How do i make my Bison 301 bb gun more accurate?

When i got it a month ago, it shot straight every time. Now the BBs start to either drop or go to the sides. i rested the gun on something to ensure it was straight and it still veered or. The hop up is on the lowest as im shooting it inside my house. Im not sure how to adjust the sight and google didnt come up with anything. Help anyone? Thanks

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