The recent thread about reverse engineering WWII German equipment reminded me of something I wondered about before.
During WWII there were plenty of anti-tank planes about on both sides. They were armed with cannon that, from the books I’ve read, did a pretty good job of peeling open tanks. Would it have been a.) feasible and b.) practical to mount these on armoured cars or other tanks and use them as tank killers?
The important thing about WW2 antitank planes is they’re shooting down from the sky onto the top of armoured vehicles, where tank armour tends to be a lot weaker - tanks are mainly armoured to take frontal hits, then the side, with the rear and top generally being weakest.
Also the planes can maneuver so quickly that the tank cant practically engage it.
Both those advantages go away if you put the guns on to an armoured car instead.
Otara
During the Post-WW2/Cold War era, there really was experimentation with putting anti-tank weapons on light armored vehicles.
An armored car was armed with a battery of Recoiless Rifles, in an array around the turret.
This was not pursued, but only because RRs were phased out as primary anti-tank weapons.
In the Reagan 80’s, modified APCs were mounted with wire-guided anti-tank missiles, to much greater effect.
The modern Bradley AFV, a successor to the APCs, carries anti-tank missiles externally.
There is, currently, a smorgasbord of armored vehicles that do carry antitank weaponry, usually in the form of antitank guided missiles (ATGMs). IIRC, there were some dedicated squads in WWII that carried portable ATGMs as well. They were light, mobile, and easily hidden from their large, hunkering enemy targets. The only problem with these (from what an older gentleman was telling me) is that the ‘bazookas’ were so light, they didn’t carry enough punch to knock out a tank with a single hit, so sometimes they’d have to hit a German tank twice or three times to be effective.
But aircraft were much more effective antitank platforms. This gave birth to the modern A-10 Warthog.
Tripler
Dude, trust me. Warthogs are cool.
You mean in addition to the tanks main cannon (which is primarily designed for killing tanks)? 
Some were and some weren’t. Especially in the early war, a fair few AFVs were dedicated infantry-support vehicles and carried short-barrelled 75s that lobbed HE shells quite handily but, in some cases, weren’t even chambered for AP, or had too low a muzzle velocity to be all that effective even if they did. And then at the other end of the scale you had our Matilda, which had a 2-pounder main gun that was primarily effective only against armour - it couldn’t fire HE (and in any case 40mm HE wasn’t likely to scare anyone), and while a 40mm shell will ruin your day if it hits you, a near-miss won’t do dick.
There were even a few tanks about the place that were armed only with machine-guns - fine for infantry support, but not likely to worry any kind of armour.
A few halftracks, armoured cars and flat-bed AFVs were tricked out with anti-tank guns. They were more mobile than anti-tank artillery but that was what they were mainly for - not to go looking for trouble but to serve as mobile defensive positions (they didn’t have the armour to slug it out with tanks in a protracted fight, and weren’t meant to).
I think the OP is talking more about quick firing lower calibre cannon planes carried, based on the misunderstanding that they were equally effective against tanks as the slower firing higher calibre weapons they tend to use.
As I poorly explained, they generally werent - they didnt have to be because the angle they were firing from meant they had to penetrate much weaker armour than the armour that ground vehicles are likely to encounter.
Also ammunition supply is much less of an issue with a plane as it has to land and refuel much more often than a tank anyhow.
Otara
Note that rockets made ot like gangbusters as anti-tank weapons on WW2 Aircraft.
More accurate than bombs, carrying more explosives than most/all of the aircraft cannon, they could mess up a tanker’s whole day with one pass.
I should have expanded somewhat on the subject. I meant instead of, perhaps letting the tank have thicker armour in place of the space saved (with less crew and a smaller gun) or perhaps have a faster moving vehicle.
It always made me wonder when I read through all the WWII RAF books my grandfather left to me, but then he wasn’t into the ground war much, so I didn’t read anything about tanks 
Thanks for the answers all.
No, you were pretty much on the right lines - some “tank destroyers” did indeed sacrifice some armour, maybe being open-topped and in some cases having no rear armour, mostly in the interests of making it easier to have a higher rate of fire. There were some German AFVs about the place in 1940 that had a rapid-firing 3.7cm gun and minimal armour - about halftrack standard, proof against small arms but not much else. Used correctly, they could worry the heavier French tanks. But as stated, most of the value of the aircraft AT weapons was that they could be fired against the more vulnerable roof armour, and didn’t have to engage the thickest (and possibly sloped) armour at the front of the tank.
I couple of years ago I read a detailed article by somebody who had done research on aircraft effectiveness against tanks during WWII, and according to the statistics, aircrafts were in fact not very effective against tanks, when calculating the number of antitank aircrafts, attacks on tanks by antitank aircrafts and knocked-out tanks by aircrafts. The figures were in fact surprisingly poor, which could explain the lack of interest of implementing aircraft carried antitank weapons on vehicles. But since I can’t remember the source, you might as well dismiss these two cents.