BLR magazines: caliber specific for a reason?

Well, I know that for the BARs, a .243 mag is the same as a .308, so I wouldn't be surprised if magazines for cases that are in the same family are the same.

I too doubt they are different, but it makes it easier for people to order if they see their specific caliber.

My mags. are marked .243, .308, .358.

I can't say for sure about your Browning, but do know they are not interchangable for all rifles. On the Winchester Model 88 which is chambered in those calibers, the .358 mag is different due to the angle required by the larger diameter bullet.

The base cartridge cases for .243, 7mm-08, .308 are all the same.

I've personally used .243 and .308 marked magazines for .308 Win in a current production BLR. They were/are identical except for the markings as far as I can tell.

Brownells sells a single Browning SKU magazine for the following cartridges:
243 Winchester, 6 mm Remington, 7 mm - 08 Remington, 308 Winchester

http://www.brownells.com/magazines/rifle-magazines/magazines/magazine-prod1924.aspx

Length is the key, IMHO

I've got a long-action BLR in .270 Win., and it feeds just fine with magazines marked 30-06. Given the similar size and shape of the cartridge body, that's not much of a surprise. I know a lot of the short-action BLRs are now being chambered in the WSM calibers, and I've no idea what, if any, changes they've made to the follower and feed lips to accommodate those short, fat critters. There may also be some differences in the long magazines to accommodate the few belted magnum cartridges they offer.

I know that my Remington 760 in .257 Roberts does just fine with magazines marked 243 Win and 6mm Rem. I'd say you're probably safe so long as you stick with something with roughly the same form factor. Magazines for cartridges that are pretty much identical except for bullet diameter, like the 270 Win, 30-06 & 8mm Mauser should be totally interchangable.