Carbine system was a product of the Xm177 with a 10" bbl. The 14.5" carbine came later when they wanted to make a carbine using the xm177 gas tube and handguard, and to mount a bayonet 14.5" would be the right length.
16" carbine is simply a civilian offering to make use of standard carbine parts, to save money from developing and making new standard for the relative small commercial market back in the days. The reason we are stuck with some of the things we have pretty much boil down to the choices made 40 years ago at the development of xm177 and the mounting of M203, as well as the standard bayonet for m16. From that point on every variant needs to work around these items.
Interesting enough, the commercial mid length was first developed by Armalite ( probably with the engineering help of LMT?) so theoretically a bayonet can be mounted and it looks more "natural" to consumers as a product differentiation ( people always complained of the weird look of 16" carbine back then, hence the practice of wielding extended flash suppressor to 14.5). The advantage of the midlength was than applied backwards on the 14.5.
16" carbine is simply a civilian offering to make use of standard carbine parts, to save money from developing and making new standard for the relative small commercial market back in the days. The reason we are stuck with some of the things we have pretty much boil down to the choices made 40 years ago at the development of xm177 and the mounting of M203, as well as the standard bayonet for m16. From that point on every variant needs to work around these items.
Interesting enough, the commercial mid length was first developed by Armalite ( probably with the engineering help of LMT?) so theoretically a bayonet can be mounted and it looks more "natural" to consumers as a product differentiation ( people always complained of the weird look of 16" carbine back then, hence the practice of wielding extended flash suppressor to 14.5). The advantage of the midlength was than applied backwards on the 14.5.