Cody Firearms Museum in Wyoming is great but unless you are in the area, forget it.
+1 on Springfield Armory Museum in Springfield, Massachusetts. It always looks like it is about to close down but it isn't and it is splendid. At least half a day -- they have closed the special tours of the collection behind closed doors.
In the same region is the reknowned Higgins Armor Museum. Worcester, Massachusetts.
The National Firearms Museum (NRA Headquarters, Virginia, across the river from Washington, D.C.). HELLO EVERYBODY!!! Gives the Armouries at Leeds a run for its money. Well, I spend a day per
floor at Leeds but you'll probably want the better part of a day at NRA. Yes, in Leeds, the place to go, like you probably, I stay at the Holiday Inn across the mall and am at the museum when they open till they close for days. Taken a class there. I could live in the bookstore alone as long as I get to eat the prepacked sandwiches the two cafeterias sell. Ooo, and the bar/fancy cafeteria! Supposed to go to The Armouries at Portsmouth next tho.
United States Army Military Academy -- West Point Museum. Highland Falls, New York. Amazing gun museum.
Frazier History Museum (Louisville, Kentucky) -- it is the US branch of The Royal Armouries and USED to be called the
Frazier Historical Arms Museum -- enough said!? between demos and performances of living history the rest of the museum is an all-day event. I spent days here too, like Leeds (almost a floor per day with regular breaks to see the presentations that are so diverse and compelling -- there are quite a few per day and it would take weeks to see them all as they do so many. A truly special place in Western Civilization my friend. I have been introduced to Mr. Frazier by new-found acquaintances who knew of our coming and study there. Am a member of the Museum, and we even sent the living historians there Christmas gifts. They are TRAINING LEEDS how to do what they do!!!
Rock Island (Illinois) Arsenal Museum, Armor Park, Confederate Cemetary -- if you're around there.
Fort Ligonier (Western Pennsylvania) Wait till you see the new historic artillery park outside the fortifications and the little modern museum on site too. Very nice.
Fuller Collection at National Park Service Chicamauga and Chatanooga (Tennessee) Battlefield Park -- a gem in the middle of nowhwere (well, sorta). If you are an historic arms guy (primarily US) this is something else. I was at the battlefield on group tour. A fellow tourist mentioned they had some guns there, my being the group gun "expert." I saw a sign about the Fuller Collection. "Claude Fuller, the famous historic arms author by any chance?" I asked a Ranger. Of course: this was his personal, donated, collection. OMG!!! This place brought a tear to my eye, made my heart race, and I was lightheaded all the while I was there. Probably just that I missed lunch, but then I almost missed the bus too... Need to get back there someday. It is not big, but it is dense and rich. I would need a full day there, and a stool to sit at this point. Who even knew this was hidden there:
http://www.youtube.com/watch?v=HtlUDF8hzIk
FYI, most popular museum in US is the Air and Space Museum of the Smithsonian on The Mall in Washington, D.C., a majestic town (which might be even better in January 2013). The entire mall and surounding area itself, the architecture and monuments, are art. And some of those monuments should be shared by you too (incl. Vietnam)! Someone mentioned Wright-Pat(terson) AFB Museum -- guy stuff planes and such -- the real deals right there!