Muzzle rise, muzzle flip or muzzle climb refers to the tendency of a firearm's or airgun's muzzle (front end of the barrel) to rise up after firing. It more specifically refers to the seemingly unpredictable "jump" of the firearm's muzzle, caused by combined recoil from multiple shots being fired in quick succession. It has an adverse effect on maintaining accuracy with using automatic weapons or rapid-firing semi-automatic firearms, as a moving muzzle can throw off the shooter's aim, causing subsequent shots to miss the intended target.
The primary reason for muzzle rise is that for nearly all guns, the bore axis (longitudinal centerline of the barrel) is above the gun's center of mass, while the contact points between the shooter and the gun (e.g. grips and stock) are often all below the center of mass. When the gun is fired, the bullet motion and the escaping propellant gases exert a reactional recoil directly backwards along the bore axis, while the countering forward push from the shooter's hands and body are well below it. This creates a rotational couple, exerting a torque around the center of mass that causes the gun to pitch upwards, causing the muzzle to elevate.
Muzzle rise can be reduced, though generally only through trading off other qualities. Methods include
A number of firearms have been designed specifically to address the issue of muzzle rise. The Jatimatic submachine gun is an example of a firearm where the bore axis is inclined against the bolt and the rest of the firearm in order to redirect the recoil force slightly upwards. The KRISS Vector submachine gun uses a more elaborate mechanical articulated mechanism which allows the block and bolt to recoil not just rearward like most other firearms, but back and then 'vectored' down off-axis along a rail system behind the weapon's magazine well to reduce muzzle rise and felt recoil.
The Hudson H9, introduced in 2017, is based on the M1911 but has the recoil spring down in front of the trigger guard, resulting in a barrel very close to the top of the shooter's grip, with the creator claiming that, "the recoil impulse from that recoil spring is directed into the meat of your hand."