Xausa, you have a top quality rifle, it was my first rifle in 1968. Xausa, I try to expleen, you could change barrels in different kalibers when you stay in the same group of kalibers.
There are initially 3 groups the first with "stossboden" 12,2 follow by the group with "stossboden 13,1 and the thirth group 13,58. Now your 7,6 X 66 Vom hofe just not fit one of these groups. Marc.