Point of impact (POI) changes of the bench.
Today I shot my .40cal Isaac Haines flintlock after installing the Jim Chambers White Lightning flash hole liner. The results were very favourable and with the new liner, true to all claims, ignition was very fast though possibly not faster or only marginally so than previously.
My MLer target shooting these days is exclusively offhand at 50m and as a result I use only a light charge of 30gr of Swiss 3Fg producing in the region of 1400fps with the .395 94gr ball. Accuracy runs 3/4 to 1 inch at 50m. For hunting or in the past when I have shot out to 100m the charge is increased to 45-50gr for velocity of 18-1900fps and much tighter grouping whereby I can cut cloverleaf groups at 50m. Today I sighted the rifle for a 50m zero with the 30gr charge then increased to 45gr to check the POI just to be sure and I am pleased I did.
Off the sandbags the 30gr punched at 6 o’clock dead center just as it should. The 45gr, accordingly, impacted 1 1/2 inches higher BUT the same distance off to the left. Most unusual as nothing other than the charge had changed so for a second reference the 45gr was shot at a full 100m. Same result. 3-4 MOA left shift. Nothing screws with a shooter’s head more than this sort of thing so I cast my mind back to when I first built the flinter and was doing a lot of regular shooting with it. All the bench shooting was done ‘damped prone’ with front hand resting holding the foreend and elbows on the bench. With good breathing control this position was a real winner. This revelation I loaded up ‘Isaac’ and shot the 45gr again this time ‘damped prone’. BINGO. Two shots cutting smack bang in the middle of the target.
Lowest shot is 30gr charge. Left pair, 45gr off the sandbags. Center pair, 45gr damped prone.
So, the rifle was moving during firing such that the fall of shot from sandbag shooting, with the heavier charge, was swinging left. Given the barrel length of 38 inches and sedate velocity, by modern standards, the additional recoil and/or torque I suspect was enough to move the rifle muzzle off target. Once I knuckled down and held the rifle solidly and consistently the POI came back to exactly where it should be.
Shooting certainly can be a black art.
Keep your powder dry. Jeff.