In the MEC manuals ...it'll show you how to close up that hole in the center...its a combination of crimp depth ..and getting any "swirl" out of the crimp fingers - in the crimp start, crimp finish and final radiusing of the shell in stations 4, 5 and 6....
I know the sizemaster is a single stage press...and I haven't used one for a long time ...but your adjustments are in the same "crimp start", the "finish crimp", and "final sizing" processes on your press.
http://www.mecreloaders.com/document...sizemaster.pdf
page 10 of the Sizemaster's manul ...tells you how to make those adjustments to Crimp Start, the Final Crimp, and the final Sizing or radiusing of the finished shell.
Yes, if I were you, I'd work on the adjustments to remove that hole and make sure the crimp is as close to a factory shell as I could get. Measure the depth on a factory shell ...vs your reloads / my hunch is the biggest issue is in the depth.