Reliable means not only "goes bang every time," but also keeps running with replacement parts being available, and having enough ammo, at a decent price to practice enough you can't get it wrong.
A2's and 16" midlengths will do that. Full power ammo does it, not cheap import, which works for training. Stoppages and how to handle them are part and parcel of being the reliable operator.
Polymags, or tested GI mags that will survive being dropped on the feed lips loaded, which means Polymags. They don't have to be Magpul, but metal mags are NOT reliable. Many use and shoot them, they cannot be trusted because they simply will bend, dent, and deform in rough use, which is what training will do to them. And for some reason, we won't crush a bad metal mag, but a cracked polymag is no problem.
Reliable is about all the parts of the system, not just the gun. And the most unreliable part is the user, then magazines, then ammo. The gun does a pretty good job of it.