Brno 300 series extra barrel sets

[gunbug]

Hi: I have a Bruno 300 series combo with 7x57R over 12ga & 5.56x50 R Mag over 12ga. I read somewhere that extra berrel sets were made in 9.3x74 &375H&H etc. Anyone know if this is true and if it is where i can get a set? Regards Dan