Apparently he visited the place before, several times, casing the joint.
Ok...
...Apparently he cased the bar with very admirable precision (how does one find out if a guard is carrying concealed or not? how many guards are there? what is bar policy regarding guards carrying concealed?)— or else he took a large chance, and was at least somewhat over-prepared. His preparation would perhaps (?) seem to indicate that he was prepared for at least the possibility siege in contrast to a quick shootout.
...Or, maybe...
Maybe someone else cased the joint, and gave him the info(?)
Which would be more likely, I wonder?
I would be tempted to presume the worst if any potential antagonism might be involved (in general, not specifically anything involving weapons). The most simple way I could think of to find a bar with unarmed guards— not that I would ever have any motivation to do that— would be to have trustworthy confederates do the difficult legwork in advance. the more trustworthy confederates involved, the greater chance of success with the “goal.”
disclaimer, i am no one special and i have no insights— just curiosity.