I also use the 1.375 tubes for my pedersoli sharps
I thing I found out that helps is since I use a cardboard wad between my powder and filler, the punch I use to place the wad before I pull it out I go around the edge a little to make a tapered edge on the top of the tube. Kinda like using a bullet head. Seems to work while there are a few that dont like to go in easy most pop right in and I use a small dab of clear nail polish to hold the bullet in. When I lube them I make sure the lube goes onto the edge of the tube to help hold the bullet in. I use a Tennessee bullet mold .544 Christmas tree bullet.