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question about parasitic mass   General Military Discussion

Started 7-Dec by smg762; 734 views.

From: smg762


whats the formula for working out how much energy is wasted by a sabot?

do you quite literally work out the muzzle energy of the sabot and then add it to the projectile?

so if one had a 20 grain bullet at 2000ft lbs, and a 10 grain sabot, would the rifle need a total of 3000 ft lbs?

my other question is that apparently tank sabots weigh almost as much as the projectile. this seems inneficient. surely they could use a soviet 'ring' sabot which weighs far less

my final question is unrelated.....does bullet weight affect recoil?

would a 30 grain .556 have less recoil than a 70 grain? by a long way?

  • Edited 07 December 2021 12:24  by  smg762
In reply toRe: msg 1

From: roguetechie


Ok so the way sabot parasitic mass works is you are imparting muzzle energy equally to the sabot and projectile so your muzzle energy is divided over the weight of the sabot + projectile weight. So if your sabot weighs 50% of your total launch mass, yes you're losing HALF of your muzzle energy.

When it comes to tank gun sabots they do go to a lot of trouble to minimize the weight of parasitic mass but you are also flinging a fuck off huge and very high length to diameter ratio dart that's a pretty tiny fraction of the bore diameter. To a degree you're just going to kinda need a sizeable amount of sabot to physically obturate the bore etc.

And yes bullet weight does affect recoil, this is another part of why the whole small caliber high velocity approach to small arms was pretty revolutionary.

The reason why this is the way it is has a lot to do with both the free recoil equation and the muzzle energy equations. 

I don't remember the exact relationship between launch mass and velocity to recoil/muzzle energy but the short version is essentially that you're better off throwing a smaller projectile at much higher velocities than a heavy projectile at much more modest velocities from a recoil perspective.


From: smg762


right. but dont you think they could use a soviet ring sabot which is half the size and weight of a western one.

in practical terms, would a 30grain 556 offer much FELT reduction than a 60 grain?

In reply toRe: msg 3

From: RovingPedant


Russian penetrator darts are a little different to western penetrators. That might preclude a ring with tails riding the bore.

Maybe accuracy, maybe aerodynamic drag, maybe something else?

In reply toRe: msg 3

From: roguetechie


Yes, a 30 grain 5.56 would offer less felt recoil even going substantially faster.

Where you run into issues going with bullets that small is that you EITHER have enough fragmentation mass for terminal performance in humans OR enough penetrator to punch intermediate barriers and etc but not both.

It's sort of a catch 22 that sidewinder explored heavily in the early hellcat designs with the 37 grain 3500 fps round that was meant to run at 1500rpm.

The issue he ran into was the one I listed above. You could make a killy bullet for it or one that had satisfactory penetration but not both simultaneously.


Recoil is something that we have multiple extremely good options to deal with that you can even stack onto the same platform. Between a good brake/smuzzle and sidewinder's simple stock tube recoil mitigation system you can get felt recoil in an 8 pound all up including can optic and laser platform down from somewhere between 7.62x39 and 7.62 NATO unaided felt recoil down to just a bit more than 5.56 with Nato birdcage level of felt recoil.

Even running above 600 rpm that's well within the controllable range assuming your gun has decent ergos.

On the Soviet style ring sabots thing, what I'm going to say is that American Abrams style sabots are actually very competitive these days in total parasitic mass  % while allowing you to have a dart that the base of all but sits on top of the primer cup it's so long.

In the world of tank guns, what we currently have is pretty near the best compromise you're going to find at a very reasonable weight. Especially when you understand that tank guns blew past the 100k psi peak pressure level decades ago.

Parasitic mass is only one part of the very complicated equation that is what makes a good modern tank killer round, which is so esoteric and so competitive that basically none of the real equations for any of it are unclassified and the modelling involving demarre etc are decades behind what actual militaries use when designing and evaluating modern tank gun ammo.

To a large degree what we have in the public sphere are very educated guesses and standardized equations that should at least in theory come up with consistently wrong answers for all current generation apfsds rounds.

Being utterly brutally honest, there's far more we know we don't know about modern apfsds than what we even think we might have a handle on in the public sphere.

There's also the fact that modern tank gun apfsds must be cross optimized to have at least nominally satisfactory performance across a whole range of armor and era combinations each of which can perform and respond to a hit wildly differently.

When you actually look at the wild variation in even the theoretical ways different nera arrays alone perform, it's kinda a fucking miracle that we don't have to load tanks with rounds specifically designed for what tanks they're expecting to face.


From: smg762


Just to clarify, cbj ammo has a 31Grain bullet at 600ft lbs. The Sabot is 8 grain so what would the total energy be with the sabot.

Likewise the SPEW darts were 10grain with a 10grain sabot. They had 550lbs energy so what would the total energy be that the cartridge was kicking out? 

Lastly my own design has a 27grain bullet at 1500ft lbs....with a 5grain Sabot what would it be?

And would the recoil of a 30grain 556 be drastically lower? Enough to make full auto feel like an MP7?

  • Edited 27 December 2021 17:58  by  smg762