This is a remix of marcusmoles 1/100th scale early war British 2 pounder ATG. https://www.thingiverse.com/thing:2297522
Marcus's excellent file give you the option of a very detailed ATG, a simplified model (which is what I used) or a gun with a built in base.
I upscaled the model to 28mm (1/56 scale) and thickened the frontl gun field, as I found this quite fragile on my first print so added a plate on the back side of the shield.
I find printed thin gun barrels fragile so I removed the actual barrel, and created a 2mm void into the gun breech. This allows you to add a piece of 1mm (I/D) aluminium tube to act as the gun barrel. Its O/D is 1.6mm, so when glued in with some epoxy it created a snug fit in the recess.
If you prefer to print you barrels out, the gun barrel can be placed in easily using Tinkercad or a similar program.