Need help w/ammo

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The powder rate really has very little to do with the impulse. The powder has burned, the bullet has left the muzzle, and the pressure has dropped, all before the toggles hit the ramps. By the time the toggles hit the ramps the upper is coasting and already beginning to slow down.

Insofar as the powder rate makes any difference in impulse, a slower powder will give more impulse because you are using more of it. But now you are into hair splitting country.

Formula:

impulse = v*mb + 1.5*v*mp

In the English system where impulse is in lbf*s (pound force seconds), v is the muzzle velocity in fps, 1.5 is a rule of thumb factor for the average powder gas velocity, mb is the mass of the bullet and mp is the mass of the powder. The mass must be in slugs, so you divide the mass in grains by 225218 to convert grains to slugs. As the bullet is much heavier than the powder charge it tends to dominate. For the Luger the result should be in the neighborhood of 0.5 lbf*s.

The impulse occurs in a millisecond or less so the force pushing the upper back is on the order of 500 lbf or more. This is far greater than a 30 lbf spring so you can see why the spring stiffness has very little effect on the upper's velocity. The spring stiffness must be such as to bring the upper nearly to a stop by the time it reaches the rear most position with the toggles up. Too weak and we are battering the action when it stops at the rear, too strong and the action stops before it is fully back and fails to pick up the next round.