Friction has two differing values; static and sliding. Typically, static friction is higher than sliding. If an increasing force is applied to an object sitting statically on a surface, the object will not move until the force rises to above the force of the peak static coefficient of friction. The object will usually experience an acceleration due to force above the sliding coefficient of friction. If the force then drops back below the peak force of static coefficient of friction (the object outruns the force source), the object will decelerate. This is called stick-slip friction and is a common event. Stick-slip is the cause of tire squeal.
The wad and shot travel down the bore may not be quite as orderly as we usually envision. In the case of a bore and shot size prone to shot bridging, it may be that shot bridges form and collapse a number of times. The bridge causes a stick and the collapse leads to a slip. If the stick phase is severe enough, it may cause a mild gas hammer. The hammer may well collapse the bridge leading to a slip phase. If a second bridge forms, the foregoing cycle may be repeated. Rivelling? Maybe.
DDA
