Solid plug or shot. Doesnt matter. The eventual result is the same. Personally in the examples I have observed, the really bad splits were with solid lead. On one gun, the split went nearly all the way up to the grip.

I have had some where the wood is not split, but the buttplate is being pushed up off the butt surface. And some I find lead in have not yet caused a problem. I will remove it anyway.

B.Dudley