thats right you cant mistakenly do that, you would know something is wrong.
>>what you may see is a lower receiver stamped for 5.56 NATO assambled with a 7.62 upper and barrel<<
5.56 magazine wells can be modified to fit 7.62 magazine there a couple examples on the range right now, also a .375 win magnum necked down to a .223 projectile.
up here its common to ammo up with a full mag, +1 one in through the out door
considering that a stripped designed for purpose AR10/LR308 lower is maybe $140 from a decent quality manufacturer it would be exceptionally rare for someone to hack up a AR15 lower and magazine well to accept AR10 magazines, I have never heard of anyone actually doing that.
if you have a photo example of someone actually doing that on an ar15 magazine well i'd be very interested to see what the hack job looks like
you should check out modular lowers, hydra and i think viper-A [or vipera] have re thunk the lower into two pieces, result being exchangeable magazine wells to match the upper config.
why would someone actually chop a lower? because its not available, or its a do something winter project to chase off the cabin fever.
the last paragraph is not exactly correct, a lower receiver for 556 NATO STANAG magazines has a different length magazine well (AR-15 derived design, not AR-10) and cannot take the dimensions of an AR10 magazine/7.62 NATO magazine.
you can mistakenly chamber a .300 blackout in a 556 nato rifle upper because the blackout is the same length cartridge as a 556 nato round and will feed.
you cannot mistakenly shove a 7.62x51 NATO/.308 winchester round into a rifle lower/upper that is not an AR-10 derived design because there would be no way to even seat the magazine.
even if you were to somehow mistakenly mount an ar10/762 NATO upper and barrel on an ar15 lower the only way you could try to get a round into the chamber would be to shove it in sideways through the ejection port