JHP vs Hard Cast

[hylander]

O.K. here goes:(at the risk of looking stupid)
.44 Mag. as example
I am under the understanding that a Cast bullet needs to
fill the Cylinder of the revolver say .430 bullet in a .430 Cylinder, to be of best accuracy.
So why does a JHP not have to be the same.
Most JHP are .429 coming out of Cylinders .431

[dakotashooter2]

Pretty simple really. Actually we are dealling with two issues. The cylinder and the bore. You want a lead bullet to be a slip fit in the cylinder and oversized in the bore. You NEVER want a cylinder tighter than the bore as it will swag down the bullet and cause leading. With cast bullets you want a tight fit  to the bore to prevent gasses from getting around the edges of the bullet and melting it, which causes leading. Since jacketed bullets have a harder surface this doesn't happen with them. You also want a deeper "bite" from the rifling in a lead bullet to keep it from stripping at faster speeds. You can get buy with a soft undersized lead bullet better than a hard undersized  bullet at slow speeds because the soft bullet will expand to fill the bore. usually.005 over bore seems to work best.