Come to think of it, the 5mm round itself seems inherently overpowered when compared to 5.56mm - The 5mm is supposed to be lighter and lower powered round than the 5.56mm, based on it's weight - However a 5mm out from an Assault Carbine does more damage per round than 5.56mm out from an R-91 Assault Rifle and also the 5mm gets the -10 DT effect for free.
However, when we take a closer look at the crafting components for 5mm and 5.56mm, we find out something very strange - the 5mm round requires more lead and rifle powder than the 5.56mm and somehow manages to pack all that into lower ammo weight per round.. Weird.
IRL, there is no 5mm round exactly like the one in Fallout. Closest thing in existence for real would propably be the FN 5.7x28mm PDW round (used by the P-90), which has improved bodyarmor penetration but makes notably smaller wound channels when compared to 5.56x45mm.. In FNV terms the 5.7x28mm sacrifices base damage and gains a DT penalty versus bodyarmor when compared to 5.56x45mm, same should hold true for the 5mm round which seems quite similar in principle if we go by cartridge weight - But in the game it doesn't.
I think this all translates to two main problems - 5mm rounds take a bit too much resources (lead, powder) to handload - Common sense would suggest they should take a bit less than 5.56mm. And second, the R-91 (Infiltrators and Perforators included) and Chinese Assault Rifle weapons do too low base damage per round - They should be doing something like at least +20% to +30% more than 5mm rapid fire weapons. Might be worth considering if anyone is going to do a weapons rebalance mod for TTW.
@McWarlord: Yeah, it would be slightly overpowered. Not game breaking or anything like that, it might be difficult to feed the 5mm rapid fire weapon in the early game, running out of ammo might be a frequent occurrence. Instead of actually changing the R-91 to 5mm, IMHO it would be a better idea to simply place some 5mm Assault Carbines into the early game areas in poor condition.
Another ramification of R-91 5mm mod would be that the enemies using R-91s would easily get through players bodyarmor especially in the early game.