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After a few inches of travel the bullet is being accelerated bythe high pressure gas behind it only.
The powder has been completely consumed.
Some fast powders before the heel of the bullet clears the case, but rifle powders normally within a few inches of bullet travel.
The bullet will continue to accelerate as long as the pressure is above the friction of bullet to barrel until it either exits or exceeds the speed of sound in the driving gas.
No object can be driven faster than the speed of sound in the driving gas.
Most rifle calibers can gain additional velocity with a longer barrel (.22 RF being a common exception),
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