38-40. Large pistol primer. Requires a 0.401 diameter bullet (it’s not actually a 38 caliber bullet), typically a 180 grain lead.
The 38-40 is slightly necked down to allow for more propellent with a smaller projectile than the 44-40 to gain a faster/flatter trajectory. The 38-40 was produced as a flexible cartridge in 1874 by Winchester, suitable for rifle or revolvers. It was the 3rd most popular caliber between 1873-1941, so original and replica firearms can be acquired that use this cartridge. Cowboy action shooting has brought this cartridge from a vintage black powder novelty to usable cartridge. It allows for a lower recoil for a handgun, making it easier to stay on target between shots.
Expected velocity is 1,300 fps from a 24″ barrel rifle or 900 fps from a 7.5″ revolver. Starline’s brass has fixed the traditional issue for 38-40 having thin necks and have made a stronger case. More information to include some reloading tips at Starline Brass’s website.