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Very old problem affecting all weapons and 'mechs. Some weapons aquire some kind of "affinity" for certain hardpoints. Workarounds inlcude but are not limited to: -selling and rebuying all affected weapons (read: sell ALL erppc)
-loading affected weapons into unused 'mechs until the "special" ones are out
-not giving a ****
-uninstalling the game
Luckily for you, though, with the BLR it's not too big a deal apart from looking ugly. All shoulder weapons are at least at cockpit-height so if you can see the target you can hit the target (barring any vertical obstacles on your sides) with all shoulder weapons.
PS: sorry for the formatting, it won't let me edit it...
Edited by Ironwithin, 22 January 2014 - 01:33 AM.