I have read a few different posts on why you should buy a factory gun because it's more reliable and comes with a warranty. Someone had posted that in a carbine class, 100% of the rifles that broke were home builds. My question is this: what is it about a home built AR that makes it less reliable? I realize that professionally trained armorers test factory guns and the factory itself has tolerances that are most likely tight enough to keep the gun running, but where, read what part, does that reliability come from? I know first hand that if you do not assemble the lower properly it will fail (hammer springs go over the trigger pin :banghead. Other than that, it seems the only other part that would cause a home-built to fail would be the BCG. I realize many home assembled AR's run flawlessly so I don't need to know how many people haven't had problems. What I want to know is what will cause them to fail under high stress, such as a carbine class. For the record, assume the "home built" had the barrel group installed on the receiver by the distributor/manufacturer. Edit: also, this thread is for comparison of factory and kit AR-15's using a DI gas system.