In the field during AIT, we use to throw our rifles in a 55 gallon drum that held the artillery stakes. Literally, the M16A1's would get dropped in the drum muzzle first until there were 5-10 in there. Some would even get thrown in there, depending upon whether or not we were under time pressue. I can still remember the sound of the banging metal and plastic. This was SOP for the training units and I had never heard of a problem due to that mistreatment. No broken front sight posts or assemblies, etc. Despite this my rifle still managed to let me qualify expert. These were all very worn and banged up rifles on a training base.
Now, when we used REAL M16A1's (and not rubber ducks) for bayonet practice against the tire and steel "enemies" at another unnamed school, that was a different story.... We had zeroed prior to this and when we reached the qualification range (pop up targets, so very difficult to see your POI re-zero while firing the Q course) I could barely get hits on the 50 meter targets, much less the 100-300 meter targets. Needless to say that I did not qualify expert that time. Had I been able to see my POI (the range was very green, not dry) I could've probably adjusted.
So, in my experience if you throw your rifles in a pile in the corner of your safe, you're probably okay (though I don't advise this practice). OTOH, if you spend an afternoon vigorously whacking a large tree at various angles with the barrel, you might experience some shift in your zero -- though the weapon will still continue to function without malfunction unless you've actually managed to bend something at the chamber/upper receiver point.
My friend and I both agree that we generally pamper our AR15's and do not subject them to the abuse they're designed to take. If the Krime Bill sunsets with no replacement, I might be willing to abuse a new $750 evil featured Bushmaster. But I'm going to continue to pamper my $1,500 preban, especially if I can't replace it. The rifles of most AR15.com members will never see 5% of the abuse that military rifles see.
Military rifles, however, have a very hard life. Based on what I have seen, the M16/AR15 is more than able to take heavy amounts of abuse. I have been very impressed.