gatnerd

Military Guns and Ammunition

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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.

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NGSW Phase 2 Consolidation and info   Small Arms <20mm

Started 30/8/19 by gatnerd; 555583 views.
stancrist

From: stancrist

24/4/22

EmericD said:

The ogive of the 6.8 mm GP is much longer than the SMK (in both actual dimensions, and proportionally to the bullet length)...

The boat-tail is also significantly longer.

Thanks, Emeric.  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.

stancrist

From: stancrist

24/4/22

I don't have a problem with the infantry having organic small arms for fights at 600+ meters.  IFV support may not always be available.

However, I agree with the author that there is no need to penetrate body armor at 600 meters, that 100-200 meters would be enough.

But, I do not get the fascination in this forum with neckless ammo.  The appeal of polymer cases is understandable, but why neckless?

Gr1ff1th

From: Gr1ff1th

24/4/22

Increases ogive space substantially for a given OAL, and with seemingly (until now) no downsides

stancrist

From: stancrist

24/4/22

And yet, both of these have 51mm case lengths, loaded with the same bullet.

nincomp

From: nincomp

24/4/22

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.

stancrist

From: stancrist

24/4/22

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.

EmericD

From: EmericD

24/4/22

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!

nincomp

From: nincomp

24/4/22

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

EmericD

From: EmericD

24/4/22

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...

stancrist

From: stancrist

24/4/22

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.

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.

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