I've seen exactly one rebarreled BLR in about 45 years . It can be done with some versions , but is extremely difficult with others . The pre-81 models have screw in barrels with bolt locking seats machined into the receiver . Post 81 have both ( not sure how to tell which is which ) screw in and press fit barrels with bolt locking seats machined into the barrels . The alloy receiver non takedown models I'm not sure of , but also have locking seats machined into the barrel . The take down models are just that and would need to have locking lug seats machined into a extension of the barrel . Any of the above would probably cost the same as , or more than , a new BLR . I've worked on quite a few BLR's over the years as well as owning a few . Great little rifles , but not a lot of fun to work on . About half of the BLR's I have fixed over the years had the same problem . Their owners had taken them apart and couldn't put them back together . Indexing the rack and pinion gearing between the bolt and lever can be a pain . I can do it , but I'd rather not .
There are other models that would be a lot easier to work on , the Savage 99 first and foremost . It is a , IMHO , far better design than the BLR . It is a lot simpler and very easy to work on . What cartridge did you have in mind for a conversion ?
AB