Also, Ogre intergration may change a lot of things. Or if you want to use Blender2Crystal, you will have a lot more headroom to work with.
On my PC with a 7900GT video card I can handle about 34,000 faces at 60fps.
Some people have not as fast computers that can’t even handle 20,000 let alone 20,000 at 60fps, so it’s totally up to you. There isn’t really a single answer for that question.
20k tris is probably a good guide line if you want it to play well on everyone’s computers. 426 quads is probably too high though, even if the gun will be in the player’s face the whole time. Here’s an AK model I made a while ago. I took the texture from a photograph, and it looks awful. The model is very high, too high, but see if you can decimate it down to the right amount of faces. It’ll be good practice.