Probably was a Glock.
They are fine handguns I must admit.
This pilot should be suspended.
If he can't be trusted to properly handle a sidearm then how can he be trusted to pilot a plane full of people?
So the remaining question, is the pilot a gun nut or an imprudent owner of a firearm? (or is that the same thing?)

"When the going gets weird, the weird turn pro."
Hunter S. Thompson

"We know that there was never any danger to the aircraft or to the occupants on board," Alter said.
...
Officials did not say where the bullet hit.

The service declined to release additional details.

I'm pretty sure if I was a passenger on that plane I would be pissed.

Pissed yes, but not as much as the folks taking off yesterday and the main cabin door fell off. I suspect an inspection deficit.

Embarresed to say, I don't remember. Heard the story on the news and can't remember where it was. Private jet took off, door took off, jet returned to airport. No injuries.

"This is an extremely safe and reliable weapon," said Greg Alter of the Federal Air Marshal Service. "It's not going to discharge on its own, is the bottom line."

The pistol — a .40-caliber semiautomatic H&K USP — discharged Saturday aboard Flight 1536 from Denver to Charlotte, as the plane was approaching to land. Photos obtained by The Associated Press show a small entry hole in the lower side of the cockpit wall and a small exit hole on the exterior below the cockpit window.
...
And it's also not something that goes off by itself. ... Someone would have to squeeze the trigger."

Well, dang! How did it discharge? Will we ever know?

A U.S. Air pilot "accidentally" discharged his/her weapon on approach to Charlotte. No one was injured and the flight landed without further incident.

In totally unrelated news:

Charlotte, Pedestrian shot while walking dog, strangely enough, bullet entry wound apparently was in the very top of persons head and exit wound was in persons foot. No suspects or motive known.