Touchback or Safety?
 

A has the ball 2nd and 4 on their own 5 yd line. A1 takes the snap and hands to A2.
A2 fumbles and while the ball is loose B3 attempts to recover, dives on and muffs, and the ball goes into and through the back of the endzone.

Since the fumble by A2 was grounded, the covering official must determine if the muff by B3 supplied a new force that caused the ball to enter A's end zone. If so, it would be a touchback. 8-5-3c

As suggested, this is purely a judgment call. As pointed out the key factor is whether the loose ball has been grounded. If so, "After a fumble, kick or backward pass has been grounded, a new force may result from a bat, an illegal kick or a muff." (NF: 2-13-1)

NF: 2-13-3, "The muffing or batting of a pass, kick or fumble in flight is not considered a new force".

NF: 2-13 is one of those rules it is worth rereading, carefully, from time to time.

touchback or safety
 

Correct me if I'm wrong, but unless the offensive team caused the ball to go out of bound in the end zone (after the fumble they would have had to had some action to touch the ball again to direct it out of bounds), it can only be a touchback if the defensive team knocks it out of bounds. Or, a touchdown obviously if they recover it in the end zone.
Don't forget on that call if the offense is last to touch it before it goes out of bounds it would be a safety I believe.

We had a call like that 2 x's this year and it's a tough call with 7 people batting that thing around trying to decide where and when(and by who) the new action takes place. Either way, 10 yards is a long way to bat that thing around. It would be so much easier if it was just clearly kicked out!

Hey, if anyone disagrees, I'm here to learn to!

Thanks

NCAA it is a safety as the impetus is charged to the fumbler

It has nothing to do with who is the last to touch it.

If A1 fumbled the ball and it was rolling toward the end zone, B's touch might not be considered a new force. If the R judged the fumble put the ball into the end zone, then it would be a safety if it goes through the EZ and OOB, irregardless of who touched it last.

Remember, when judging impetus/force, you must judge if it has enough force to get to the goal line, not if it had enough force to roll out of bounds.

Ex: If the fumble in the OP had enough force to get to the goal line but not the end line, then the muff by B was not a new force.

If the officials rule that B provided a new force that put the ball in the end zone then it is going to be a touchback or a touchdown. As long as there are no fouls, it will never result in a safety. If B falls on the ball in the EZ, it is a touchdown. The ball going OOB due to A or B muffing the ball or A falling on it will result in a touchback.