Another consideration, possibly the reason European defense contractors have the closest thing to the OP’s specification:
Against a peer-level opponent, firing an artillery piece gives an opponent with counter-battery radars a return to sender address. It’s feasible for the enemy force to use enough automation to return fire with their own artillery and kill the enemy artillery after it fires the first time.
Thus the need to fire as quickly as possible - and to use a remotely operated artillery device. It’s going to die to enemy shellfire regardless, it might as well have an empty ammunition magazine when it gets destroyed.
This would explain why the Swedish and German artillery linked above have only 10 round magazines.
Airburst munitions significantly predated cluster munitions and nations still developed cluster munitions. There’s also a newer problem with airburst artillery. Russia has fielded a new electronic warfare platform in the last few years that can detonate electrically fused detonators like those used in airburst munitions. It’s seen some use in Ukraine. Here’s a storyof Russia considering sales to Iran in 2015. Current airburst fuse designs will become increasingly ineffective if that capability becomes common.
Thermobaric warheads provide similar broad lethal areas. Russia’s put a lot of effort into developing thermobaric warheads for their multiple launch rocket systems since the end of the cold war. They’ve also fielded a thermobaric warhead for the RPG-7 launcher. I don’t recall if they have fielded them for their tube based artillery. Russia isn’t a party to the cluster munitions convention; they just like thermobaric warheads. I don’t recall seeing widespread development efforts or fielding from parties to the convention. ISTR warheads for “bunker busting” and a US mortar round but nothing yet for NATO standard tube artillery.
The US isn’t a party to the convention. There’s some self imposed limits that take us way down another path. The wikipedia entry on the convention covers allowed exceptions for parties pretty concisely.
NATO parties to the convention have been starting to lean towards procuring new systems like the PzH 2000 and Archer that let them fire rapidly enough to achieve multiple round simultaneous impact. I wouldn’t personally call that subtle. IMO it’s more like Nuke LaLoush in the Movie Bull Durham - " I want to bring the heater. Announce my presence with authority." … and then run away. Running away is really important in a world where counterbattery radars and fast digital fire control networks are becoming the norm. Survivability moves are called that for a reason.
There are breech loaded mortars that accept magazines for ground mount. There are also breech loaded mortars in vehicle mounts. A good crew with rounds prepared can drop a lot of rounds quickly from traditional muzzle loaded weapons. You start running into the same limits of gun heating, the sheer bulk of ammunition needed, and the problem that the effectiveness drops off pretty rapidly after the first round(s) land.
There’s recovery time, but there’s also employment. Machine guns are typically used something more akin to very long ranged shotguns than they are like bullet hoses, a-la Hollywood. 4-5 round bursts, etc…
But howitzers/guns aren’t used like that. They’re carefully aimed. So that re-aiming time is built into any kind of auto-loader scheme. And yeah, these sorts of things DO exist- the 5" gun on the *Burke *class destroyers fires 16-20 rounds a minute fully automatic. That’s roughly a round every four seconds, most of which I suspect is getting the gun back on target after each round.
That’s not the sort of thing you could pack into a M109-sized vehicle though, and the M109 crews get about four rounds a minute anyway (every 15 seconds), and that’s accounting for loading varying sized powder charges, etc… the naval 5" gun fires fixed ammunition, so there’s not as much versatility there in terms of shell size, range, etc…
Can I toss out the old Mk19 automatic grenade launcher, which fires a 40mm round at a cyclic rate of 400 rounds per minute? It’s no howitzer, but those grenades make a reasonable pop when they arrive on target and they can really spray.
Various naval guns of similar caliber have fired at rates of at least 45 rounds per minute per barrel, ie loading cycle of ~1.3 seconds. The Russian AK-130 is an example still carried on many ships. Some Western equivalents in that caliber range have disappeared (the widely used US Mk.45 is more like 20 rpm, Italian OTO 127 is close at around 40). Some OTO 76mm shipboard guns fire much faster but that’s more in the range in early to mid 20th century field artillery caliber than post WWII standard of 105-155mm.
Anyway you could put devices like the 40+ rpm naval guns on a big vehicle chassis. But they are heavy and complicated and need a lot of cooling water (typically high rate of fire naval guns use open circuit sea water cooling, then flush out the system with ship’s fresh water after firing is finished). The main need for such a high ROF is for aerial targets. For field artillery use the slower more limited burst capability of many recent gun-howitzers guns is good enough.
The British cruisers HMS Tiger, Lion and Blake all had 3" (75mm) twin mounts that could fire up to 90 rounds per barrel per minute - these were intended for AA but were limited in usefulness because of their fire arc.
The loading machinery looked like a bottle factory with all the rounds shuffling past on to the breech.
The biggest “machine gun” I think I’ve ever seen would have to be the British 1 pounder “pom-pom”. It’s literally a water-cooled Maxim gun scaled up to fire 37mm shells.
Well, I was wondering if it would be easier to lift one (complex) howitzer into a region and then have it do its own “fire for effect” rather than a bunch of M777s which would salvo fire.
Not an answer to the OP. But I just finished Alamein by Jon Latimer. In it he describes how the British 25 pounder howitzer had the same fearsome reputation among Germans that the German 88 anti tank gun did among the allies. The Germans were convinced that it was, as the OP described, a automatically loaded “machine gun” howitzer. POWs who asked to see it were amazed that it was in fact just a manually loaded regular artillery piece.
Multiple round simultaneous impact (MRSI) systems are the major route for enabling one gun to do that. They are even heavier mechanized elements where sling loading isn’t an option.
I also think in the OP you underestimate just how fast a well trained gun crew can pump out the rounds during fire for effect. Sustained fire takes time. The crew can prepare all the rounds in advance of firing the first round for a fire for effect, though. It doesn’t produce the same effect as an entire battery or MRSI since there’s still a bigger lag between rounds. It’s not 30 seconds between round impacts, though. It’s a time gap that lets a troop target throw themselves prone where they are. It’s not enough time to sprint to better cover in response to the first round.
It’s a little surprising that their explanation for that was some advanced weapon as opposed to just having more guns & ammo and tactics that emphasize using firepower rather than manpower.
Well, yes, but did the British accomplish that with having many guns, or training to fire quickly?
Reading about the war of 1812 and the battles between the American “super frigates” and the British, the guy who won had either trained his gun crews to perfection, or killed all the officers in his first broadside.
Yeah good training and tactics. The point the author was making, was although the British never matched the Germans in armored ground warfare (Alamein was basically a battering ram, which the British and commonwealth troops ground down weakened Germans and incredible cost,), they absolutely did match them, and beat them, in artillery and air warfare. Not just by having more guns and planes (though that helped), but actually with superior training and tactics.
To me that’s pretty understandable. Its much easier to accept that the enemy has better equipment than you, than to accept that they are “better at soldiering” than you.
A problem with this is that unlike ammunition for the gatling guns or main battle tank guns, the 155mm ammunition is not one round (i.e. the projectile combined with the propellant and primer) but is a separate shell, a bundle of propellant (which can be adjusted in size) and a separate primer (similar to a blank for a gun)
This means that the loading mechanism is more complicated and slower.
As previous posters have mentioned, the PzH2000 does have such a loading system. While a faster fire rate is certainly an advantage, the main advantage is that you need fewer soldiers to man it.
A well trained M109 crew can put out 3 shots within 15 seconds and a total of 6 below 1 minute. After that, effectivity of the artillery is much lower and danger of counter battery fire increases. That’s when you pack up and go to the next firing position.
… and then run away. Running away is really important in a world where counterbattery radars and fast digital fire control networks are becoming the norm. Survivability moves are called that for a reason.