Speaking of the one with the funky head, how does one remove it?  My .243 project gun needs an ejector pin.  Do you have any pics of the case you managed to fire?  What it blew out to look like might help determine what it is, a chamber cast might be in order.  6.5x257 Improved might be a possibility, though still a wildcat, one would have been able to make it from more readily available cases than the once hard to find 6.5x55.