material at the neck is apparent if you consider the mechanical properties of polyethylene (PE) based polymer versus cartridge brass.  Existing cartridges and their cases have been optimized for brass or steel.  Polyethylene (PE) based polymers have much lower tensile strength and require a greater thickness to come close to matching even thin-walled brass.

If polymer cases need a greater thickness of material at the neck, how do you explain https://youtu.be/G1DCJsMKJb0?t=30 ?

nincomp said:

The reason that I posted a comparison of the 6.8 TVC to existing cartridges of similar length and diameter is to show that the odd design of the neckless polymer case apparently provides a velocity advantage over a traditional one.

That would be true only if "OTBE" were true of your comparison. 

The problem is, all other things are not equal in your comparison.

1.  It's unlikely the propellant types and chamber pressures are the same.

2.  It is certain that the bullets are not the same type, diameter, or weight.

3.  It is obvious that the cartridge case materials are clearly not the same.

nincomp said:

The 270-08 cartridge exists as a wildcat cartridge but has no commercial ammo.  Even if TV made a polymer cased 270-08, it would almost certainly have a significantly lower velocity than the 6.8 TVC.

That's a questionable assertion.  Such a conclusion cannot reasonably be made on the basis of your "apples to oranges" comparison.

As discussed in this forum, polymer cases do not lose as much energy via heat as brass cases, which IIRC translates to higher velocity.

In order to determine what effect neckless design has on velocity, you must compare cartridges which differ only in neck configuration.

stancrist said:

Were any of those rounds True Velocity ammo?

Yes they were.

stancrist said:

If polymer cases need a greater thickness of material at the neck, how do you explain https://youtu.be/G1DCJsMKJb0?t=30 ?

The magic of the Internet.

Now, take the 100,000 or about M240 in service in the US Army and try to duplicate the video, I'm pretty sure that the result will be much different.

Firearm munted radar chronographs hitting the market , I imagine doppler radar units  will be quite mainstream on many artillery and autocannon munts in the future

stancrist said:

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.

Dimensions of the 135 gr SMK according to Bryan litz:

• BOAL: 1.25" (4.51 cal)
• Ogive: 0.726" (2.62 cal)
• Shank: 0.304" (1.10 cal)
• BT: 0.205" (0.74 cal)
• rounded part after the BT: 0.015" (0.05 cal)

Dimensions of the 6.8 mm GP according to the single picture available, probable error of 0.01" (1 pxl on the picture):

• BOAL: 1.41"
• Ogive: 0.825"
• Shank: 0.380"
• BT: 0.205"

EDIT: those dimensions lead to a bullet mean density of 9.4 g/cm3, or a 120 gr bullet if the density is 8.4 g/cm3.

Because the actual neckless version of TV should be good to 100k psi and allow longer ogive projectiles, plus what emeric Said about case necks.

It simplifies things and makes for a round more conducive to what we're trying to do.