Thinking WW2 era and around those.
Instead of breech loading, why not make it like AA guns and have like 5 round magazines? I know in real tanks there’s a definite preference between autoloaders and manual loaders, but it seems like especially for the smaller 37mm and 57mm AT guns semi-auto would work fairly well.
My very minimally informed guess.
A semi auto uses some of the force of the round to actuate the munition cycling. This would decrease the velocity of the round. Basic armour piercing rounds favour high velocity.
Manual versus auto loaders do no have a negative or positive effect on the velocity of the round. Just how they are delivered and loaded. More human action versus complex machinery.
There may be reliability issues as well. But I have no information on that. A larger round does require more energy from the discharge energy of the round to cycle through a semi or full auto mechanism. There is a 57 mm Bofors naval gun that operates in auto mode. Seems it is just a naval gun?
My guess: the recoil in these weapons is so great that you have to re-aim before every shot anyway, and aiming process is so long that there’s more than enough time for another crewmember to reload the gun while doing it. A magazine wouldn’t make the process any faster and just add more complexity.
Autoloaders as they are understood today for tank guns of up to 125mm didn’t exist in WWII. The AA guns you are describing generally ran from 20mm to 40mm and were fully automatic, not semi-automatic, the ubiquitous Bofors 40mm L/60 being about the upper limit in size for automatic AAA. These AA guns had three things you didn’t want in an AT gun: they were heavy for their caliber, they had a high silhouette, and were thus very difficult to conceal. As the war went on, guns in this caliber range very quickly ran into the problem of not being powerful enough to penetrate the armor of ever larger tanks being developed and deployed. The 37mm was a perfectly fine AT gun in 1939, but by 1941 when encountering Soviet T-34 and KV series tanks the Germans began referring to it as the “door knocker” due to its inability to penetrate either of these tanks even at very close range, something only partially addressed by the Stielgranate 41, literally a shaped charge stick grenade muzzle loaded into the barrel and propelled by firing a blank charge like a rifle grenade.
A bit ironically given the OP, the larger caliber AT guns developed as the war went on were often derived from larger caliber, breech loading, single shot AA guns. The 88mm Flak 36 AA gun which was pressed into service as an AT gun to deal with tanks was eventually adopted into a proper AT gun on a low silhouette carriage as the PaK 43, note how much lower to the ground the PaK 43 was and thus actually concealable, unlike the Flak 36 where concealability to ground targets wasn’t a consideration in its design.
Similarly, the 90mm gun used by the US M26 Pershing and the M36 tank destroyer began its life as the 90mm M1 AA gun, and the 85mm gun used by the Soviet T-34/85 and SU-85 tank destroyer was originally an AA gun, the 85 mm air defense gun M1939.
I don’t think there is much velocity shedding from an automatic weapon. If it is gas operated the gas port is near the muzzle and is still at full pressure when the round exits. If it is recoil operated the same thing has to happen. The breech is locked to the barrel as it recoils a short distance then it opens. Once again, the round has cleared the barrel.
The cycling action is very quick when it does occur. There are rifles that are designed to fire 2 rounds in such quick succession that they are both in the barrel at once.
I thought the Ukrainians had an armored car with a semi auto anti tank gun.
Vehicle-mounted autocannon are a different thing than a magazine-fed semiautomatic towed gun. A vehicle can devote a lot of space and weight for the magazine and the loader.
Thanks.
Using the terms semi-automatic and autoloader in the context of tank/anti-tank guns is probably going to lead to a lot of confusion, since for small arms, semi-automatic means the action of the firearm automatically loads the next round from the magazine into the chamber. Per wiki:
A semi-automatic firearm , also called a self-loading or autoloading firearm (fully automatic and selective fire firearms are also variations on self-loading firearms), is a repeating firearm whose action mechanism automatically loads a following round of cartridge into the chamber and prepares it for subsequent firing, but requires the shooter to manually actuate the trigger in order to discharge each shot. Typically, this involves the weapon’s action utilizing the excess energy released during the preceding shot (in the form of recoil or high-pressure gas expanding within the bore) to unlock and move the bolt, extracting and ejecting the spent cartridge case from the chamber, re-cocking the firing mechanism, and loading a new cartridge into the firing chamber, all without input from the user.
This is not what is happening with the autoloader of a tank. An autoloader for a tank (or artillery piece or naval gun) is a mechanical system that replaces human loaders to automate the labor of moving a shell from the magazine into the chamber. The action of the gun does not cause the next round to be automatically loaded, the machinery of the autoloader does. Again per wiki:
An autoloader or auto-loader is a mechanical aid or replacement for the personnel that load ammunition into crew-served weapons without being an integrated part of the gun itself. The term is generally only applied to larger weapons, such as naval weapons, tanks, and artillery; that would otherwise have a dedicated person or persons loading them.[1][2]
An autoloader extracts a shell and propellant charge from the ammunition storage rack/compartment and loads it into a magazine or belt, if the gun has one, or directly into the chamber of the gun if it does not. It often replaces a human loader. Automation can streamline and speed the loading process, resulting in a more effective design.
ETA: Even the word ‘magazine’ is being used differently in the context of the two. For small arms, the magazine is something holding rounds for the gun that is attached to the chamber and replaced with a fresh magazine when one is emptied. For tanks/artillery/naval guns, the magazine is the physical space where rounds for gun are stored in the vehicle or ship; it’s not something detachable.
Exactly.
Even in a magazine-fed cannon, the magazine is either permanently attached to the weapon, such as the 40mm Bofors anti-aircraft gun:
or a dedicated space in the vehicle, like the autoloader magazine of a tank:
The only type of magazine that makes sense in the context of a towed crew-served weapon is the first case: an open box or frame to hold a few rounds of ammo, served by a loader at one end and fed into the breech of the gun at the other.
But that’s extra weight for a weapon that has to be muscled into place and, beyond the narrow limits of the aiming screws of the mount, aimed by adjusting the placement of the trail itself. The only advantage would be the ability to fire a burst of rounds (up to the number of rounds in the magazine), but antitank guns are basically artillery: you still have to sight the fall of the shot and make aim adjustments.
Anti-aircraft guns are expected to spray shots in the general vicinity of their target, especially since the shells can detonate by proximity to the target (close is good enough); hence, magazine feed makes sense. But proximity detonation is useless against tank armor, so spray-n-pray is no way to kill a tank.
Vehicular antitank guns autoload in order to remove a loader from the vehicle’s crew, or because they’re light high rate of fire autocannon, more like heavy machineguns than tank killers. (And also useful against aircraft for the reasons mentioned above.)