I have agonized about buying a bolt carrier group. There is a lot of information out there and it all sounds important. Some companies match bolts with barrels, Noveske lists “properly” staking their carrier key as a feature, which implies some carrier keys are not staked correctly (and indeed some are not but anyone can do it themselves). And then there is the full auto/semi auto listing or even enhanced semi auto, with a price difference to go with it.
JP "Full mass" BCG on top and "Low mass" below
The quick answer is for the best overall deal, get the spikes tactical BCG – you used be able to find it on sale for just under $120 using google shopping to find a vendor. It is solid, MP tested and will last. But it is also backlogged 8-12months. For a good deal at a good non sale price, get the Bravo Company auto BCG ($150). When they are in stock, they don't last long
If money is no object, get the JP enhanced bolt and “full mass” bolt carrier with QPQ coating. If you just want a different improved bolt carrier group, get any MP tested NiB BCG. There are some good ones out there, but FailZero has well documented stress testing of their EXO (NiB) treatment and they say theirs is the best. However Barnes Precision does the same treatment but makes the bolt in house so if you want NiB they are my top recommendation until I can test them all. I am also still waiting to test an NP3 BCG also- but the hype at Robar is working on me - it looks good and I am interested, though there may be an issue with the treatment's hardness as the round count gets in to the 10-20k+ range. Currently Wilson Combat carries them.
Marking on a Spikes HPT, MPI bolt
There are some other companies that offer enhanced bolts or carriers, LMT and LWRC for instance, but I have yet to test them and from what I researched, they do not support the increase in cost as well as the brands I have listed. Additionally some (like the LWRC) are for gas piston and not interchangeable with DI systems, or for that matter other gas piston brands.
For an interchangeable system, Leitner-Wise does, you can buy it as the JP Enhanced bolt. If you have the money it is worth the extra cost. It is cheaper to get it as part of a matched barrel with JP, or as complete BCG. The bolt is better because it is made of 9310 steel, not Carpenter 158. Knights makes an improved bolt also, but it only matches their chambers, thus severely limiting your options for barrels. LMT is also worth a look for their interchangeable improved bolt.
Yes. It does. Bolts fail - I have owned/used many, many bolt carrier groups and worked alongside literally hundreds of people with similar experience levels as me. Working in a gun carrying profession, it is of immediate interest to everyone if someone's weapon has a part failure. While I have never seen a bolt carrier fail, I have seen hundreds of bolts fail over the years. Depending on your gas system, how well the bolt was made and what alloy it is made of, a bolt can last well over 50,000 rounds. Or they can fail after an average of 3000 or less. Yes, it pays to inspect your bolt during cleaning, but you don't always see warning signs (without xray anyway) before catastrophic failure.
Just remember: Bolts are a high movement, high stress working part. Eventually they will all fail, even the JP and Knights. People who think they won't, just don't shoot as much as those who know they do. Many a guy on many a forum will never shoot enough rounds to wear out a bolt, but that does not mean they last forever.
Again, JP. Or at least until I can compare it to an NP3 BCG. While I mentioned that I have never seen one fail, the QPQ coating really does provide lubricity and allows the rifle to run smoother and with less lubricant. Additionally, if you are interested in tuning your rifle for competition etc, you can buy the low mass BCG which is very effective at helping to reduce recoil when used correctly. In a personal test of the effectiveness of the QPQ and the JP system overall I ran 55 mags (1650 rounds) without adding any lubricant before I began to have malfunctions.