I have em ...they work ..heck the price of one of em , is cheaper than 1 high end shell , and great for if you have to reinstall you our sight for any reason
It is a great tool to use if your shopping for a gun
They have a margin or error of less that a few inches , if you rotate in the chamber you'll figure out the center , if you wrap the shell front and back evenly with tape that margin will tighten up significantly
This does not guarantee that you choke threads are not installed off center or misaligned , a dial caliper will help indicate that by measuring the wall thickness all around