Mine has run just fine regardless of what I fed it. Either something is messed up with the gas block or gas tube alignment or there is something wrong with the chamber.

If you are handy with rifles, you can check these things out and maybe fix it yourself. If not, I would recommend using the life time warranty S&W provides.

As far as AR platforms in general and the Vietnam experience, the Army hated the rifle and set it up for failure. They used the wrong powder in their ammo contrary to Armalite/Colt's recommendation to start. They also told the troops they didn't need to clean their rifles and provided no cleaning equipment for them to use.

The powder they used burned more dirty and left a lot of residue in the action. This caused malfunctions and jambs. Not cleaning the rifles exacerbated the problem.

The original M-16/AR platforms had some other issues as well that got addressed over time but the above was the main problems, to the detriment of the troops all because the Army was force fed a rifle they didn't want.

Modern M16/M4/AR platform rifles are very robust and reliable as long as they were built correctly.