When a firearm goes off there is primary and secondary recoil. Primary recoil begins the moment the bullet begins to move and increases as the bullet accelerates down the barrel. The rifle is trying to move back away from the bullet with the same amount of energy the bullet uses trying to get away from the rifle. Through formulas this rearward energy transfer can be measured in foot pounds and rifle rearward velocity. A good example of this is a recoil operated semi-auto handgun. As the bullet moves, pressure is exerted back thru the barrel making the barrel slam rearward until it hits the stop and then the extra pressure continues to push back against your hand. No spring in a rifle so the pressure is immediately against the weight of the rifle and your shoulder.
Secondary recoil is the expanding gasses pushing back against the end of the barrel.
If you rest your barrel directly on a hard surface, especially with a lightweight rifle, your bullet will impact high. In most rifles for practical hunting purposes this is mitigated by resting on the rifle forearm.
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