Pros of lightweight BCG:
* Less recoil from reciprocating components
* Faster rate of fire
* Less gas required for functioning, which means you can reduce the adj gas block opening, allowing your muzzle brake to be more effective at reducing recoil, and also less carbon fouling/less frequently needed cleaning
* Overall rifle is lighter, allowing you to carry your gear for a longer duration of time before fatigue sets in, or to carry more gear for the same time
* When firing unsupported, lower weight allows you to remain on target for longer durations before fatigue sets in
Cons of lightweight BCG:
* Lower mass BCG has lower momentum, which increases risk of bolt not closing all the way when there is extra friction, such as releasing the charging handle with the BCG, or not pulling the charging handle all the way back, or too much fouling in the chamber/not enough lube, or cold weather conditions (stronger recoil spring may help with that, although that increase in velocity can potentially cause other issues as well)
* Higher velocity BCG increases risk of slam fires (titanium firing pin supposed to help with that)
* Faster unlock time, which in theory means higher chamber pressure and more port pop when used with suppressor
* If BCG is moving too fast, there is potential for outpacing the magazine spring, resulting in failures to feed (this is unlikely to occur from simply using a lightweight BCG, it would only occur if you are doing a lot of other race gun modifications in combination)
In my opinion, the pros of a lightweight BCG far outweigh the cons, with the potential exception of when you want to make a very quiet suppressor host