What kind of fire arms are you looking for? Present day, historical, futuristic? For present day, a big film I go to is HEAT. The gun fight in that movie is one of the best I have seen/heard. A go to for sound design ideas. For over the top fun, "The Expendables has some really good bits too.
For a historical movies, "Saving Private Ryan" and "A Band of Brothers" has some real good reference to design as far as audio, some of the best for a period piece. As well as
"The Patriot" and "Glory"
Another movie that as some really good design is "Star Trek - Into Darkness" For scifi, it has some really good variations of SFX. Another is The Matrix, I know you might think it is a scifi flick but they use a lot of normal fire arms in that movie. The sequence where they go rescue Morpheus is just brilliant!
Sorry for being all over the board on this but I am sure when I get home and look at my collection, I could add 10 fold to this. I hope this help in your quest. Let me know if you would like more, I would be happy to help out. Oh and as a tip/technique I try to imply when designing gun SFX. A little thing i picked up from the video game world, there are 3 sounds for every weapon:
1:The mechanical - The actually working of the weapon be it the shell loading to the hammer clicking or the revolver spinning.
2:The "Boom" - Pretty self explanatory. Be sure to give it the style and characteristic of the weapon
3:The human element - The actual use of the weapon. From it being handled by an individual (loading and unloading to fire the weapon)
Hope this help with you ventures! Best of luck!!!