6.5 BR brass is 30-30 brass necked down. So I would not call it difficult to get. And it will also be cheaper. The only thing more expensive is the original purchase of the dies. And that is not that bad.
Both cartridges are great cartridges. I go for the 6.5 BRM for one reason. The brass headspaces on the rim. The 6.5X55 is not a rimmed cartridge. And IMO a rimmed cartridge has several advantages in a break open action rifle. So I believe the 6.5 BRM is a better choice. Both cartridges shoot very well. But when shooting a rimmed cartridge in a break action rifle you do not have the normal head spacing problems that most break actions have, and extraction is much easier, faster, and positive. The rimmed cartridge also alleviates the misfires that some people encounter on shoulder head spaced cartridges. Tom.
Both cartridges are great cartridges. I go for the 6.5 BRM for one reason. The brass headspaces on the rim. The 6.5X55 is not a rimmed cartridge. And IMO a rimmed cartridge has several advantages in a break open action rifle. So I believe the 6.5 BRM is a better choice. Both cartridges shoot very well. But when shooting a rimmed cartridge in a break action rifle you do not have the normal head spacing problems that most break actions have, and extraction is much easier, faster, and positive. The rimmed cartridge also alleviates the misfires that some people encounter on shoulder head spaced cartridges. Tom.