[m&p45acp10+1]

You might try turning your sizing die down another half turn. Cam it over, and hold it for at least two seconds. Then take the case out. See if you can push a bullet into the case. If not then go to flaring. You should be able to barely start the bullet into the case. Now when seating. First get the depth you want. Then adjust for crimp. You should be able to run your finger nail down the bullet, and feel a slight catch at the case mouth.