Also mate don't bother with the chrony, just get zero spot on and head out to 3-400 odd and shoot a group without dialling or holding over (be sure to have a target set up to take 12-24 ish inches of drop so a tall target). Measure the drop in inches or mm if you are a MIL guy and feed speeds into an app until the drop matches. Far more accurate than the lower end chronys.
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