Sounds interesting. Basing is pretty dull as it is IMO. Different bullet types would add some variation.
However, i don't think this idea would work well in spar; all i want to do in spar is focus on dodging, not constantly switching ammo types because i've run out of one, everyone would just buy the 'best' ammo type for spar; it's a small arena, tactics are limited to dodging, positioning, and shooting in a timely manner, varying bullet types would just be an annoyance, but that's my opinion.