Lately there seem to be quite a few posts from people that are new to the AR in general, or new to this site in particular, that are starting to educate themselves and are feeling a bit of buyer's remorse. The "damn, I bought a $900 Bushmaster when I could have had an LMT for $1k" feeling is nothing new as people begin to educate themselves, and is not unique to the firearms enthusiast.
First of all, it's important to note that the below only really applies to guns intended for defensive, or offensive, purposes. If you really only ever intended your rifle as a plinker there isn't much need to do any of the below because the worst thing that will ever happen if your rifle fails to function will be that it will end your day at the range and several coke-cans will be spared that day. If, however, you envision ever needing to use this rifle for any kind of so called "hard use", then you owe it to yourself to at least do the following.
I believe that "The Chart" is having a negative impact in this regard. People are coming here and finding out that the gun they bought is sitting on the middle-to-right-side of the chart and are wondering if they made a bad choice. The answer is "probably not".
I have noticed quite a few threads in which Bushmaster, Rock River, Stag, etc. owners are starting to ask what they should do to their rifle to try to "move it to the left" on the chart. Some are even asking if they need to scrap their rifle entirely and buy a new one from the left side of the chart. The answer is "probably not".
Whether you have had the carbine for years with thousands of "trouble free" rounds through it (real-world rounds, not internet rounds) or you just bought the rifle, have hardly fired it, but are already starting to feel that buyer's remorse, don't fret. There are some very basic things you can do to improve your carbine and ensure better reliability.
1) Check the carrier key for proper staking. How do you know if it's properly staked? Go here and educate yourself. If yours doesn't look right, follow the instructions to correct it or send it out to have someone else correct it.
2) Check the extractor spring and insert to insure that the insert is black. Go here to learn about extractor inserts, springs, and O-rings. This is something anyone can install themselves.
3) Check the receiver extension end-plate for proper staking. How do you know if it's properly staked? Go here and educate yourself. If yours doesn't look right, follow the instructions to correct it or send it out to have someone else correct it.
4) Buy an "H" buffer if your carbine doesn't have it. Go here to learn why. Go here to see the different kinds of buffers. This is something anyone can install themselves.
5) Buy a properly tested, staked, constructed bolt carrier group (BCG) with the proper extractor insert. The bolt itself should be Magnetic Particle Inspected (MPI), Shot Peened, and Proof Fired. You can buy a top quality BCG from Bravo Company, G&R Tactical or LMT for around $130. You can either change out to this BCG right away, or use the one that came with your rifle until it breaks and then switch it out. The choice is yours.
Hopefully the above is helpful. Obviously, if you have that "thousands of trouble free rounds" DPMS, you may feel perfectly comfortable with leaving it alone. At the end of the day, that's what it's all about, how secure YOU feel in the thought that you may use this carbine for self, family, or society defense.