Is the holster closed (stitched) across the bottom so that the barrel is bottoming out? If so the holster is the wrong fit for that Ruger send it back for a swap for the correct size. This is the first option and best one.
If the barrel is not bottoming out, you can try wet forming the leather and getting a little deeper fit. Dip holster in warm water, drain and drip dry. Set in a plastic bag in frig for over night. This is called casing the leather. Now oil the gun and wrap in saran wrap, carefully push/force (within reason) the gun into the holster until it bottoms out and the strap snaps easily. The strap will stretch a little and the gun should insert a little further into the holster. Set in a warm (not hot or heated) dry place on a towel and let dry completely on the outside with the gun installed. You can get a decent "custom" fit by using a little finger massaging of the leather around the gun while it is still damp. Remove the gun and allow the holster to dry completely on the inside before using - very important. Don't hurry this give it a couple of days to dry fully before using.