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...

nincomp said:

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.

Yes, I understood all of that.  What I'm saying is that there is no readily apparent reason for GD's use of the neckless design over a conventional necked case.

It isn't needed for pressure, because the 6.8 TVCM reportedly is normal pressure.  It isn't needed for ogive length, because COL in a necked case is the same.

nincomp said:

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 see no reason to draw that conclusion.  TV's 7.62 NATO seems to work just as well as their 6.8 TVCM.

YouTube

True Velocity Ammunition is Ammunition Made Perfect.

True Velocity Ammunition (TVA) is an advanced technology company that manufactures the most precise, lightest, and most accurate composite cased ammunition a...

nincomp said:

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...

While we don't know for certain what a sectioned 6.8mm Sierra MatchKing looks like, I think it's far more likely to be constructed like the 6.5mm Sierra MatchKing (posted earlier) than a Lapua Scenar of any caliber.

And unlike the Scenar -- which has an extremely large nose cavity -- the sectioned 6.5 SMK has only a very tiny air space in the tip.  Which means it does not have the lightweight tip that your stability argument is based upon.

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