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.

Thanks.  So there is a significant difference in ogive and shank lengths, but the GP boattail is not longer than that of the SMK.

roguetechie said:

Because the actual neckless version of TV should be good to 100k psi...

Whoa!  How did we get from 80-85k to 100k ?

roguetechie said:

...allow longer ogive projectiles...

That seems unlikely, at least without a redesign of the front portion of the case. 

It also seems irrelevant, since there is no indication a longer ogive will be used.

roguetechie said:

...plus what emeric Said about case necks.

Yet in post #2529 you criticized the Army for defunding polymer ammo, and praised the Marines for "charging ahead" with polymer ammo.

So, if case necks are a significant problem with polymer ammo, why are the Marines spending so much money and effort on developing it?

roguetechie said:

It simplifies things...

Actually, it complicates things.  It requires redesign of SIG's guns, as well as development of new, high-pressure ammo.

If conventional-configuration (necked) 6.8x51 polymer ammo could be used, it would require only ammo development.