Yes, that's a larger diameter than an anti tank rifle, which usually was between 12.7mm and 20mm. But compared to, say, a 20mm Lahti L39, this most likely uses a lot less propellant in the cartridge. A lot. Thus less muzzle energy, less recoil, less injuries.

For comparison, Rock Island had a four gauge and a .950 JDJ (that's ~24mm) in one of their auctions, and they took them out for some shooting. Here's the clip: link. And yes, that's Ian from FW in the background.

Bloody hell, .905 is what, 23mm? Better put some shock absorbing butt plate on that sucker or it'll split you in half.

Kinda reminds me of that old Finnish AT rifle, the Lahti.