Sorry, I don't understand the "nowhere to go laterally" bit.... Since the lead shot only fills the tube 3/4 full, it can move in any direction except down, as gravity is already holding it down inside the damper.... However, should the barrel initially kick "upwards", it would accelerate the shot up, but the shot, from it's own momentum would not follow the barrel down immediately.... You have to remember, the movements we are trying to damp out are tiny.... A 1" change at 50 yards (150 ft. = 1800 in.) would only be 1/900" in a 2 foot barrel (just over 0.001")….I don't know how well a weight mounted in rubber would be in acting as a harmonic damper, but I would think it would be similar to a "limbsaver", which is a rubber weight you slide along the barrel until you get your smallest group.... All I can suggest is to build it and see what happens....Bob
That’s a nice lightweight bullet . It must be pretty short at that weight , less than .4?
these powders are likely to be rough particles that might bind and pack