I started with with bore riders and to date have found them to be more accurate than other designs at higher velocities. With the bore rider it is already past the throat and in the bore almost half its length before primer ignition. Depending on alloy and pressures a portion of the bullet will be deformed. At the higher velocities bullets recovered from the most accurate rounds show swagging or "nose slump" to about 3/16" from the nose. I believe the initial pressure spike actually does the swagging. The portion of the bullet that is already closely fitted in the bore is swagged in one movement to fit the rifling, the portion in the throat is expanded to the throat and then into the rifling. This is why a loose fitting bore rider is seldom accurate because of the axial alignment. I believe it is also what makes a bore rider more accurate at high velocities ( when neck of bullet is swagged into rifling). Powder coating provides the barrier between the alloy and the barrel. This is one reason that the lubed bullets run into problems because on the heavier bullets as much as two thirds of the bullet went down the bore before the first grease groove. These are just my theories trying to explain what I have found so far.
Is sizing required--I have found very few bullets perfectly round and when PC is added it is not perfectly uniform. Also when I slugged my barrel the best I could measure it was .300, now I am sizing the nose portion to .301 or .302 (depending on barrel) and they slid in easily, Before I started neck sizing I could not shoot heavy bullets because they would not fit in bore. The sequence I follow is cast, set gas check, size to .310, PC, resize .310, and then size neck. There are extra steps but I believe the accuracy gained is more than worth the effort. This experiment has opened my eyes to the potential of cast bullets. I do not believe we are close to the limit yet.