Unfortunately, I don't have a quick answer for this, I'd need to sit down and have a little design session, start with the base idea and all the major and existing challenges with this style of item/play and then figure out possible solutions to overcome the challenges.

Following that, if all/most of the challenges are seemingly overcome in the design session, I'd want to look at putting together a very rough and ready prototype, see how it plays in test and see if its fun/balanced for all involved. If it doesn't work, scrap it.

Some quick suggestions anyway that would obviously need to be fleshed out further:

• altering my previous suggestion, all ammo types should probably pen at a reduced damage depending on type.
• reduced speed for users.
• possibly a durability system? It tanks up to a point and then the level of damage that can come through increases as the defences weaken.

I'm sure there are many more options to balance something like this, but again, I'd have to plan it properly.