fionashanahan, If really stuck, give a touch of penetrate oil down the jaws of the chuck, put the drill in reverse, firmly grip the chuck with gloved hand and gently squeeze the trigger up the point that you cannot hold on. If it still doesn't budge and you're not able to hold the chuck against - Ryobi P202
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To Retighten A Loose Chuck; Maintenance; Chuck Removal - Ryobi P202 Operator's Manual [Page 10]
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Ryobi P208 One+ 18V Lithium Ion Drill/Driver with 1/2 Inch Keyless Chuck (Batteries Not Included, Power Tool Only) : Tools & Home Improvement
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Unlocking the Drill Bit Chuck on my Ryobi Hammer Drill