Like most countries, Germany had a standard-issue antitank rifle when World War II began - the Panzerbuchse 39. It fired an 8x94mm cartridge with a small very high velocity armor-piercing bullet. And like the other AT rifles from the 1930s, the PzB-39 became obsolete quickly as tank armor improved during the war. However, while most countries simply scrapped their antitank rifles, the Germans opted instead to convert the guns into dedicated grenade launchers.
Because the PzB-39 was already designed for a very high pressure cartridge, it was ideally suited to handle the stresses of firing large anti-tank grenades. Rather than relying on simple kinetic energy to penetrate, the grenades could use shaped charge technology to be vastly more effective than AP bullets.
In converting the PzB-39 into the GrB-39, the barrels were cut down, grenade launching cups attached to the muzzles, new sights designed for grenade use, bipods lengthened, and the folding stocks were fixed in place. Most of the PzB-39 rifles in service were subject to these modifications, and the resulting GrB-39 guns were able to be reasonably effective through the end of the war.