question about the "armor-piercing fin-stabilized discarding sabot" round

[roscoe]

I was reading some news and there was discussion on a thread where the above-mentioned round was mentioned. I am aware how this works in a smoothbore gun (like the M1's 120mm), but apparently they are used in rifled guns - it is one of the rounds available for the the 25mm on the Bradley.

I get the concept of the sabot (I still have some old 30-30 'Accelerator' rounds). But how does it work with a fin-guided round? Wouldn't the spin tend to offset the intent of the fins? A little spin is apparently desired, but anything coming out of a 25mm at near 4000 fps would really have to be spinning.

 

[12Bravo20]

The 25mm M791 armor piercing round works on the same principle as the 120mm sabot round. It is just a big dart.

Here is a photo of the M791 sabot round.

 

[rust collector]

The fins seem small and likely work more to keep the projectile properly oriented through drag rather than rotation such as arrow fletching does. More tech details here: https://www.globalsecurity.org/military/systems/munitions/m919.htm

 

[mcb]

Its not fin guided in the sense that it is being steered in flight but it is rather fin stabilized. A twist fast enough to stabilize a projectile as long and thin as these long-rod high-density penetrators would be way too fast for standard AP and HEAT rounds these medium caliber cannons also use. The long rod pentrators may actually be so long and thin that it would not be possible to spin them fast enough without other issues causing problems. So spin or no spin that type of projectile require the fins to be a stabile projectile.

 

[Jim Watson]

On the other hand, there are HEAT shells with ball bearing bearing bands so the shaped charge does not rotate in a rifled barrel.

 

[roscoe]

Are the fins making them more accurate? More accurate than other sabot loads? I recognize that they are much longer than regular sabots.

I do know that, at least in 30-30, the sabot loads are definitely NOT accurate.

 

[Jim Watson]

The fins are the only thing providing stability and accuracy, the modern tank gun is smoothbore.

Commercial varmint loads are not in the same league.

 

[Mk-211]

Here's some real 50 BMG sabots, aftermarket is worthless junk.



What makes these superior is the way they're built. If you look at the rear of the projectile, you can see a disk that holds it in place.

As the sabot travels down the barrel, it unscrews and releases. You then have a .35 caliber tungsten penatrator going down range.

 

[lysanderxiii]

I was reading some news and there was discussion on a thread where the above-mentioned round was mentioned. I am aware how this works in a smoothbore gun (like the M1's 120mm), but apparently they are used in rifled guns - it is one of the rounds available for the the 25mm on the Bradley.

I get the concept of the sabot (I still have some old 30-30 'Accelerator' rounds). But how does it work with a fin-guided round? Wouldn't the spin tend to offset the intent of the fins? A little spin is apparently desired, but anything coming out of a 25mm at near 4000 fps would really have to be spinning.

With the 25mm ADFSDS rounds, the rotating band is plastic teflon and not cemented to the sabot. So, the rifling engages the rotating band, but the torque is not transmitted to the sabot, so it does not rotate, all the rotating band does is seal the gap between the bore and the projectile. There are many rifled auto cannon that fire ADFSDS rounds, and they all use this method of disconnecting the rifling from the projectile.

Here's some real 50 BMG sabots, aftermarket is worthless junk.

View attachment 1135472

What makes these superior is the way they're built. If you look at the rear of the projectile, you can see a disk that holds it in place.

As the sabot travels down the barrel, it unscrews and releases. You then have a .35 caliber tungsten penatrator going down range.

No, it does not.

The disk in the back is to prevent blow through. The inertia of the penetrator would punch through the base of the sabot, the solution was to increase the contact area for the plastic with a steel disk. Other than the area multiplier, they work exactly like the old Remington Accelerators (Remington was involved with the development.)

 

[Mk-211]

Here's a better picture.

 

[Jim Watson]

Oh, yeah, the Serbu Buster ammo.

 

[CapnMac]

The physics eggheads sorted out, that at AT velocities, what was wanted was a rod penetrator of 1:15 to 1:18 proportions.

Now to stay on course, the thrust vector has to remain in the center third of the rod. So, you want enough fin to keep the rod on axis that much.

The 25mm rounds are not optimized to that level so they can be shorter gaining mass, whic hcan "make up" for not having quite as uch velocity.

The tank killer rounds are running in the 4000fps range at the muzzle, so when the rod penetraor touches the armor the conversion of potential energy to kinetic energy creates a state change in both materials. The length of the rod helps "propagate" the state change and keep the effect advancing through the struck plate. Conservation of momentum and all that.