The screenshot is a Eevee render, and the cannon is aligned both in angle and vertically with the top’s seam.
I will use this later when i learn about modifyers to apply the Boolean one, so the meshes don’t intersect with eachother once i do that.
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Welcome to this site.
Nice simple gun turret.
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Thank you! 
Depending on how meshes intersect. But is no problem for blender!
Only when having faces on the same spot (for example after duplicating), then you get strange artifacts. you will learn. Have fun!
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very nice.
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Thank you for the input