has driven me back into reloading. I shoot almost exclusively light 7/8 oz loads in old 12 ga sxs. The only primer I could find was the Cheddite and the hull I chose was the Rem Gun
Club hull. It takes an extreme amount of pressure to seat this primer. I used to use W209 primers and don't recall this being an issue.
You have a double whammy going on. Gun Club hulls are steel base and harder to resize and take more force than STS\Nitro 27 brass washed base hulls. I have thousands of 20 ga. GC hulls but will not use them until I totally use up my old 20 ga. AACF hulls. Which will likely be never considering how many I have yet.
The Cheddite primers in Remington hulls even STS and N27 take more force to seat the first time than W209 primers. My 12 ga. MEC 9000 is attached to one of Ben's first automates, made before he sold the patent to MEC and it became the 9000E. It has well North of 100K rounds though it and is worn, I should send it in for a rebuild if I ever stop using it. Cheddite primers would not always seat fully when I first switched to them, run your finger nail across the base and make sure they are fully seated.
I called MEC about it and they said to install a second clamp on the reprime tube to hold it securely in place. And replace the 1" spacer on top of the spring with a half inch spacer that they sell to make room for the second clamp.
He told me ALL MEC presses leaving the factory since January of this year come this way. He cited 2 reasons. The increase in use of foreign primers and increase in use of steel base hulls.