After reading all the posts, my first suggestion is to obtain the Hornady Case gauge setup, the proper modified case and attach the bushing and base to a dial caliper. slip a bullet in and insert the gauge, push the bullet till it stops, then remove and measure.. Compare that to a factory round.
It sure sounds to me like the throat in that chamber is way small, or even non-existent.. I'd also call Cooper and find out what round they used to test-fire.
Ex- USN (SS) '66-'69 Pro-Constitution. LET'S GO BRANDON!!!
