Anyone here a locksmith? (rotary lock)

Or anyone know rotary combination safe locks? I went to look at a used gun safe this afternoon. I got there and it's a real nice 'National Security' brand safe, looks like 21 gun or so storage, scratch & dent special. Everything looks good till he went to demonstrate the combination is correct. Wouldn't open. I tried it and got it to open a total of 2 times out of maybe 10. The guy was a nice guy and I don't feel like it was a hose job, he runs a small safe company, but deals mostly in punch key safes that are all under warranty, this was a trade in.

Being the cheap bastard that I am I offered him what I thought the box was worth in the event I had to replace the lock entirely.

I know a little bit of how these locks work. When you turn the dial to the numbers in the sequence you can feel the tumblers catching...on the two times it opened. The times it didn't open I could feel all but the fourth tumbler catching. I think that the last one is stuck somehow and maybe needs some graphite lube or something. I left the door open from the last time it worked so I can remove the lock assembly if I needed to take it somewhere.

Any thoughts? I paid less than half of what I think it would be worth in good condition, and I just can't see a decent mechanical rotary lock breaking. I am thinking it just needs some lubrication or something. Would I be hurting anything if I pulled the inner door and used graphite lube on the tumblers?