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.

stancrist said:

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

You answered your own question.  The necks are indeed a significant problem.  That is why it takes such large expenditures to develop it and find a solution to the problem.  The 6.8 TVC gives us a good idea of an optimized polymer case looks like.  The section that holds the bullet requires more thickness than a brass one.   It got rid of the Achilles' heel of he polymer case, the neck. 

We learned that the polymer case of Textron's CT cartridge flowed foreword upon firing and sealed the breach.  That makes me wonder about what is going on with TV's case during firing.

Unfortunately, existing chambers have a neck area that requires a thin case wall.  Even worse, that thin neck gets hot because it is in an area of the barrel that is exposed longest to the burning propellant during firing.    Previous attempts with polymer cases have shown that bonding the shank of a bullet to the thin neck material commonly results in the necks being torn off.   One logical solution would be to bond the bullet farther back the case.  Make the shoulder area of the cartridge solid polymer and bond it there.  The thin material at the neck now becomes just a tube to fill up the gap between the bullet and the chamber wall.   An extension of the chambers's freebore in essense.   Just wasted material and length serving as a guide for the bullet.  The propellant capacity of the case is reduced, however, because the area near the shoulder is now solid polymer.  The effects of the flow and movement of the polymer under pressure during firing might still prove a problem.

It would resemble the 6.8 TVC case with a hollow tube attached to the front. 

I don't know if a polymer case body with a solid shoulder area would be injection molded in one piece.   I've looked at pictures of TV's 308 ammo and even viewed TV's videos in slow motion, but I cannot see into a fired case.  To be honest, if no pics show up soon, I am tempted to buy a some, just so I can section it.   For what it is worth, the 6.8 TVC ammo case is made in two pieces, with the neck welded onto the body.