[shortwave]

I agree with Webleymkv.

I just wouldn't be sanding/modifying a part on a brand new gun that I thought was the cause of the pistol malfunctioning or not operating properly unless I was told to do so from the manufacturer.
Unless I was willing to be okay with the manufacturer voiding my warranty.

Too, you said the slide would release almost on its own with the standard mag. before you started sanding on the release. You may have a hard time convincing the gunsmith at S&W of this if he/she realizes you have been sanding on the release.