Sabots allow a fin stabilised projectile which means you can have a higher L/D ratio.

Technically you can have fins on the tail of a full-bore projectile, but needs a longer tail/bigger fins to be effective. Or you go for deploying fins, which are a nuisance in large calibre and probably impractical in small ones

Perhaps perfect sabot açcuracy (in small arms) could be achieved if you tried to take the exact proportions of a tank APFDS, (projectile weight versus muzzle energy).....

..and EXACTLY scale them down.

Tank flechette penetrators are 4.5kg each....sabot is roughly 3kg...

I dunno the energies though. Perhaps someone who knows could then do the math and work out how much a rifle projo would weigh.

Clearly more than 10grains as used in the ACR trials

Problem is tanks have no weight targets to reach....everythings very heavy

To match those proportions would probably mean a package weighing similar to 308 battle rifles

smg762 said...

Perhaps perfect sabot açcuracy (in small arms) could be achieved if you tried to take the exact proportions of a tank APFDS, (projectile weight versus muzzle energy).....

..and EXACTLY scale them down.

If you could scale the tolerance as well? It would still be around 0.3 mrad ballistic dispersion, assuming that the aerodynamics scales too, which it probably doesn’t.

smg762 said...

Tank flechette penetrators are 4.5kg each....sabot is roughly 3kg...

I dunno the energies though. Perhaps someone who knows could then do the math and work out how much a rifle projo would weigh.

Clearly more than 10grains as used in the ACR trials

If it is pure scaling and you are using the same materials (Tungsten heavy alloy or depleted uranium in small arms ammunition?) then it will be the cube of the ratio of calibre. So 120mm to 5mm would result in 4.5kg to 0.3g (about 5grains).

Of course if you are scaling to achieve a 5mm diameter projectile it would be different.

On a related note, my understanding is that “sub-calibre” is where the projectile is smaller than the bore. If you refer to a small calibre then it’s small calibre. Not too sure if there is a standard definition on where the break point is.

smg762 said...

Problem is tanks have no weight targets to reach....everythings very heavy

Don’t kid yourself that tanks have no weight limits. Quite apart from everything adding up to gross vehicle weight which affects mobility and transportability, the ammunition needs to be something that the crew can lift.

Yes i originally had 2 questions...one was about 18cal bore riding bullers and barrel wear. By sub caluber i was reffering to that.

I dont know if your conclusion of 5 grains is right...120mm is the tanks barrel. Tank flechette is 30mm.

Thats a big difference in size.. thats like using a 762barrel and shooting 2mm projectiles....if u scale it down.

To ascertain the projo weight for our small arm we would take the tanks muzzle energy, and work out the percentage difference between it and roughly 1k ft lbs (our rifles energy)

Then apply the same percentage reduction from the tanks projo weight  to our rifle projectile. To find out the rough ballpark ideal weight.

ill try and find some typical tank muzzle energies

EDIT

Here are the figures.

Tank flechette weight 4.4kg or 67902 grains.

Tank velocity of flechette...5200fps

Resulting muzzle energy...4077600ft lbs.

Percentage reduction from tank energy to rifle energy (1200ft lbs)...  about 99.970%

These are figures from one of the Abrams tank rounds.

My maths is poor so i still cant use this data to find out what the projo weight would be for our 1200ft lbs rifle..

What RovingPedant wrote is totally independent of external shape.

Be it a flechette or a conventionally shaped bullet, its mass changes by the 3rd power of diameter. Halving diameter -and all other dimensions- reduces the projectile mass to one eighth. Doubling it make the projectile by the factor 2*2*2 = 8 heavier.

Similarly, the cross section (dominating air drag) changes by the 2nd power. If you halve the diameter, cross section reduces to one quarter.   

smg762 said...

I dont know if your conclusion of 5 grains is right...120mm is the tanks barrel. Tank flechette is 30mm.

Given the assumptions stated, it is.

