Mark Serbu, the owner of Serbu Firearms, has released a video where he shoots an RN-50 rifle chambered in rather unusual .50 Spotter-Tracer caliber. This cartridge looks like a shortened wildcat based on .50 BMG.
However, it is actually a military cartridge used in M8C spotting rifle. The latter is a coaxial rifle chambered in .50 Spotter-Tracer and mounted on the 106 mm M40 anti-tank recoilless rifle.
Originally, the idea behind this combination is that the .50 Spotter cartridge ballistically duplicates the trajectory of the 106 mm ammunition. The M40 operator would shoot the .50 caliber gun first and once he confirmed the target engagement, then he could immediately send a follow-up 106mm round. The cartridge is loaded with a special projectile which works as a tracer and also has an explosive charge in it. So that bullet allows to see its flight trajectory and relatively easily spot the impact point.
By shooting the spotter round first the AT gun operator makes sure his calculations are correct. Even if he misses the target, he loses just a .50 cal cartridge and not the 106mm one. Also in a case of a miss, he won’t compromise his position as much.
For a modern civilian use, the RD-50 rifle chambered in this caliber will be a softer recoiling .50 cal rifle shooting same .50 BMG projectiles. That sounds like a lot of fun without displaced shoulders at the end of the range trip. Well, the question is how easy are those .50 Spotter-Tracers obtainable compared to .50 BMG and if their prices are comparable.