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Old January 12, 2019, 04:36 AM   #20
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Join Date: November 26, 2016
Posts: 795
Originally Posted by Aguila Blanca View Post
The overlooked factor is that the lighter bullet accelerates faster and exits the barrel sooner.
The light bullet generally goes slower than the heavier bullet (see the link in my previous post), so I'm not sure it will exit the barrel sooner.

As soon as the bullet leaves the barrel, there's nothing there for the column of expanding gas to push against. From that point forward in time, any gas left in the barrel and chamber basically dissipates without adding anything to bullet velocity or recoil.
The exiting gas does add to the recoil, in accordance with "for every action there is an opposite and equal reaction" - just like you said. The gas exits at speed, faster than the bullet exits, producing a forward force. This creates a reciprocal backward force.

Formulas that calculate recoil require the weight of the powder to be included in the calculation of the recoil force in accordance with the conservation of mass. The powder weight is considered part of the ejecta. e.g.

And there's this:
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