The 75mm Regimental Gun M121 is a modern, light-weight artillery piece that combines portability, speed and firepower into a single package. Its mass is only about 900 kg (1,984.2 lbs), thanks to a number of weight-reducing Aluminum and even Titanium components. Unlike the Pak 40, however, it is not mounted on a fixed axle - the gun, recoil tray and shield are separated from the carriage and trail by a turnstyle pivot, allowing limited traverse. The M121 also has a modern split trail that can be closed and locked together for transport or separated during use, making it very low to the ground - comparable to its Pak 40 counterpart - and easier to load. The long-barreled, rifled gun uses single-piece metallic cartridges (like giant bullets) and a vertical falling wedge breech that automatically opens upon firing and extracts the spent cartridge. These charge cartridges are detatchable and come with several propellent bags - color-coded red, white and blue - that are removed before loading to vary projectile arc or range when the gun is being used as an indirect-fire artillery weapon. Full Charge is only used with anti-tank ammunition or for very long-range shots. The M121 has a rapid-action hydropneumatic recouperator and a very long recoil tray that doubles as a loading ramp, as well as a larger tubular muzzle brake than the Pak 40 that allows stronger propellant charges to be used (and thus higher obtainable muzzle velocities).
The 75mm Regimental Gun M121 and its counterpart, the 105mm Infantry Gun M130, is the principle means of fire support afforded to infantry regiments, with two companies of M121s and one company of M130s in each regiment. The M121 is not as powerful as dedicated 105mm anti-tank guns (which have now been replaced in most service applications by M65 Assault Guns) or the 125mm towed anti-tank guns, but can knock out T-55s at close range. As an artillery piece, the low-angle and high-velocity nature of its shells, combined with their small size / weak explosive yield, gives it mediocre performance against entrenched enemies at best. The blistering rate of fire, however - equal to that of the French Canon de 75mm of Great War fame - and high accuracy goes a long way towards making up for this, and it is positively deadly against assaulting enemy infantry without the protection of cover. Using specialized Tungsten APDS - Armor-Piercing, Discarded Sabbot - ammunition, the M121 can puncture the front armor of a T-55 from a range of about 700 meters, or about double the effective range of American shoulder-fired HEAT rocket launcher weapons. Confirmed kills have been reported from greater ranges.
In service: 1957-Present
Manufacturer: United States Ordnance Department, Watervliet Arsenal
Weight: 900 kg / 1984.2 pounds
Length: 6.8 meters
Barrel Length: 50 calibres
Calibre: 75mm / 2.95"
Breech: Semi-automatic vertical sliding block
Carriage: Split trail
Elevation: -5º to +40º
Rate of fire: Up to 25 Rounds per Minute (15-16 sustained)
- Direct Fire (anti-tank ammunition): 1,800-2,000 meters
- Indirect Fire (HE shell): ~8,000 meters
Maximum Range (IIndirect Fire, HE shell): ~12,000-13,000 meters