Artillery requirements for jungle warfare are about the least well understood military problems even today, and particualalry for anyone hailing from North america or western europe. its a safe generalization to say that in the main, people from those parts have basolutely no idea of what they are talking about. The overweaning temptation is look for bigger, heavier, meaner guns at every step. That is absolutely the wrong way to approach the problem. even when we do get some appreciation that we have to unlearn our desire for ever more firepower and meannes, we still tend to fall into the trap of not wanting to compromise on firepower too much. Thats only a slightly less dangerous hole to fall into. in general, western equipment for the jungle during the war was far too heavy and unwieldy to be considered ideal.
The Prussian military theorist Carl von Clausewitz said that artillery is the weapon an army can least do without...and nowhere is that more true than in the jungle.
The Australians along the Kokoda track did entirely without artillery except for a few days from 21 September 1942. When the Japanese were on Ioribaiwa Ridge, at the limit of their advance, two 25lb guns of 14th Field Regiment were dragged up to Owers' Corner from where they were able to fire on the enemy. They gave crucial fire support in the battle, and immediately outgunned and outranged everything else on the battlefield. but after the main battle, the Australaians again found themselves fighting without artillery support, as the 25 pounders they had were too heavy to manhandle into and through the Owen Stanleys.
In 1943, the Australians introduced the the 25 pounder short, and married it to the ubiquitous jeep so as to give some much needed firepower, but the system still remained overly heavy and cumbersome.
The Japanese experience of war in China in the 1930s had taught them that in remote road-less regions the only artillery they would have was what they carried with them. On first landing in Papua they had 17 artillery pieces. These were of three types; 75mm mountain guns, 70mm infantry guns and 37mm guns which could fire an anti-tank or an anti-personnel round. All three could be taken apart and carried by horse or man. When the Japanese advanced into the Owen Stanley Range the carrying of the guns and their ammunition had to be done by men alone. One fifth of their force was needed to shoulder the burden of the disassembled guns and several thousand rounds of ammunition.
The great labour involved was, in the first half of the campaign, rewarded by victory in battle, time after time. The Australians found themselves at a severe disadvantage without artillery support. The Japanese artillery had several times the range of any Australian weapon but in jungle war the gunners usually cannot see the target.
This problem was solved by forward observers. These men advanced with their infantry until they could see the Australians then, by field telephone, directed the fire of their artillery on to the target.
However, in the jungle, transport is difficult, and having artillery that is heavy or requiring large numbers of men to transport is simply untenable. This makes all the heavier German artillery weapons, even their lighter mountain guns, basically not ideal in any remote jungle situation. German guns are notable for their accuracy and stability, which they achieve in part by their relatively heavy weight.
British Guns are similarly handicapped. The Australians found the 25 pdr QF guns difficult to position and use in the jungle because of this, and very manpower intensive. They developed a partial solution in the 25 pdr short. Still heavy and too buly for jungle conditions, but an improvement.
The best weapon in the jungle IMO proved to be the lowly Japanese 75mm mountain guns. Easily broken down into manp portable loads, simple in the extreme, but able to pack a useful punch. There were two 75mm mountain guns that I know of, the Type 91 dating back to 1909, and the Type 94 dating back to 1934.
On the last day at Isurava, six Japanese guns were engaged and at Ioribaiwa there were eight, including their three most powerful ones, 75mm mountain guns. The greatest concentration of Japanese guns during the Kokoda phase of the fighting in Papua was at Oivi-Gorari where 13 were in action. In the disaster that overtook the Japanese there, all were lost.
The Japanese 70mm Infantry gun was extremely useful. Far better than mortars, because of its range, idirect fire capability, limited ATG capabilities and easy transport, they were greatly feared.