also, 99% of the time(minus the current ammo shortages ) 45 colt ammo is a lot harder top find typically than 12 Ga ammo
a 12 ga shotgun is a way more popular caliber/gauge than the colt 45, and thus typically sold in way more pl;aces and in stock
5 colt is honestly these days more of a novelty caliber, that many shops DON"T typically stock or stock in amounts/selections they would normally stock 12 ammo!
its just right now, with craziness of ammo demands that the more common calibers are sold out, leaving them less popular calibers in stock, which might be why your finding /seeing 45 colt ammo over 12 ga in stores right now!
and not sure what you mean by a lever action advantage here? what are you considering an advantage over what other type of rifle/shotgun???
as IMO there really isn't much of an advantage of a lever action rifle over any other type, its more about personal preference
as yrs ago, levers were able to MAYBE hold more ammo, offer faster follow up shots, or be shorter in size
but in the real world, most all rifle types have some form or another in smaller in size, larger capacity of ammo held, and so on
so, to be honest, picking a TYPE of rifle, is more about personal preference, or unless there are restriction or /laws that require use of a certian type of rifle to be legally used!
all rifle types have pro's and con's
a lever action is no different! than other types to be honest!