the ball 15 a firing lever 31 also pivotally mounted at 32 for movement about a horizontal axis a link arm or stop member 33 connected to the lever 31 normally cooperates with an extension or lever of the stop arm to hold the stop arm 29 in position as shown in fig3 so that the ball 15 cannot move down the guide member 11 forward movement of the lever 31 will disengage the link arm or stop member 33 from the stop arm 29 or extension thereof which will allows the stop arm 29 to rotate clockwise and lie substantially flat along the base of the guide member 11 as shown in dotted outline in fig3 which will allow the ball 15 to move down the guide member 11 a further microswitch 34 is provided in the carriage body 13 in the path of movement of the firing lever 31 placed along the bottom of the guide member 11 is a friction pad 35 this pad 35 can be made of cloth rubber foam plastic or other suitable material that will absorb any lane conditioner carried into the mechanism on the ball 15 and provide friction for the ball 15 to drive itself out of the guide member 11 an optical switch 36 see fig2 is provided on the side of the guide member 11 to sense the presence of a ball 15 on the breech 18 when the breech 18 is slid into the guide member 11.