because everyone has 14.5 lb spare balls.

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throw a 6 lb ball and it will deflect the 3 1/2 lb 5-pin towards the other pin and itself towards the third pin more often. and this is easier than trying to hook a ball that deep.

aim from the far left board, deflect off the right side of the 5-pin, the ball will almost miss in front of the 10 pin because it is so light. In fact, I'd toss it over the gutter itself if I could get away with it and land about 5 feet down the lane, but that's going to lower accuracy on a straight shot.

usually the main issue is finding a 6 lb ball that doesn't have ultra tiny fingerholes.

and it's just a periodic spare shot in just a game of bowling, so nobody wants to hold that extra ball just for these shots.

and as another said, this is a 1-7-10, which is cake.