There are many ways in which to knock down your poly count a bit. If the program you are using is like Milkshape, there might be a command to reduce the number of polies on your model. If it doesn't, there are a few steps that you can use next time you model, or even try to put into this model if you want to edit it.
1) When making a barrel or whatever else that is round, use as few sections as possible and try to keep the pieces (this defines whether it is a hexagon or an octogon) to a minimum.
2) Whenever possible, use squares and simple objects like pyramids and such to make your model.
3) Textures work wonders at showing detail. If you follow the above step, you can make your model look completely round even if it is one giant block!
BRSSEB has some great tutorials on how to make weapons. I believe he also has some tips on keeping your poly count low, but I haven't read it in a while so I'm not sure. Anyways, check it out at the following site!
http://ofp.info/brsseb/BUNTRO :beat: