I would say a laser boresighter. preferably the chamber mount kind. Other than that:
you can take fishing line and either tie it through the peep sight or create a brace that allows the line to sit centered in the peep. Run the line down the top of the barrel, through the muzzle and down the bore, back to the receiver. Pull it tight and tie off. The inside-top curvature of the flash hider should keep the muzzle end of the line centered. stretch the line so that it balances on the front sight it self. Inspect. The line will allow a better visual to see there is an offset between the front and rear sights. IE: the line should be perfectly straight with no bends, left to right as it balances on the front sight. very low tech, I know.
you can also go though a box of shells. sight at 100
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