QuintusO said:

You certainly can do it that way, but there's really no reason to. The TV-NGSW neckless format has superior propellant capacity and compression characteristics.

For new design weapons and ammo, no argument. I was thinking in terms of ammo fired from existing guns with no chamber modifications. As I recollect, a neckless version of the 7.62 x 51 might suffer from some disadvantages, to do with the bullet needing to jump into the leade?

nincomp said:

I am curious to see how much of the bullet's tail protrudes into the propellant space of the neckless designs.  I have often read that doing so with a normal cartridge can reduce accuracy.  Maybe this reduction is too small to be an issue for a military cartridge?

There is an accuracy advantage in having no ullage (i.e. the propellant exactly fills the available space). This is because propellant burn is more consistent. I recall seeing a presentation about the .50 BMG which was found to differ in chamber pressure, MV and accuracy depending on whether the ullage was at the primer or bullet end. One of the less well-publicised advantages of polymer cases is that their wall thickness can be adjusted for each loading to ensure that there is no ullage.

The long jump to the leade is a potential issue, no doubt.  There may be a bullet design that has a high BC and good self-aligning properties, but I don't know.    Ideally, new barrels would be fitted, which would add to the expense of the project.

I was aware of ullage being a problem.  I had not heard of customizing the wall thickness to minimize void area, but it makes perfect sense.

The bullet intrusion into the propellant area is a separate issue.  Again, its effect may be so small that it is only noticed by competition shooters, but I have read about some shooters who insist that only the boat tail of the bullet should protrude past the neck.  Some do this by careful choice of bullets, others go as far as having the chamber throat cut to get the perfect geometry.  Of course, the throat will slowly change with shooting, so the optimal conditions don't last for long.  Some shooters will routinely remove a small length of the barrel's base, then recut a new chamber slightly ahead of the previous one.  Of course, eventually the barrel will get too short and be sold, discarded, or used as a very fancy stake to hold tomato plants upright. 

So essentially you're potentially proposing modern heeled bullets then?

I have a feeling that this would be somewhat incompatible and or expensive/requiring more manufacturing steps due to the way modern jacketed ammo is made. But that's just my first impression on it.

Heeled bullet illustration below.

autogun said:

A thought crossed my mind: as I understand it, one problem with polymer versions of metal cases is that the necks have to be very thin, hence TV preferring a neckless design for their clean-sheet 6.8 mm NGSW. A alternative approach might be to design a bullet with a reduced diameter at the case mouth, so the neck can be thicker. Not sure if this would allow enough contact with the bore to keep the bullet stable, though.

You mean, just like the Mle1898D bullet with a 8.15 mm shank riding in a 8.3 mm bore?

That's one option, for sure, but you're still stuck with the very short ogive length of existing military cartridge.

Another option is to use a neckless design, and have 3 "petals" around the bullet. When the bullet starts moving, the petals "open" to allow proper bullet guidance. 

nincomp said:

I am curious to see how much of the bullet's tail protrudes into the propellant space of the neckless designs.  I have often read that doing so with a normal cartridge can reduce accuracy.  Maybe this reduction is too small to be an issue for a military cartridge?

Short necks are no different to long necks. The longer necks are just mechanically stronger and support the projectile better. With the polymer neckless case, the projectile is welded to the case so that isn't an issue. It will allow the the flat body of the projectile to be smaller or even allow a more coke bottle shaped projectile which will will reduce friction in the bore and drag.

nincomp said:

There have probably been designs that tried to tuck the neck backwards into a metallic cartridge body to get the same effect, but I have never seen one. 

It's theoretically possible but would make filling the case problematic, and while it might tension correctly the external neck is much more accessible for testing and correcting errors. 

EmericD said:

You mean, just like the Mle1898D bullet with a 8.15 mm shank riding in a 8.3 mm bore? That's one option, for sure, but you're still stuck with the very short ogive length of existing military cartridge.

Sure - but I was only trying to solve one potential problem without needing to alter the guns: the weakness of thin polymer necks. I wonder how the 7.62mm TV holds up when being hammered through a high-rof MG?

You are of course correct regarding the short ogive length offered by existing military cartridges. They are from a time when nobody thought that small arms would ever have any important role in combat again.  

But I cannot resist to remark that the 6.5x55 Swedish/Norwegian of 1894 offers 25 mm ogive length, compared to the "big" 23 mm of the new wonder cartridge 6.5 Creedmoor. Pity, that Norwegians and Swedes gave up their superior cartridge (sectional density)  for standardizing on 7.62 mm NATO, only to find out that 6.5 mm is now viewed as respresenting the future. 

JPeelen said:

But I cannot resist to remark that the 6.5x55 Swedish/Norwegian of 1894 offers 25 mm ogive length, compared to the "big" 23 mm of the new wonder cartridge 6.5 Creedmoor. Pity, that Norwegians and Swedes gave up their superior cartridge (sectional density)  for standardizing on 7.62 mm NATO, only to find out that 6.5 mm is now viewed as respresenting the future. 

That was a Mauser thing after the 7.65x54. Everything from Mauser had 25 mm for the ogive. The US in their wisdom reduced it on the 30-03/6 and no one else thought it was a good idea at the time. 

Red7272 said...

Short necks are no different to long necks. The longer necks are just mechanically stronger and support the projectile better.

From what I could infer from the "stay out of the propellant" articles, the concern is that the turbulence of the combusting powder at the base of the bullet could effect the initial engraving into the rifling.  None of the articles specified whether the concern was simply bullet alignment or some other evil caused by propellant turbulence on the unprotected bullet base. (Wow! that almost sounds off-color :0  )

As for metallic case designs with the neck within the backwards necks.  Yes, I can see any number of problems when put into mass production.  It still would not surprise me if it had been tried and become one of those ratholes into which money was poured.  There is a big difference between having an IDEA and having A GOOD IDEA. ;)