Gas pressure loaders are fully and semi-automatic firearms in which the gas pressure generated by the ignition and combustion of the propellant charge is used to carry out the reloading process. They are in contrast to recoil loaders. In recoil loaders, the recoil energy is used to accelerate the bolt rearward and open it at the final position, causing the ejection of the empty case from the chamber in the final position. The gas pressure escapes through the muzzle and the open breech, whereupon the breech is accelerated forward again by a spring. When the breech is closed, a new cartridge is inserted into the chamber and the gun is ready to fire again.

In gas pressure loaders, however, part of the propellant gas is diverted to repetition of the breech, which takes place through a hole in the barrel wall. The energy of this diverted gas pressure is used to unlock and open the breech of the rifle. One advantage of gas-operated loaders is that the breech lock can be safely and easily maintained by design until the gas pressure in the barrel falls below the critical level, because the gases do not actuate the self-loading mechanism until after the bullet has passed through the gas extraction hole. Due to the inertia of the mechanism, the bolt is unlocked with a sufficient delay.

The gas pressure causes the self-loading mechanism to be actuated by the gas until the bullet has left the barrel. In English, this time span is called "dwell time".

Gas pressure loaders are preferred for firing powerful ammunition.

The disadvantages compared to recoil loaders are the increased weight and the larger space required for the gas piston and gas linkage. In addition, the system can easily become fouled.

Gas pressure loaders do not play a significant role in hunting.