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I'm not sure this is the right forum, but now I'll try.
I'm working (long term) on an ATL in the early 1940s and inspired by OTL Danish Army 20mm Madsen Autocannons mounted on MC sidecars I wan't to take the concept a step further - mounting a weapon similar to the OTL French 25mm ATG on the sidecar of a heavy MC. Incl. mounting the OTL 25mm weighed a little more than 400 kg, but what would the barrel incl. recoil mechanism weigh - 100 kg? A 12-13mm MG weighed 25 kg, a 20 mm autocannon about 50 kg - would 100 kg for a 25 mm and using it from a MC sidecar be realistic?
I don't wan't automatic function, just a simple sliding breech like on the OTL French gun. I guess a muzzle brake would be handy as would be a folding down spade to lessen recoil.
BTW the 20 mm Madsens on sidecars took out 11 armoured cars and two PzI in the brief clashes on the morning of the 9th of April 1940.
The OTL French 25 mm has often been ridiculed but apparently its performance was much similar to the German 37mm - ie effective vs most tanks in service until 1942.
In an early 1940s scenario with masses of relatively light tanks being thrown into battle I could imagine 25 mm ATGs on MCs as a very effective and cheap way to counter armoured movement.