Well, good for him I say. I'm sure being absolutely terrorized by two armed thugs is not any kind of fun. Especially when they are waving guns and beating on you. So, kudos for the clerk. Downside: apparently it wasn't on his person. That gun under the counter isn't going to do you much good when you're stocking groceries or in the bathroom when the bad happens. It also doesn't do a lot of good when you're getting monkey-stomped on the ground. I'm hoping the clerk is re-considering his choice of firearm placement. I know if I worked a convenience store I'd look like I had Batman's utility belt on under my shirt and for some reason I would be carrying, like 3 "wallets." All-in-all, good outcome. Lessons to learn. Also, I'd love to see if there is store video released soon.
A little more in this story.
Also, notice that firearms aren't about "stopping power" on an assailant, they are about the power of stopping the assailant. Neither of these thugs dropped immediately to the floor, but they both stopped what they were doing to try and exit post-haste and that's what matters most.