Here's a guy who successfully buried a gun for 15 years. To summarize he oiled everything with rust preventative oil, sealed it in a mylar bag with dessicants, then put in a PVC pipe and sealed on both ends. Said it worked just fine.

https://www.backwoodshome.com/bury-a-gun-and-ammo-for-15-years/

Here's a guy who successfully buried a guy for 15 years. To summarize he oiled everything with rust preventative oil, sealed it in a mylar bag with dessicants, then put in a PVC pipe and sealed on both ends. Said it worked just fine.

https://www.backwoodshome.com/bury-a-gun-and-ammo-for-15-years/