Shotshells vary in size, but only slightly. TacStar side saddles are somewhat tight, though they have never cause me grief when I have used them. I decided against them for different reasons.
There's no need to insert the shells all the way and with use, it will loosen up. This is anticipated from the factory. Better tight than to have shells fall out. Yes?
Get yourself a length of 7/8" dowel at the hardware store, (actually 7/8" dowels are just under that, so even better), and a cheap 20$ heat gun if you don't have one.
Cut yourself however many dowel pieces as your saddle will hold and force them on. Careful not to burn/melt the saddle, gently heat the shell holder a little at a time to relieve the stress so it conforms closer to the dowel's diameter.
Leave the dowels in over night.
It'll never set to the same size diameter as the dowel, but it will open up just enough to make it right.
If the dowels don't all fit on at the same time, do every second one and repeat alternating.
Don't be a tool and try this with actual shells. Shells are not fat enough to make this work anyway. And take the holder off the gun if you do this.
You could try some talcum powder, or some graphite on the inside of the holders for now.
But really, I think you're being too anal about it. So just use the damn thing and it will loosen up on its own like it's supposed to.