Then I stand by what I posted - you have a rifle with what would today be considered an abnormally small neck. Mike a fired case from one of your factory rounds. Compare with your reloads. Post both numbers for us. Interim solution will be to thin the neck walls to maybe .005", use a smaller boolit, until your reloads have a neck dimension small enough to enter the chamber. Take apart one of the factory rounds and measure the neck wall and boolit diameter, too. Post the numbers for us.
Any boolit you use in this rifle will have to be very soft, so that will obturate quickly. The original factory rounds that I have examined were not only dead soft, they were swaged, not cast. Much cheaper process for manufacturing them in military volume. ('course the patching added cost, but....)
Powder should be Holy Black, or a fairly fast smokeless. (Here finally is a use for Trail Boss, which I otherwise abhor.)