3d printing for beginners…
What filament? 220-225 seems about typical of ABS. The gun had exrusion issues. Your temperature was probably wrong. Why not flip your gun on its side or upside down? Just 1 wall thick is not thick enough. For a nerf gun, try 2-3mm thick walls. For a pellet gun, try 3-6mm thick walls. I am guessing on the wall thickness. Make sure to have at least 30% infill overlay on your walls.