From http://neverenuffammo.com/ June 12, 2012 / Matt 

Along with your barrel, your Bolt Carrier Group is the “Heart” of your AR rifle. So choosing the right one is very important to the overall quality and dependability of your rifle.

The “Mil-Spec” or industry standard for BCG’s are as follows …… Carpenter 158 steel, shot peened, gas key secured with grade 8 fasteners, chrome lined and parkerized. While there are some variations and options like different coatings, these standards will be the same for most BCG’s on the market.

One of the first things you want to look for in a BCG is that it’s been MPI and HPT tested. MPI stands for “magnetic partical inspected” and HPT stands for “high pressure tested” …… basically these are tests to help determine if there are any issues with the BCG such as cracks or hairline fractures that could cause failures. These tests are also use on barrels as well.

Now many companies test their BCG’s in these ways but some companies “Batch” test their BCG’s which basically means that for every batch of BCG’s they produce, they only test one BCG. Definitely something you’ll want to check into when purchasing. Companies like BCM, Daniel Defense, Spike’s Tactical & Palmetto State Armory are examples of companies who individually test their parts.

Next you’ll want to check the staking of the gas key on your BCG. Now this can be pretty hard to do if your buying online so do your research on the different companies to find out who has a good reputation in this department.

For those that don’t know what the staking of the gas key is, I’ll explain. The gas key is the part of the BCG that comes into contact with the gas tube coming from the gas block on the barrel and it is what the gas blowing back from the barrel comes into to contact with to cycle the action of the bolt. Because of all the pressures associated with this and because the gas key is attached to the rest of the BCG by two small hex bolts, it’s inportant that those bolts are properly “staked” into place to prevent them from backing out or loosening due to the pressure and causing a failure.

The next thing you’ll want to take into consideration is wether or not your getting an AR15 style BCG or an M16 BCG. The differences are noticable at the rear of the BCG where the top and bottom of the M16 style BCG are the same length adding weight and stability to the action and also slightly slowing down the cycle rate which of course is preferable for full auto rifles. The AR15 style BCG has the bottom rear of the BCG cut back to reduce weight and cost of the BCG.

Now while most civilian shooters won’t notice the difference between these two types of bolts, now days the two kinds of bolts are so comparable in price that you might as well just go for an M16 style BCG.

One final thing to keep in mind is the chrome lining. Some companies advertise chrome lining on their BCG’s and some don’t, but keep in mind that chrome lining in the body of the BCG where the bolt rides and inside the gas key is an industry standard. So while you may not see it advertised, I’m not aware of any company currently manufacturing a BCG without chrome lining.

So thats its, those are the basics. There are some customized options out ther like specialty o-rings and ejector springs that you’ll sometimes find on some BCG’s but for the most part those are upgrades or customizations and aren’t a part of your standard BCG.