It may be in the eyes of the buyer/seller as well. I bought a lovely Brit BLNE from a Seller of good reputation. The cones had been lengthened. I asked if it affected proof, and was told no. Couple years later when I was trying to trade the same gun with the same dealer, I was told value was diminished cuz the cones had been lengthened and it was out of proof...Heated discussion commenced..........