Where is Sakamoto Ryoma's gun?

I'm kind of back from travelling.  I've only used 6 days of my 7 day JR pass though so may do a day trip tomorrow.

Everywhere I've been in Japan has a replica of Sakamoto Ryoma's gun.  I think I've seen 500 replicas (give or take a few), but where is the real one?

I can understand with historical documents and those kind of things, you might want to display replicas so the originals don't get damaged or destroyed, but this is a gun.  It's not so frail.

My hand (real), gun (replica)