In my humble, non-scientific opinion, 22" on a 6.5x47 Lapua would be just fine. The smaller case capacity of the Lapua would work fine in a shorter barrel. In my opinion, larger capacity cartridges like the magnums benefit from longer barrels. I can't answer the velocity drop question with any authority. However, in similar sized cartridges I have seen in the neighborhood of 40fps per inch of barrel removed. It has also been my experience you may actually, although not promised, see an improvement in group sizes simply because the barrel will be slightly stiffer being shorter. Although, 4 inches might be insignificant to the average shooter off a bench.
Take it for what it's worth, all comments can not be backed up by any "real" data but are rather my opinions.
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