best time travel gun

curious as someone who has never owned a gun what would be the best gun to travel back in time with, basically the strongest most efficient gun that you could also perform maintenance on and create ammo and such . obviously depends on the time period but i was thinking something much pre industrial/medieval ish after i had a dream i was being chased by a knight, but in general something that would be mythically overpowering to any potential foe that could be encountered