Finding something that will work through the magazine is precisely my concern. I want to find the smallest bore diameter that will allow a clean re-bore and re-rifle. Then that bullet will be fitted to the 6.5x54 case to form a new cartridge - keeping all dimensions identical with the original with the exception of the bullet diameter.

With that in mind, I plan to mock-up a couple of cartridges: one with the 140 gr. .277 bullet, and one with a 140 gr. .284 bullet. If the dummy cartridges can be inserted into the magazine, then I will know if I am on the right track.