In military parlance “X” is experimental. As far as cartridges go there are some very interesting experimental cartridges that were researched and developed during our history. The military had their wildcats as well.
Henry & Spencer Rifles
US military cartridge development began with the Henry and Spencer rifles. Both were rimfire. The Henry was carried by those who bought or captured their own. The Spencer went on to serve in the Indian Wars after the Civil War. They were both under powered and a bit anemic. But in the muzzle loading era they were the assault rifles of choice. The Spencer held 7 rounds where the full-length Henry held an astounding for the time, 15 rounds without reloading.
Trapdoor Rifle Reproduction
After the war the ‘Trap Door’ conversions of the standard rifle became the rifle of choice for the military. The first officially adopted center fire was the .50-70 that soldiered on from 1866 to 1873 when the Army adopted the .45-70.
High Velocity Bullets
High velocity (compared to black powder cartridges) was the .30 U.S. or 30 Army with the civilian designation of .30-40 Krag. A small bore at the time the Krag utilized a jacketed round nosed bullet. Then in 1897 the Germans introduce the 8x57mm Mauser. The cartridge was so astoundingly right, that its features have been incorporated into nearly every rimless cartridge since. Even the 30-06 was influenced by this rimless design. Between the wars the US Army were experimenting with a 7mm design. The US development was the .276 Pederson (7x51mm), in fact the first Garands where build around this cartridge. General Douglas MacArthur as Chief of Staff nixed the .276 Pederson and specified the .30-06 for Garand.
Vintage Military Ammo
Fast forward to the Vietnam War. The M193 cartridge for the M16 only had a 55-grain bullet, not well suited to thick jungle as all pointed bullets can be deflected by bush. The heavier 7.62×39 Soviet cartridge could penetrate jungle bush more reliably than our 5.56x45mm cartridge. In 1962 the US Army saw an article referencing a 458 Winchester Magnum cut to a 2” case length with the article appearing in the 1962 June issue of Guns and Ammo. Frank C Barnes and author of the original Cartridges of the World, in conjunction with the US Army developed the 11.63x33mm Belted. Loaded with a 500 grain full metal jacketed bullet the round was chambered in some bolt actions with heavy barrels and sent to Vietnam for testing. The users complained about the weight even though they were equipped with suppressors. A lighter shooter with a sports barrel profile in the 19-22” may have been more effective in winning over the troops. All the remaining ammo was destroyed in the 1984-85 time frame.
6.8 SPC Ballistics
A more successful cartridge that is finding civilian acceptance is the 6.8 SPC (Special Purpose Cartridge). Designed by the SOCOM (Special Operations Command) of the US Military. COL (cartridge overall length) allowing its use in the M16/M4 family of weapons magazine dimensions. They took the .30 Remington case and necked it down to .270 caliber. With a 115 grain bullet at about 2600 fps it was successful, partially. Kind of like the 276 Pedersen revisited.
458 SOCOM or .50 Grendel
For road blocks, they need a car killing round, and the 5.56 NATO is inadequate. Civilian development was around the 450 Bushmaster, but final development was with the 458 SOCOM or .50 Grendel. Either of these rounds into a engine block will have dramatic consequences. Kinda effective on folks as well. Limited to 150 yards or less they are designed for security troops operating roadblock and rolling inspection points. They all operated within the standard dimensions of the M16 magazine. Different uppers and modified magazines lend to the logistical support as they use common components with the service weapon. The barrel return spring and magazine is all that is needed to convert these to car killing monsters.