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But in actual practice?
and 120mm mortar ammo can weigh 45kg for two rounds in a wooden crate. Not including fuses or propelling charges.
http://mirex-ds.com/products/ammunition/Ammunition Mortar Round 120mm High Explosive.pdf
64 rounds + 64 fuses + 76 propelling charges weighs 1557kg. You are not going very far without engines, horses or a LOT of men.
Germans might have shifted to dual 20mm AA guns a lot sooner, perhaps they thought the improved rate of fire of the M38 was just as good?
In any case the 30mm AA guns in use at the end of the war were the MK 103 guns and not the slower firing MK 101 guns. While the MK 101 may have been better than the Flak 38 you start getting into target effect vs weight of weapon/carriage.
One point of easy change was the 37mm AT gun, It was the first of it's generation (pretty much, first guns were issued in 1928 ) and as such had the poorest performance. As such it should have been replace the soonest. Russians stuck a 45mm barrel on the same carriage in 1937 and got roughly the same performance using standard ammo as the Pak 37 got with tungsten cored ammo. HE performance was better too
The story of the 37mm being kept for the MK III tank but with a larger turret ring is well known.
Germans do have the same problem as many other countries. The very rapid increase in the size of the armies made it very hard to change weapons as interruptions in production meant delays in issuing weapons to many troops.
I forget which but they used Czech weapons to almost totally equip either 10 or 20 divisions. From pistols to 15cm howitzers. Mucking about too much with basic weapons in the late 30s could mean several divisions sitting around waiting for weapons.
The FACT that the howitzer would be harder to hit with doesn't seem to enter into these calculations though.
Neither does the FACT that the German Pak 75 used a propelling charge about 28% the weight of the propelling charge in the Full AP round with a corresponding reduction in barrel wear.
While the 10.5 How was available earlier it was a much bigger gun and might have trouble fitting into a MK IV turret.
according to Tony Williams table. 75-77 MM CALIBRE CARTRIDGES the Germans used at least 12 different 75 mm rounds not including the 1891 left overs and assorted 77mm leftovers OR assorted captured guns (French 75 counted in with the 12 already), so some forethought might have helped. Having different ammo sometimes prevents mistakes though. Using different propelling charges in the same case for different guns might lead to even more confusion and accidents.