More than likely a 9 twist unless it's a large shank barrel. Does the action have 3 screws (not counting the rear trigger guard screw) and a small ejection port?
You can also determine the twist with a cleaning rod, a tight fitting jag, and two pieces of tape along with measuring tape and sharpie. Just measure the distance for the cleaning rod to make one full revolution.
If nine twist, 77SMK is about the heaviest you can shoot reliably, unless you're shooting atop Pikes Peak where the air is thinner. Otherwise, any 69 grain will suffice.
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