btw the model has around 250 triangles. Its probably a bit high but its a very "round" gun...
250 tris? It's about 500 vertexes! I can't even imagine, where they can hide...
The round part is just an octagonal tube with 4 segments, it's 40 vertexes. The rest is just some boxes... It seems that you've lost some unoptimized vertexes inside the model.
It is a common weapon, it should be optimized as much as possible without loosing quality.