In general, modern firearms are very reliable. It's rare for a firearm produced from a major manufacturer to have problems that aren't related to substandard ammunition or magazines. If a firearm should encounter a problem, most manufacturers have a policy to repair at no cost to the owner, shy of perhaps shipping costs. I've experienced this with both Smith and Wesson and Ruger in the last 5 years. Neither resolution was speedy, but in both cases an issue was remedied to my satisfaction. The Ruger firearm repaired was even one that I had purchased used. It's hard to find manufacturers that stand by their products like that.
Neither of my problems were caused by the manufacturer, btw. My issue with the Ruger Vaquero was traced to aftermarket springs installed by the former owner. It seems one of those failed. It's amazing that Ruger fixed it at all, much less for free. My issue with a S&W involved an improperly sized barrel included with my pistol, a barrel provided by a vendor. Yes, S&W failed to quality check that part, but once it was brought to their attention it was replaced without issue.
I have guns that I have thousands of rounds through and haven't needed to replace any parts. Most firearms made today are very durable and reliable. This thread isn't about those- it's about guns that aren't offered by manufacturers, guns that people commonly assemble themselves, AR-15's.
The AR-15 and M-16 series of rifles were designed to certain specifications. It's a very durable and reliable design. Since it's inception, some changes have been made. The barrel was first shortened to 16", then even shorter. The buffer, buffer tube (aka receiver extension), and gas tube was shortened to make a carbine that could use a collapsing stock. Many people say the shortened systems make the gun run a bit harder, but in general out of the box AR-15 carbines are very reliable and durable. But we are humans, we like to monkey with things. We now see even more variants.
NFA firearms are becoming increasingly popular. Perhaps it is because as more information is available on the internet, more people realize it's possible to own firearms and accessories that need to be registered as part of the National Firearms Act. From 2005 to 2011, NFA processing more than doubled, and 2012 is turning out to be a record year, exceeding 2011. I own a couple of NFA items myself, a Short Barreled Rifle and two Suppressors. The short barreled rifle (SBR) is an AR-15 and one of the silencers is attached to it. This modifies the system and makes it potentially less reliable and less durable. It's something I have to accept.
My SBR has a 12.5" barrel. I've never fired it unsuppressed, and my suppressor is currently permanently attached (which, doesn't even require it to be a SBR, since the perm attached can is considered part of the barrel, but that's another topic). The combination puts additional gas into the action when a round is fired, more gas than is needed to properly operate the firearm. This is often referred to as being "over gassed". The suppressor creates back pressure since it inhibits a smooth flow of gasses from the end of the barrel. The AR-15 platform works pretty well in this situation, but it does run harder than a standard 16" carbine.
So people do things to change the system. I've added a stronger action (buffer) spring, and a heavier buffer. This absorbs some of the energy created by the extra gas. It's an attempt to balance it out. And it's been exceedingly reliable. I've had one jam, caused by an old crappy magazine.
Well, it's been exceedingly reliable as a system. If I switch it to a standard AR-15 lower (which I can do, since the 12.5" barrel plus permanently attached silencer are 17" in length) it doesn't run as smooth. Without the heavier buffer and spring, it's not reliable. I've deviated from a standard, and it's limiting.
This weekend I saw three AR-15's have problems, and all of them were non-standard builds. One was mine, as mentioned above. One was a new rifle that would only cycle reliably with higher pressure 5.56 ammo, but not with 223 ammo. It was undergassed, because it was a 16" barrel equipped with a rifle length gas system, commonly referred to as a "Dissipator" configuration. Putting the gas port so close to the muzzle resulted in less gas making it back to the action. It's an easy fix, either drilling the gas port a smidge larger or attaching a muzzle brake should do the trick. This was a brand new rifle, it may smooth out over time as well. But likely, it's going to need a modification to become as reliable as a standard configuration.
The other AR-15 that had issues was a pistol built with a 11.5" barrel. This gun was assembled by it's owner, and it's had some growing pains. First it had ejection problems which required a stiffer spring to be installed in the bolt. The problem now is the pistol buffer tube, in combination with a non-standard end plate (which has a sling attachment) doesn't fully engage the buffer retainer pin. One has broken, and the replacement part came loose during shooting. Both times it rendered the rifle unreliable. The fix is a different pistol buffer tube or a different endplate.
I just wanted to share my experiences. Out of the box, AR-15's work great. When you start messing with them, you increase the likelihood you'll decrease reliability and durability.
This doesn't just apply to AR-15's. If you build a rifle with a tighter chamber you can get better accuracy, but you may not be able to safely fire all commercially available ammunition. If you build a super-accurate semi-auto target 22, you may end up with something that doesn't cycle properly with all types of ammo. I see people who shoot competition pistols have problems all the time, they have tolerances and springs set on the verge of proper function with a specific type of ammo. It's a tradeoff they make to reduce recoil and enable getting back on target faster.
So keep that in mind, you can have the coolest looking, fastest shooting or most accurate gun at the range, but you are likely
impacting it's reliability or durability. Like most things in life,
it's a tradeoff. If you want the most reliable and durable firearm,
stick to well-tested designs.
If you like to tinker though, figuring these things out can be a lot of fun! I've learned so much about the AR-15 platform by mucking around with it.