Probably suggested before, but a good idea to balance the game a little is to make a spar where you are forced to use a temporary M4. Infact, I'd say that would be best as the kind of spar used during the GST. That way, the victorious gang is the one with the most skill, instead of the one with the most money.  

Well yes. This was mentioned before I think. Perhaps there is a special spar room which gives you a temporary weapon. Not all weapons should be featured in "insert gun name" only spar or it becomes like a "try before you buy" type. Which can hurt era's economy a bit. (possible understatement)

I'd say handgun, and m4 are good guns for these kinds of spars.