Edit: Well Rhade, I think you have the right of it. I had seen the 75mm gun on the AT-1 before. However, most of the pictures I looked at had the 45mm gun I drew, so I went with that. I now have a version with both guns uploaded.
Well, nevertheless your AT-1 is too big. It's look like T-26 with different turret, not SPG. And 75 mm ? soviets never use that caliber on T-26, maybe you think about 76.2 mm gun. And that gun it's for SU versions. Again check your picture with page I link you, and you see what need to be fixed. Practice, practice and again practice, with every one of designs you get better.