Here’s an idea: Making AR-15 carriers with integrated gas keys. Of course, if you are just milling carriers out of round stock on a horizontal machine as in a military BCGs, then this would considerably increase the amount of material you need to hog away to create a carrier. However, if you are making the carriers from scratch yourself, as manufacturer Voodoo Innovations is, then this may make some sense. From their press release:

VooDoo Innovations’ NEW Integral Bolt Carriers are now available!

…

Our new Integral D.I. Carrier was designed from conception to improve the service life and durability of the direct impingement carrier. The VDI Integral DI Carrier eliminates the potential for carrier to key separation due to carrier key screws stripping, fracturing, or becoming un-staked. The new Integral carrier has a improved gas flow path with no obstructions or shelves for carbon to deposit and build on eliminating the potential for gas obstruction to seize the operation of your carrier. The 20% reduction in carrier weight in the Low Mass Carrier decreases the reciprocal mass reducing felt recoil and increasing the Tune-ability for every application.

Sincerely,

VooDoo Innovations

With a properly staked gas key, the problems the Integral Carrier is designed to solve should happen virtually never. However, I still feel the integrated bolt carrier is pretty slick, and I admit I am tempted to try it on my next rifle build.