I've used a couple of different free audio recorder/ editors in the past. Anyone can take just about any sound and turn it into a gun sound because it's so small. Adding on extras like a casing or recoil sounds takes time to find and come from the oddest places.
I made a P51 landing gear sound that took hours and hours to make trying to get the right sound desired while being timed to the actual game action. The sounds i used came from electric windows rolling up, factory machines of different sorts, doors closing, lazers firing and a few others i can't remember.
The editing tools allow you to manipulate the original sounds into what ever you want them to sound like. Cut, Paste or mix paste,... Edit the time, pitch or volume then Listen carefully while trying to match it to an action in an AH video.
I did the same for my tank guns firing, turrets turning, rockets shooting, bombs falling, plane scraping, debris hitting, Bullet Hit #4, 5 and 6 ect. All of my explodes I manipulated except one or two and never made a decent Engine sound........ but did have success with a couple of them starting and timing the smoke.
It's all very time consuming and some of those sounds were other peoples. I took what they made and made it my own by adding, subtracting or injecting other files or manipulation .... I suspect all of us interested in sound for the game do it.
I have always made personal packs out of other peoples packs by mixing and matching whatever I thought sounded best and putting them into my Sound folder. I SEE NOTHING WRONG WITH THAT and don't care what other people do with whatever i have created for myself or the community. I used to LOVE HEARING a sound someone decided was good enough to bundle into their own and distribute among the community once sent.