If you wanted a 5mm subcalibre projectile where the projectile is 1/4 the bore diameter, then that would be a 20mm calibre barrel and a 20gram (300 grain) dart.

If you plan on a MV greater than 1500m/s like the tank gun, i don’t think I’d fancy it.

Going by your ratio for energy and keeping the MV the same (mainly so I don’t have to deal with USCS units or two variables), the energy scales linearly with the mass, so 0.03% energy requires 0.03% of the mass, or 1.35 grams. You’d be looking at an 8mm bore with a 2mm projectile.

But that all assumes that you are using the same materials and that manufacturing and aerodynamics scales, which they probably don’t.

I think the thing that holds.true the most is that many variables can negatively affect the accuracy, from poor quality control per-sabot, to poor separation on firing. 

As i say, tanks aee resistant to this because everythings on a heavy scale. A 5kg dart wont get 'nudged' or swayed by a dodgy sabot.

So the closest you could emulate tank-like accuracy is to have a fairly large caluber, lets say a 62grain 224 projo, as opposed to the usual 2mm or 4mm projos.

of course tanks use petal type sabots...i dont know if the sabots in the ACR trials used those sabots. And of course an APSFS round costs thousands od dollars

smg762 said:

Anyway to eliminate the inevitable barrel wear problems, i wondered if it would help to use the advanced modern stuff, like the hammer forged barrels on daniel defence guns

Yes, that would help. From memory of various barrel testing, the following barrels maintained accurate service life to:

M4 Colt Button rifled chrome lined = ~7-8k rounds before accuracy reduction

DD CHF chrome lined = 15-20k rounds before accuracy reduction 

H&K 416 Superior Steel + Heat Treatment = 30-40k before accuracy reduction. 

So in theory at least, you could have a round with 2x the barrel wear of 5.56, and still get 7-8k round service life using a CHF barrel, and perhaps 15k rounds using the 416 barrel. 

Whether that theory holds up in practice, I have no idea. I can't think of any CHF barrels or military trials of sub 5.45/5.56 rounds since the H&K G11. 

I think you'd probably be fine with .17-.20 round based on the 5.56, shooting VLD projectiles. Increasing powder charges beyond that will start to show progressively more barrel wear. 

Thanks. My last questiion was, if u had the most extreme 416/chromelined barrel, would it be any heavier than an M4 barrel. 

 The original idea was a 4.9mm, was is also the actual diameter or the G11 4.73mm caseless. 

Anything below 4.9 is definatrly in the extremes.

i then decided you could get better range by lowering to 4.65

The problems are

#this is very close to .17, and no 17. Could realistically reach 1100ft lbs. They struggle to get past 900. Youd have to hope that my ultra heavy bullets would make this possible.

#also 17 is notorious for THROAT wear and fouling. And thats with a mere 900ft lbs.

#lastly, the whole idea was to use a 25mm, 50grain bullet.  Stabilising this would be a nightmare. Even the .17 is unstable with 37grains. (Accordin to saubier forum members).

smg762 said:

Thanks. My last questiion was, if u had the most extreme 416/chromelined barrel, would it be any heavier than an M4 barrel. 

No; per Emeric's weighing of the 14.5" 416 barrel in a thread a few years back, it was nearly identical in weight to the standard M4A1 14.5" barrel. 

In terms of projectile design, Nathaniel wrote an excellent article a few years back, featuring a number of EPR VLD's:

https://www.thefirearmblog.com/blog/2017/09/10/preference-driven-vs-process-driven-design-field-small-arms-ammunition-discussion/

The .204 Vulcan Projectile - 50gr, 0.192 G7 - seems like a good starting point to look into for sub-5.56 cartridge design.

Simply trimming a 5.56 to 42mm and necking it down to .204, and sticking the .204 50gr on top, would handily outperform the 5.56 while still maintaining acceptable barrel wear. Figure around 3100fps/1067 ftlbs from a 16" barrel.