What we see go down at class falls into these catagories.
Wear items
Specifically, extractors and extractor springs.
Bolt rings
Buffer springs
Parts guns
Parts ain't parts. If you get your stuff from gun shows and garage sales, don't expect quality
I had a guy in a recent class bragging on T1 that he built his carbine for $400 and "it is just as good as..." That feternoon it wouldn't ectract. The extractor was worn snooth and then re park'd; the extractot spring was flat (no joke) and the insert was just crumbs.
We replaced it all.
On T3 his bolt broke in half.
Hobby guns
Generally ok, but non MPI bolts may not last (we see less problems with breaking lately) and some of the new guns may not work at all. These should not have to be shot in- they should work out of the box.
Chambers are often 223, no matter what is marked on the barrel. When the gun gets hot, extraction slows down.
Worn out guns
Parts wear. Sometimes the guns are just plain shot out- more often, military guns.
Think MEAL
Magazines- serviceable mags
Extractor- serviceable
Ammunition- quality ammo, not garbage
Lube- generous lubrication
If you have that, most of the guns- even some of the very low end guns- might do well for a class or more.
On the other hand, some of the companies make absolutely great guns. They will last a long time.