A regular M4A1 shooting M855A1 ammunition in a time-displacement study was found to take about 0.006 seconds to drop from a velocity of 3 to 3.66 m/s to zero on closing.
A standard bolt carrier has a mass of 0.4366 kg.
Throwing all that together, the average force during locking is 266 Newtons, or 59.8 lbs.
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