...and why yours might be sh*t!
BCGs. There are hundreds on the market, most made by only a handful of machining companies (Toolcraft, Microbest, A.O. Precision, etc.) and then branded by their end retailer/providers. However, the mentality that they are "all the same" is both false, and potentially dangerous. Currently, a mil-spec bolt carrier group is comprised of a manganese phosphate coated chrome lined 8620 steel alloy bolt carrier (the big piece) and gas key, a 158Carpenter steel alloy bolt that has been magnetically particle inspected, shot peened, and high pressure tested, a hard chromed 8740 steel firing pin, grade 8 fasteners, and a 4340 hardened steel cam pin, and pixie dust....still paying attention?
These components make up the type of BCG that has been in military service for over 50 years, and are currently the ONLY types of BCGs that companies like Battlefield Vegas (renowned for their torture testing of BCGs in their full auto assemblies) swear by (specifically Daniel Defense, and LMT models). Proper parts, PROPER staking on the gas key, and making sure you have a FULL auto m16 BCG (NOT A SEMI AUTO/AR15 BCG) are critical to sustained performance you can rely on.
COATINGS: most of the coatings seen on modern BCGs stem from the oil/machining/aerospace industry. TiN (gold), NiB (silver), TiAln (grey), AlTin (deep black) coatings all increase surface lubricity and corrosion resistance. They were developed to to increase the lifespan of drill bits and other components that see a lot of use. However, these coatings, while aesthetically appealing, don't necessarily increase performance in a BCG. Incidentally, improper coating of the bolt face can cause issues with the lockup of the barrel extension/chamber and the BCG. Companies like Cryptic (an arm of Toolcraft) have perfected the application of these coatings through a process very similar to semiconductor coating (hence the price tag on their BCG). If anything, buying BCGs with next-gen coatings aids in cleaning, and ups the glamour level of your rifle, but ultimately doesn't make your rifle "perform" better.
HIGH MASS/LOW MASS: A complete mil-spec full auto BCG weighs in at 11.5oz, which is what the AR15 platform performs very well with. Increasing or decreasing this weight can have both favorable and unfavorable affects on the system. Increasing the bolt carrier mass (weight) will increase lock time, resulting in a smoother pulse of the rifle (this can also be achieved with a standard mil-spec BCG and increased power spring or heavier buffer). A lighter or reduced mass BCG will cycle faster, and possibly induce FTF/FTE issues, as well as wear out your system faster. This can be mitigated by both an adjustable gas system (will address in a later article) or heavier buffer tighter spring.
SUMMARY: BCGs are one of the most abused internal parts of an AR15/M4, and buying one that's up to the task is a must. Paying a lot more for a next gen coating on the basis of appearance and cleaning ease is fine, but you're not gaining a performance multiplier. And if a company is selling a BCG that they won't disclose the materials, testing, and origin of, it's a red flag they either have something to hide, or they themselves didn't do their homework to find out. This same rational applies to anything in the firearms world. Buy quality, even if it costs more than that bargain bin $89.99 BCG....even if it ships free.