I have an Orion 12 gauge.  

In fact this last time, I bought Marine kit with hand flares and the gun with a watertight case.   They're the name brand on boats.

First keep in mind, all aerial flares illuminate.  Distance and time just play a role.  The best times are with parachute flares.  Most commercial parachute flares are in hand held tubes although the 12 gauge guns usually come with at least one white shot non-parachute for illumination.  

Here's the problem with buying surplus or oddball stuff, these flares all are shipped under Fed Hazmat laws much like loading powder is.  What this does is pretty much like classic road flares makes the availability unpredictable and cost of individual ordering exorbitant.  Its not like the city boys goo to the range to shoot their flare guns.  Most people don't shoot them at all and folks like us may once or twice to see how it works and what to expect.  This naturally falls into that buy it when you find it and on sale if you can.  For example, a 4 pack of handhelds $20 at Walmart is $40 plus if we order it.  

The 12 gage you can convert for about $4 to 25mm.  That helps the availability thing some but by far 12 gauge is what you'll find at places like West Marine or Walmart..  

Now this fascination some have for gotta have a metal gun that looks like a gun is beyond me.  I own real guns.  Flares, I have flare gun launched, handheld aerial, hand held non-aerial.  Because my intention is to actually use these things not put them back for some fictional zombie invasion,  as they expire, I have to replace them.  Things like availability and cost become a real big deal.  

Do yourself a real favor, buy yourself some cheap flares even road flares, get out on a nice wet day, and see what they do and how they do it.  If you are lucky like I was, your buddy with his expensive hard to find flare gun will show up.  I think you will be surprised in first how inexpensive you can achieve your goal and second how much splitting hairs there is on this topic.

Tj