I'm not a BLR fan, but I know that there have always been questions posted about finding or making an extended magazine. Pastorp has mentioned it a few times.
I found an old post, from many years ago, by a guy in the UK, who said that he made one by mating (tig welding) the bottom half of an FN FAL magazine to the base of the BLR 81 factory mag. He said it wasn't very hard to do and it worked fine.
I thought that I would pass this info along. The vast majority of posts speculate that the project might start by using an M14 Magazine, but maybe the FN magazine is just easier.
People always seem to immediatly leap to "10 rounds" when they talk about an extended mag, which I think is a mistake. Silly looking an ungainly. I'm sure an 8 rounder would be much better.
I got so interested in it, that I have considered buying a BLR 81 mag and an FN FAL mag, cutting them off, mating matching the edges up, and then running down to the welding shop for a TIG weld job. Here in the Shenandoah Valley, skilled tradesmen are starved for work and on food stamps. People will gladly do anything (skilled carpentry, plumbing, welding etc.) for $20 to $25 an hour. You just have to ask.
If I go forward, maybe I'll mail it out to Pastorp, so he can try it out. :-) I figure that maybe it'll cost me about $125 for the experiment? Polishing the welding seams out of the inside of the mag would take time, but I figure that a dremel tool and some patience would do the job.
Regards, Mannyrock