Beautiful piece of work, I am not sure but is looks like you´ve largely built up the gun yourself, the double gun shield and the wheels don´t look like they´re part of the 40k range. The crew looks great as well, clean colors and especially the epeaulettes (spelling?) look great. The base is the only weak point, it´s quite big and extremely barren, so sandbags, ammo or similar would have made it more interesting.