SAAMI published COL is just a standard the makes ammo that will cycle thru actions without any problems and fits in all magazines.
For the most parts, as Larry stated, reaching the lands is an impossible feat in most Handis with few exceptions. The .45-70s are short throated, but most are long, some extremely long in some cases and trying to load just off the lands is futile specially with lighter bullets for caliber, so most just use SAAMI COL and most shoot very well.
My 7x64 Brenneke Handi has 3/8" bullet jump with a 175gr Grand Slam, that's with minimal seating of the bullet depth!! But it shoots factory loaded Federal Nosler Partitions excellent!
I haven't loaded for my .223 Ultra yet, but those that do know that it is one of the deep throats of the Handis, but it still a great shooting caliber using SAAMI COL from all reports.
Tim