The reason for the 0.355" bullets was they needed a taper in the case to aid in extraction. Also the reason for the larger web for the rebated rim.

And ALL bullets that are factory loaded are 0.355" Win and Federal and Hornady. That was the way it is designed. Saami spec allows for a 0.003" +- tolerance. So that is the reason they are allowed in most states that have a straight wall only cartridge rule for deer hunting.

I am using several lead bullets and having some good and bad. Lilgun powder which seems like everyone is using is a very unstable powder just like H110 is. They go from a nice round primer to blown in a .5gr increase. I am getting right at 1.25" at 100yds with the Lee 200gr bullet. I am going to be using different powders as I do not like how lilgun behaves. 5744 in the next range trip.

And this is in a AR platform. 16" BCA barrel. Spikes upper. Toolcraft BCG.