Somewhere on this forum I read that there is a gun shop in Windhoek where one can store his guns for a few days. I tried to do a search but with no results. Any information will help.

I believe it's Rosenbergs Gunshop in Windhoek. They would be a good place to start.

Store the guns with your PH.

BTW, once you get the guns into Namibia (and the way to get a gun in legally is with an Invitation to Hunt issued by your PH) you can keep it with you throughout your tour, even if you are not actively hunting.

Namibiahunter