So being the good husband that I am, I was going to clean my wife's new-only-shot-once Cimarron Lightning revolvers, and found out the hard way that these things have a base pin that only works one way.
The base pin has dimples on only one side that the base pin spring goes over. To help with making sure you orient the base pin correctly, the top of the base pin is flat in one spot. You must have that flat part against the barrel of the revolver. It is very easy to accidentally turn the base pin as you're sliding it into the cylinder, and that's what happened to me. By the time I had this figured out, both revolvers were stuck.
To fix it, I removed the base pin screw/spring to make my life easier, then gently tapped the base pin out a bit to free the top of it that had gotten stuck against the barrel. I then carefully made sure I slid the base pin back in correctly, and I was back in business.
Flat top of base pin:
Beveled part that goes over the spring: