Final answer:
The MK9 Spotting Rifle is associated with the BAT Recoilless Rifle and was used for aiming by firing tracer rounds. The exact length is not commonly available and would often be found in specialized military manuals.
Step-by-step explanation:
The MK9 Spotting Rifle is a specific firearm that was historically used in conjunction with the BAT (Battalion Anti-Tank) Recoilless Rifle. The MK9 was designed to help the gunner aim the primary, larger recoilless rifle by firing a tracer round that followed the same trajectory as the larger round would. However, the exact length of the MK9 Spotting Rifle is not readily accessible in public domain sources. For precise specifications, one would typically need to consult military manuals or other specialized references that may provide detailed dimensions and characteristics of such weaponry.