It has to do with mass. The heavier the bolt the easier it can pick up a round (especially in adverse conditions).
You also might have to "tune" the gas block (most go adjustable) and buffer/ buffer spring to make up for the lightweight BCG (not really worth it for the few ounces you'd save). Thus why you mainly see lightweight BCG on 3gun/ competion builds, where a high rate of fire in semi-auto comes into play. Now there are some people that have no problem with running a lightweight BCG in a defensive AR15, but it's definitely a minority.
If I was to go the lightweight route... I'd probably be looking at the JP Enterprises Low Mass Operating System (LMOS):
https://jprifles.com/1.4.7_LMOS.php
From the JP site:
Quote:
For the no-compromise competition rifle, our Low Mass Operating System (LMOS?) offers the most significant improvement in the felt impulse of the rifle compared to any modification other than the addition of a compensator. By reducing the reciprocating mass within the operating system, the bolt velocity is increased while simultaneously reducing the overall impulse experienced by the shooter with each shot. Consequently, LMOS? equipped rifles have virtually no reciprocating mass feel and move less during cycling, allowing for much faster sight recovery and follow-up shots.
While use of the LMOS? components is recommended only for competition and sporting use, this is only precautionary as their reliability is excellent, particularly on rifles with adjustable gas systems. Like all our bolt carrier designs, LMOS? carriers feature a 100% increase in bearing surface for smoother operation, improved alignment and lower wear in the upper receiver as well as the longer Mil-spec. cocking pad for optimal reliability. In fact, Team JP shooters have been using the LMOS? system for over a decade without any reported operating system malfunctions at a match, and most every shooter that implements this operating system feel that it is proven to be a great performance improvement. Some shooters have even reported fewer unexplained flyers in their groups after switching to the LMOS? system.
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*Edit: Forgot to mention the flash hider vs. muzzle brake... The fireball/ blast from a muzzle brake in low-light, or night can completely obstruct what you are shooting at. Another obvious reason is that the fireball/ blast basically lets anyone who can see it, know exactly where you are. Lastly, the concussive nature of most muzzle brakes are extremely brutal.