I want one of these (or three) for my Russian battlegroup

http://www.missing-lynx.com/gallery/russia/kosmomolets35sz_1.html

Luckily with the recent release of the main unit, this should be relatively easy, with a bit of artistic licence that is. Would look great one the gaming table methinks .

I was going to say it looks like it would roll over backwards when it fired, but I've just seen the rear view showing support struts. 

See also the SU-76P for a similarly interesting piece of Russian kit.

Have found myself idly wondering if I had the skills to do a conversion of a Stug III into an SU-76i ... but decided not

http://tankarchives.blogspot.co.uk/2013/12/su-76i.html

If you want a SU-76i start with the Pnz III, which is what the Russians did.