I’m trying to make sure it fits the p250 40 caliber compact before I buy
Referenced product: MAG-MOD-F-9-21
The 21rd magazine will fit in the P250 .40 grip module but it is a 9mm magazine so it will not accept .40 S&W ammunition. If you have a 9mm slide and are running 9mm ammo you're fine. Full size magazines will fit in compact (and sub compact) grip modules but there will be a gap between the grip shell and the magazine floor plate / extension (in this case).