20mm antitank rifles with HEAT rounds?

[Ulzgoroth]

As others have noted, small caliber high speed rounds and HEAT are a terrible combination.

Originally Posted by Glimmerman

I see. Bazookas' backblasts need bigger sniper nest and shots are more easier to see. So 20mm sniper rifle with AP rounds works against armored cars, but not against tanks.

By TL8 and probably even the latter parts of TL7, you could have a LAW with a confined-space safe countermass rather than a shooter-cooking backblast. Still probably going to be more obvious about what you're doing, though.

The WWII British PIAT anti-tank launcher took an interesting alternate approach - it was spring-loaded. No backblast, no flash, and not much noise. Of course, you don't get a lot of range that way.

As for what a 20mm rifle can do, it depends on what tank, what angle you can get, and how close you can shoot from. A good 20mm AP round probably could penetrate most WWII medium tanks if fired into the sides at close range, almost all of them if fired into the lower sides (behind the treads) which were sometimes flatter and thinner on the better-protected machines, and could probably penetrate any operational tank in the war if fired down from above into the rear deck. That last, obviously, is rather hard to arrange...

EDIT: It's probably not going to do anything to any MBT that a regular-caliber rifle wouldn't do just as well. You could rattle the hull a little bit or try to snipe unbuttoned crew, but trying to penetrate is a waste of time.

[johndallman]

Originally Posted by Ulzgoroth

The WWII British PIAT anti-tank launcher took an interesting alternate approach - it was spring-loaded. No backblast, no flash, and not much noise. Of course, you don't get a lot of range that way.

There was a propelling charge in the base of the projectile. The spring basically just got it far enough from the operator that he wouldn't be hurt when it went off. It was a "spigot mortar" rather than a spring-gun.

[Ulzgoroth]

Originally Posted by johndallman

There was a propelling charge in the base of the projectile. The spring basically just got it far enough from the operator that he wouldn't be hurt when it went off. It was a "spigot mortar" rather than a spring-gun.

You're right, the spring wasn't the propulsion...but I don't see your attributed role attested. It looks like the spring was just a (rather oddly heavy) firing pin mechanism.

(In a spigot mortar, the propelling charge is fired while attached to the launcher almost exactly like an ordinary gun mechanism, except that the tube is part of the round instead of being part of the launcher.)

[Anthony]

Originally Posted by Ulzgoroth

By TL8 and probably even the latter parts of TL7, you could have a LAW with a confined-space safe countermass rather than a shooter-cooking backblast.

You only need it recoilless if the round is heavy enough to cause a problem. While I'm not finding a HEAT grenade at the moment, the M443 HEDP grenade exists (though it seems to have safety issues) and can be fired from an M203 grenade launcher. Not that 2" penetration in RHA will do much to a current tank, but it's still the equivalent of about 8d(5) in GURPS.

[Fred Brackin]

Originally Posted by Anthony

. Not that 2" penetration in RHA will do much to a current tank, but it's still the equivalent of about 8d(5) in GURPS.

DR 140 on _frontal_ armor in modern tanks may be unknown but most armor on all other facings is lower than that. See the T-72 in High Tech among others.

[Ulzgoroth]

Originally Posted by Fred Brackin

DR 140 on _frontal_ armor in modern tanks may be unknown but most armor on all other facings is lower than that. See the T-72 in High Tech among others.

Current (main battle) tanks in the sense of ones that are up to date commonly have ERA on the sides, which will probably be enough to settle that.

Current tanks in the sense of ones that you can find in service with less-fortunate militaries around the world...yeah, not necessarily.

[sir_pudding]

Originally Posted by Ulzgoroth

Current (main battle) tanks in the sense of ones that are up to date commonly have ERA on the sides, which will probably be enough to settle that.

Current tanks in the sense of ones that you can find in service with less-fortunate militaries around the world...yeah, not necessarily.

The M1A1 doesn't need or use reactive armor and is probably not to be grouped in the less fortunate category...

[Ulzgoroth]

Originally Posted by sir_pudding

The M1A1 doesn't need or use reactive armor and is probably not to be grouped in the less fortunate category...

The TUSK kit adds reactive armor to the sides.

I don't know whether or not the side armor without that would be sufficient to stop those grenades. I don't know of estimates for Abrams side armor performance.

[clu2415]

The penetration of a HEAT warhead is directly related to the diameter of the warhead. If you look at the panzerfaust or RPG-7, you'll see the bulge to make room for a higher diameter.

When you fire ammunition containing explosives out of a rifled barrel, you have to design the projectile with a thick shell to stand up to the stresses of spinning induced by the rifling. Higher the muzzle velocity, the faster the spin and the more shell you have to add, which eats into your space for explosives.

When you combine an already small diameter projectile with the need to thicken the projectile walls, you end up with a very small HEAT mechanism. I suspect that the two factors combined are enough to make it impractical.

The 40mm HEDP (HEAT with extra fragmentation) can work because it's lobbed at low velocity (therefore needs minimum extra shell) and it has enough diameter to make a potent HEAT warhead.

[Fred Brackin]

Originally Posted by Ulzgoroth

I don't know of estimates for Abrams side armor performance.

I do. They're in Ve2. The "TL7 MBT" there has been explicitly identified as the M1A3.

The DR happen to be exactly DR140 and laminate for the sides and rear. I did not remember the laminate part. So they just barely stop 8D(10).

The top (including turret) and undersides as well as the treads are all less than that and not laminate either.