Sa80a2 or M16a2?
If you mean the SA80 you could get a UGL, Susat (Eotech and ACOG are also being implemented), maybe custom make a rail system so you can have a pillar grip or if you can find one, one of those side mounted torches.
For the M16a2.... nothing, thats why i like it.
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Edit: unless you mean what brands of Sa80A2, I know a guy with a STAR SA80, seems like a decent weapon, the hop-up could do with getting sorted though. ICS have just brought one out, not sure on its performance but if it doesn't have a split gear box and is anything like there MP5's then it shou