When we increase that surface area and decrease the weight of the projectile (Flechette plus Sabot in this case) we get a substantial increase in velocity that is difficult to understand from a lead-based logic foundation.
The misunderstanding here is that the above statement is not true. The pressure vs velocity vs surface area relationship is the same regardless of the construction of the bullet/sabot/dart/what have you. There is no special lead-based physics that suddenly do not apply when we hurl plastic sabots. The sabots total area is the same regardless if it houses a 1oz .50 cal wasp waist slug, a .70 cal lead 1oz slug or the aforementioned subcaliber dart so we are not increasing surface area. The only thing here affecting the outcome is the weight of the projectile. If all other factors are equal, it is true that as weight goes down, the velocity will increase. But it is not a linear relationship, not in rifles, pistols or shotguns. Dividing the weight by two may only increase velocity by 20%, and halving it again will affect it even less, up to a point where the return is negligible vs the reduction in KE due to reduced mass.
Once you've reduced it to 80 grains, the only option is to add pressure, either by adding more powder, adding barrel length, or using more powerful powders. Again we run into a non-linear powder capacity vs velocity relationship, where the amount of powder burned skyrockets to achieve a relatively small increase in velocity.
This is in addition to other problems to this end of the solution.
That large sabot housing the flechette really eats into powder capacity, even with a 3.5" shell. The light weight sabot more than likely does not provide enough resistence to let all that powder burn efficiently, further reducing velocity gain from the additional powder.
Prior to this there was never a proposed dart diameter or weight.
You might be able to achieve enough pressure to obtain the spec velocity from the 12ga bore, but the pressures would require a completely redesigned hull and weapon.
My question is why a shotgun that can launch 438gr projectiles at 1760fps cannot launch an 80gr, .15 caliber dart at 4,000fps. Am I missing something here? Did the physics of pressure and velocity suddenly change and I didn't get the memo?
Can it? Yes. It is possible. At pressures any shotgun shell or shotgun can safely tolerate? Not likely. The little ACR could do it, yes, by throwing a dart 8 times lighter, using chamber pressures 6 times higher.
Am I nuts to think that said dart would be incapable of setting off reactive armor or shattering the gunsight on a tank?
A .223 or .308 can shatter the gunsight glass, obscuring vision. .308 AP can penetrate 120mm of plexiglass alone.
Reactive armor is designed to defend against chemical HE weapons, not KE weapons. The explosive used inside is as insensitive as possible to avoid premature detonation, and the plates are desinged not to react to small arms fire or shell fragments from grenades, bombs, artillery etc, all of which are capable of producing projectiles with the weight and velocity of the proposed dart. Even the APFSDS rounds are unlikely to set off most types of reactive armor, they just punch a hole in it.
Can it not penetrate the auxiliary drum tanks that many Russian Tanks use?
Yep. So can .223/.308 AP.
Can it not create one hell of a hydrostatic shock on a soft target?
Maybe, but flechette ammunition has time and time again been found wanting in the terminal ballistics dept when it comes to soft targets, whether the researchers subscribe to the theory of hydrostatic shock or not.
Can the 'fins' that stick out from it not make some pretty nasty incisions [on the deer]?
Possibly? How big of fins are we talking about ? If its a 0.15" diameter dart, just how much larger are the fins than the body of the dart? Say 3 times the diameter of the main shaft? Those are going to be pretty small fins. They are not as large as say the cutting blades of a broadhead arrow. If they don't shear off from striking the target at high velocity, they would probably add minimally to the cutting damage. Tissue damage from fins was never mentioned in any research of Flechette wounding mechanisms. Most of the data suggests that wounding was similar to conventional small bore ball rounds when the flechette tumbled and broke up. When it did not, wounding was far less significant. This was with lighter, higher velocity and weaker constructed flechettes, that are more likely to shear and tumble.
Furthermore, will it not shoot flatter, have a BC much closer to one, and carry more energy downrange?
Yes, Yes, Yes. But how will it transfer this energy to the target when it reaches it?
Switching gears again from deer to tanks, the most daunting problem for this new uber-round is not pressures, velocity mechanics, increased weapon bulk, accuracy, etc, but the fact that the round will not damage armor significantly better than what is already afield. Say we could somehow get the proposed ballistics from the round. Lets compare it to the already sub-par experimental Steyr AMR round.
Steyr AMR: .223, 308gn, 4750fps, SD 0.885, KE 15,473ft/lbs, PF 1463
Penetration: <40mm at 1000m
Uber-Slug: .150, 80gn, 4000fps, SD 0.508, KE 2843 ft/lbs, PF 320
Penetration: <<< Steyr AMR
The proposed projectile falls weigh behind in the categories that affect armor piercing, namely mass, sectional density, velocity. Its lower diameter is offset by its much lighter weight, so that wouldn't give it any increased performance. I included KE for those people who believe KE is the only factor by which bullet performance can be measure. IPSC PF is included for fun and those who love momentum. Anyway you measure it, the uber-slugs performance on armor will be vastly less than the already inadequate Steyr AMR. Also consider .50 Cal SLAP ammo can penetrate >75mm RHA out at 1500M, with no changes to the weapon other than the ammunition.
Take a jaunt into fantasy again, and say that the requisite velocity could be produced from a conventional shotgun. Now you've given the infantyman another weapon that is less accurate than a sniper rifle, causes inconsistent wound that are likely to be less severe than his battle rifle, and requires bulky ammunition that allows him to carry less rounds per load out. Be sure to let him know that it isn't selectfire, doesn't penetrate heavy armor or deliver an explosive payload. I'm sure grunts will be lining up to carry that weapon.
