20mm antitank rifles with HEAT rounds?
 

High-Tech (p. 170) mentions High-Explosive Antitank (HEAT) rounds which have minimum caliber 20mm and (10) armor divisor.

Did historical antitank rifles like Lahti L-39 or Type 97 have HEAT rounds available? Could they have those at TL7?

Howabout todays TL8 Anzio 20mm anti-materiel rifle - could it have HEAT rounds?

Re: 20mm antitank rifles with HEAT rounds?
 

HEAT rounds don't like rifles (they perform poorly if spun) and don't much benefit from high velocity; as a result, they're more common as rocket or recoilless munitions (see the bazooka and panzerfaust for early models).

Re: 20mm antitank rifles with HEAT rounds?
 

A 20mm HEAT wouldnt work.. or work very well!
Lower caliber AP is sabot instead!

Re: 20mm antitank rifles with HEAT rounds?
 

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.

Re: 20mm antitank rifles with HEAT rounds?
 

Note that the 20mm antimaterial rifle is a thing with marginal military value. Currently there are really only three weapons in this class, and two of them are modular weapons that are more typically deployed in another configuration (and one of those is a direct copy of the other too).

I was in a convoy of trucks that was hit with a 12.7mm antimaterial rifle. We didn't initially even notice.

Re: 20mm antitank rifles with HEAT rounds?
 

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

Re: 20mm antitank rifles with HEAT rounds?
 

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.

Re: 20mm antitank rifles with HEAT rounds?
 

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.

Re: 20mm antitank rifles with HEAT rounds?
 

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.)

Re: 20mm antitank rifles with HEAT rounds?
 

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.