In the first episode, the Mythrol the Mandalorian captured goes to use the vacc tube as a guise to poke around the ship and while snooping punches some buttons on a keypad. Doors on the opposite bulkhead open revealing a cabinet full of weapons. That might have been a fluke but it happens two other times in the series. When the Jawas strip the Razor Crest for parts they take all the weapons and when Din is transporting Ran's crew to the New Republic prison ship, Burg again punches the keypad and the weapons doors swing open.

I feel like it is actually a locker for the weapons for two reasons. The first being that it is good practice to keep weapons locked up even if you are a bounty hunter and need quick access to them. The Razor Crest is a pre-Empire military gunship so it may not originally be a locker, again for ease of access, but if you are transporting bounties you might want to change that. The other reason is that both the Mythrol and Burg seem to punch more than a few buttons to open the weapons cache. It crossed my mind that this could be a trap used by Din to see who he could trust and who he had to freeze in carbonite but this seems unlikely since it showed Greef Karga's goon unloading the bounties and they were all frozen but they were all criminals or at least had bounties out on them and thus all failed the test.

Why is the Mandalorian's weapon locker onboard the Razor Crest so easy to open? If it is shown that the cabinet is not intended to guard the weapons and keep them from unapproved use then I will accept that as the answer.