Modifying a hi-cap to fit another weapon is not illegal, as long as the magizine will continue to work in the gun it was originally intended for. For example, using a Beretta 96 10-rd .40 cal mag as a 13 rd 92FS 9mm magazine would be legal, unless you bend the feed-lips. Bending the feed lips would prohibit the use of .40 rounds in the mag, and would therefore be illegal.
As far as FAL mags in a BLR.....