I'm here in Meridian for T-45C TS.
That's great! You'll enjoy the South in the winter time! There are some really great days where you can see all the way to Shuqualak from 15,000 ft over Meridian. Summertimes there are really hazy and there is that layer of crud at 5000 feet that defies description.
Going back to the original question, there is Sanders on 5th street and Garretts on 8th Street. I haven't been shopping in Meridian for a while, but I have family there and my kinfolks have patronized both places if they're still in business.
You have to understand about the deep south, in rural areas and small towns most gunstores are in pawn shops and hardware stores. And expect to stop and talk for a long time.
If you want to drive a little bit, go down 45 south to Citronelle Alabama, then stop and ask how to get to Quints Ace Hardware in Saraland, another 15 miles or so. Go in and ask the lady about the guns and she points you to a dark stairway, which is guarded by a giant moose head. Down the stairway is a gun store taking up the entire basement. Plan on making it a day if you go, because driving is a lot longer than flying. (If you fly, the trip from Meridian to the Coast Guard base at Bates Field in Mobile only takes about 20 minutes at 250 knots. But the field is still about 40 minutes from the store and car rental is across the field at the airline terminal so even flying will take all day anyway.)
If you happen to fly in to Pensacola there is a place, really kind of hard to find, called Gateswood Sports 'way out on the northside on Pensacola Blvd. This is one of those really fantastic old dusty hobby places where the guy knows everything there is to know about black powder and half of everything about reloading. The last time there I got his attention though. I showed him my medieval hande gonne, something he hadn't seen before.
http://www.silcom.com/~vikman/isles/scriptorium/firearm/handgonn.html
You do know that you can't buy a pistol in Mississippi, Alabama or Florida unless you change your residence to that state? Being stationed there doesn't count for residence unless you change your drivers license and state income taxes and establish a residence and not just the Batchelor Officers Quarters.