The American with its small loading port and flat chamber entry isnt ideal for single loading. The mag sticks up a bit on my .300aac which puts rounds kinda off to the side when dropping them in. If you do it carefully it isnt hard but its not as easy as tossing them into the gun and closing the bolt.
The gun cant be loaded from the top, youll need to pull the mag and load it.
Mags are cheap tho, keeping a spare with what ever rounds you want in it is easy. If your prone and want to add more ammo, its easier to pull the mag, load, then stick it back in, than it is to try shove rounds into a tradtional box. If you just want to top up a round, then a traditional is easier, but again, if you got a spare mag run one till your try and swap. If you have time to top, then pulling the mag and adding rounds shouldnt be an issue.
As a note most control round feed guns can also be chamber loaded, by flexing the extractor. Ive never had a Winchester, but the Mauser, springfield, and ruger i had could be loaded like that...actually the ruger didnt need you to squeeze the extractor.