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nice footage of the M12
991st Field Artillery Bn and the M12 155mm SP Gun
imagine that beast in a direct fire role..
I read of two accounts where that happened. One on Pelelui when the 7th Marines were trying to take a ridge. Tried once and we repulsed. Brought in a ton of spotters and picked out the Japanese positions. Then, they unlimbered some 155s at ranges of around 800 yds and blew the hell out of the ridge. Said the blasts were so fierce there was schrapnel landing in the gun pits.
Another was going through the Sigfried Line. An infantry unit was repulsed in their attack and trying to figure a way through when an SP155 rolled up to the situation. The LT in charge asked if they needed and help and proceeded to blast bunker after bunker with this thing (even let some of the infantry pull the lanyard). Even if the round didn't penetrate, it scrambled the brains of the Germans inside. After they'd shot up everything they could see, the Artillery limbered up and clattered away. The infantry were very thankful.
If you want the most effective in making life miserable for the other side/make enemy generals say "Boy, I wish I had that"/step towards the future, I'd go with the US SP 155 Gun. Not the howitzer, the rifle. That thing was murder. And, very mobile. Better than the Soviet 152 Mobile in that it was a rifle and not a howitzer type.
A recoilless gun is a flat trajectory weapon that often cannot fire indirect. Given that jungle is such closed terrain this places a severe limitation on range of a recoilless gun, and often prevents its use altogether. A howitzer is a far more useful weapon in the jungle.
The US possessed a 75 Pack Howitzer, which was useful, but still required a mule or two, or at least a lot of men to move it. The CW had a shortened 25 pounder, which was even worse when it came to transport. The little 70mm was superior to a mortar, far more mobile than its allied counterparts, and moveable by a relatively small number of men (about 6 from memory). It was a high trajectory weapon, meaning it could be used to deliever plunging fire over the top of the tree canopy.
The only weapon the allies possessed with all of these qualities were mortars, and these were badly outclassed insofar as range was concerned