Look at the photos here. On those folks in a firing position, notice their right elbow. Notice only one appears to have his elbow well tucked into his side. (There are other images, not shown here, that make this more dramatic.)
Lesson 1. (This is far more important with a gun with recoil, e.g., 308 or 30-06.) Get that elbow secured as best you can. “Spray sticky” on the mat and elbow pad and even at the inside of your bicep; on soft ground, dig a pit with your elbow during prep (this is good during rattle battle where accessories like mats aren’t used), do what you can – because if this elbow slips, your position is dead.
Lesson 2. If you shoot some pop gun like an AR in 5.56, there is no appreciable recoil and you can have a pretty sloppy position and endure. (Some of the photos show these.) But, in doing so, you are making a habit that will hurt with a real gun. So make good habits no matter whether shooting .22LR or .30-338.
Example. I was shooting a match at Camp Pendleton. My offhand was, for me, a par, mid 90s; but my 200 rapid was a 199-9X. Then we went to 300. That damn Pendleton dusty sand ate me alive. EVERY SHOT SLID MY ELBOW. Needless to say, my day in the money was gone. A hard learned lesson.
Want to see perfect LEFT arm elbow position? Look at first photo, guy in brown jacket, no cap.
Lesson. Get that left elbow UNDER the rifle. (Magazine on wrist/forearm is a plus, but not a factor with a bolt gun and impossible with 30-rd mag.)
Other issues – note placement of right elbow.
(1) it’s off the rubber (possibly he gets better traction on canvas; can’t tell. But the purpose of the rubber on the mat is to provide improved purchase.)
(2) could be tucked closer to his side.
Ancillary comment – “size matters.” I’m 6’1 and wear a 16½-36 shirt (kinda long arms). I shoot an M14. My prone position is like this fellow’s, head forward, with nose up against rear of receiver. However, with an M14, my left hand is up against the sling swivel! Unless he’s a midget, I’m not sure how he has his hand immediately in front of the action.