Just received a 450/400 NE 3" that I had built on a J.P. Sauer SxS 12ga shotgun, and ran across an issue that I'm not sure how to go about having corrected.

I have 10rnds of live Hornady ammo left over from a previous 450/500 NE 3" and 80 or so brand new Hornady 450/400 NE 3" brass. Went to shoot the gun tonight just to see how it handled. Of the 10rnds, I was only able to completely close the gun on 4 rnds. By completely close, I mean, top lever back to center or just right of center, and the Greener cross-bolt flush with the receiver.
Took some initial measurements of the brass rim thickness. Cases with a rim thickness of .058" or less allow complete closure and lock-up of the action. Thicker rimmed cases do not allow full closure and lock-up. Measured pretty much every case - loaded and unprimed new brass that I have. Rim thickness falls between .057" and .062". From what I can tell the case rim thickness should be .065" from the case drawing I was able to locate. I could understand having an issue if the rim thickness of the brass I have exceeded .065" but it doesn't. This issue does not bode well with me and not sure how to go about correcting it. The gun closes and locks up tight when the chambers are empty - every time.

http://www.loaddata.com/members/search_detail.cfm?MetallicID=5750

What's the next step?