As others have indicated, sometimes, the firearm just doesn't fit - no matter how hard you try, or want it to. I've tried on two different occasions two different S&W handguns in the 6900 series. Neither time worked for me. I couldn't hit what I was aiming at, others had no problems killing the "X" ring. I consider myself a competent shooter, and used to shoot competitively with some success. I finally concluded that the S&W 6900 series just doesn't work for me. Same story with the Sig Sauer P-220.

Also, I'm struck by one thing you wrote: "...I also own a 6906 which is overly large for me necessitating me rotating my hand slightly to get a good trigger squeeze, but I don't have trouble shooting where I'm aiming with the 6906." Have you tried using something like a Hogue HandAll grip on your 3913: HandAll Universal Grip Sleeves - Handgun Grips - Hogue Products

Good luck,

Dave