Thought PTFE piston seals are supposed to be self sizing (to an extent) in as much as every time it slams into the end of the cylinder the seal is squeezed out to the size of the cylinder?
Edit: although this would rely on perfectly parallel cylinder walls...
Sure TR Robb used to sell slightly undersized PTFE piston seals with the instruction to dry fire until the seal fitted? (I may be wrong though?)