6.5x55 twist
I got to this thread late. My siilhouette rifle is a custom 6.5x55 which is on its second barrel. It has a Hart # 5 1/2 barrel with a 1:8 twist. The problem with silhouette is that you are shooting light loads for chickens, pigs and turkeys and a heavy load for rams, so the barrel has to handle light and heavy loads. Mine shoots 108 grain Scenars on top of a light load of Varget for C,P,T and a 142 grain MatchKing on top of a heavy load of Reloder 22 for rams.
Some of my friends have gone to 1:8.5 twist Broughton barrels for 260s or 6.5x47s (same bullets basically) because there is a lot more light shooting than heavy. I might do the same for the next barrel. However, if this was for a hunting gun shooting 140 grain bullets, the 1:8 is the way to go.
By the way, I also have a Howa thumbhole sporter in 6.5x55 because I reload for that cartridge, so it seemed like a good idea to get it. Never got around to shooting it yet, but have some big heavy Lapua Mega 155 grain bullets loaded up for it for hunting. That would be a good bullet for deer or hogs and should be OK in a 1:8 twist barrel. The Swedes use them for moose.