- Maple_Leaf_Eh
On the M40 infantry 106mm Recoilless Rifle, the .50 calibre spotting rifle is a big bolt action (?) that is mounted on top of the main tube. The round is shorter than the conventional .50 cal. Its ballistics are matched to the main projectile.
We operated M40 RR's in my Air Cav Troop in Vietnam (off of M151A1C's and two "unauthorized" M37 "Beeps"). My recollection is that the spotting rifle was semi-auto (with a big honkin' magazine), but I'm open to correction. It is correct that the round was ballistically matched to the main round, but there was not the expectation of a 1:1 correlation of a hit when fired. That would take forever unless the crew was shooting at a distant bunker (and probably unoccupied, or at least soon would be after a spotting round shot or two).
In practical terms, it was more likely to be used in perimeter defense, and at shorter ranges, though the RR crews practiced on more distant targets. I was in the Scout Section, so this was not a primary duty of mine.
Don, I am in possession of a manuscript photo of the spotter rifle* installed on the XM551 as of the last few days, but as it's in preparation to go to press by the author, I'm not in a position to post it. However, I'll let you know as soon as the publication is available, which should be very soon.
*NOT the same device as on the M40. Different caliber and designation entirely.