Mark Serbu Shoots RN-50 Rifle Chambered in .50 Spotter-Tracer

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.

Hrachya H

Being a lifelong firearms enthusiast, Hrachya always enjoys studying design, technology and history of guns and ammunition. His knowledge of Russian allows him to translate and make Russian/Soviet/Combloc small arms related information available for the English speaking audience.
Should you need to contact him, feel free to shoot him a message at


  • Hellbilly

    I have an almost-full box of these cartridges that is decades-old. After researching, I found these vintage cartridges are rather scarce and can sell for $20 each. That’s some expensive ammo.

    • datimes

      10 round box $49.95 @ CDVS.US

    • 22winmag

      Collectors overpaying aside, the bullets are on GB for about 50 cents and the brass for $4 per 50 pieces.

  • SGT Fish

    yeah, but you can just reload 50 spotter projectiles into 50bmg cases. I guess this would be real good for locations that 50bmg is specifically banned. wasn’t California like that, at least at some point?

    • Christian Hedegaard-Schou

      Yup. This rifle would be legal in CA chambered in .50 Spotter.

      .50bmg chambered rifles are explicitly illegal in CA.

      Funny enough, since a semi-auto M2 is not a rifle (Just a Title 1 ‘Other’ firearm), belt-fed .50bmg M2’s are perfectly legal in CA. (Since they are not rifles, they also cannot be assault weapons in CA), but you are limited to 10rd belts 🙂

      • mosinman

        how about a bunch of linked 10 round belts? 🙂

        • Rick O’Shay

          “That’s not a 200 round belt. That’s just 20 interconnected 10 round belts!”

          • mosinman

            Great Scott!

      • gunsandrockets

        Tracer and incendiary ammo felony crime to possess in Commiefornia.

        • Christian Hedegaard-Schou


          That doesn’t have anything to do with the rifle’s legality, however.

    • jestertoo

      CA bans all rounds except for ball, so no fun ammo there anyway.

      • SGT Fish

        good point. though you can load ball projos into 50 spotter cases as Mark does for the indoor shot in his video

        • jestertoo


      • Thomas Maker

        How do you humanely hunt with ball ammo, it would make anything under .44 cal. a non starter for all but the smallest game

        • jestertoo

          This only pertains to 50bmg. 50bmg is not about hunting (usually). It’s about either long range target shooting or anti-material. Cali bans all of the anti-material type rounds. There are not very many hunting type rounds.

    • Cymond

      50bmg is still banned in CA, but they’re allowed to have other stuff like 50 cal wildcats, 416 Barrett, 408 Cheytac, etc

      • Whitechapel Charlie

        Because reasons….

    • Flounder

      California has 50 BMG banned by case dimensions. A few other places around the world have done similar things. It is the primary reason .510 DTC exists. Which is a slightly modified 50 bmg with almost identical balistics and interior volume.

      This 50 tracer is a neat curio or novelty but that is kinda all it is.

  • Julio

    Great story. A guy finds himself with a load of surplus ammo in an odd calibre, contacts a can-do gunmaker who not only chambers rifles for it but also promotes both their businesses on blogs and YT, everybody wins, and I want one…but I expect pigs to fly before either the rifle or the ammo cross the Pond… (AFAIK – and depending on the specific characteristics of the projectiles) they would be legal here in the UK, too. Sob.

  • politicallyincorrectshooter

    I have long felt that the .50 spotter would make a better basis for a rifle round than the full power version. An improved version of the case could probably approach the BMG performance with modern powders.

    • Anonymoose

      But how is the trajectory out past 1000 meters? We have other wildcat rounds like .50 Peacekeeper (.460 Weatherby) and .510 Snipe-Tac (.408 Cheytac) that have more commonly available parent cases to form.

  • Dan

    Displaced shoulders huh? I had a 98lb 8th grade girl shooting my .50cal yesterday. She shot 10 rounds her shoulder seemed fine today. The super dooper all powerful .50bmg myths never get old. Oh wait yes they do.

    • iksnilol

      I agree, Amerikanski are weak. I mean, 20 kJ of energy and people say “ow, my shoulderowski”.

      Really, it is pathetic.

  • trjnsd

    The little M-40 “Ontos” anti-tank vehicle mounted six of these 106MM Recoilless Rifles. Neat little vehicle. Funny aside – they were powered by a Chrysler 318 Cubic Inch engine, same as in Dodges and Plymouth cars and trucks. Some young American soldiers caught on to the idea of replacing some of the engine parts, like intake manifold, carb and cam, with HI-Performace parts just like they would have on their cars back home! The results were reportedly pretty amazing…..!

    • Blake


  • gusto

    Sweden had this system on a tank IIRC

    • gunsandrockets

      I believe the original UK Chieftain tank used a co-axial .50 caliber spotting gun for assisting aim of the main gun.

  • gunsandrockets

    Found this quote, don’t know how accurate it is…

    “BTW, M48A2 ammo fired an 833 gr. projectile with 110 gr. of IMR 7383 @ 1745 fps measured 78 feet from the muzzle. Ammo data from TM 43-0001-27”

  • Thomas Maker

    For those in an AO that specifically forbids 50BMG but have no issue with other .50 cals, a bolt or single-shot rifle in this caliber and a 50BMG reamer squirreled away for a rainy day may be worth considering…

  • Wild Bill

    Cool! The 50 Blackout.

  • Dracon1201

    How about those SU15s, though? 😂

    • Henry C

      one can dream

  • mazkact

    50-70 is a lot of fun so is its Swedish cousin 12.7x44mm.

  • Thomas Maker

    Making guns is easy, it’s the steel quality and heat treatment that is somewhat more iffy when one gets to 50bmg level cartridges, and if things get rainy in a way that would see a real need for a 50bmg rifle ammo availability would be very much dependent upn how common something was and chances are access to a lathe will be possible is a situation like this