I've made citizens arrests before, but none armed. However, a long-time friend of mine once made an arrest at gun point.
The BG had broken into a building (lockpicked the door) after hours and was trying to liberate change from a soda vending machine when my partner walked into the building after finding the door unlocked. When confronted and told he was under arrest, the BG brandished his crowbar and said, "I'm getting out of here," and began advancing toward my friend (who stood between the BG and the only exit). My friend drew down and made it clear to the BG he wasn't going anywhere until the cops arrived.
When taking statements for their report, the officers repeatedly told the BG that he was lucky not to be dead, and that given the circumstances, it would have been a clean shoot if he had made ANY move other than dropping his weapon.
Note that at the time of this incident my friend was POST certified and had all his BSIS certs in order to carry a loaded firearm. Also of interest is that this occurred about 100 feet from the main PD station in Stockton, and the police response time was roughly 10 minutes from the time I placed the call to 9-1-1 (I was the shift supervisor, so I was the one making the call on his behalf after hearing his radio traffic requesting help).