
Originally Posted by
lysander
None quite.
People talk of 'military 5.56mm' as if there was single pressure limit for all military loadings. There is not.
The M193 family has a maximum allowable average pressure of 55,000 psi, with a plus three sigma pressure of 61,000 psi.
The M855 family currently has a maximum allowable average pressure of 58,700 psi, with a plus three sigma pressure of 64,700 psi. Prior to 1999, the pressures were 55,000 psi, with a plus three sigma pressure of 61,000 psi, respectively.
The M855A1 family currently has a maximum allowable average pressure of 62,000 psi, with a plus three sigma pressure of 66,000 psi.
All of these pressures are currently measured with conformable pressure transducers calibrated against pierced case, direct reading, pressure transducers. This is the same method used by most commercial ammunition manufacturers.
Basically correct, but the reason the military chose a longer lead was to keep the maximum chamber pressure down, without loosing velocity, not the desire to use different bullets.
I think you just aren't explaining it well. The longer leade, or free bore, allows the bullet to get a "run and go" at the rifling, so the work required to engrave the rifling is less. It has to do with the way propellant burns and the way it reacts to the pressure it generates. Long leades do not "allow pressure to drop to a safe level before the projectile engages the rifling", it keeps the pressure from reaching high levels in the first place.
Unless you have a really fat ogive, or have loaded the bullets extremely long, the bullet will not contact the rifling before firing.
Short leads mean short bullet jump, which leads to better accuracy, which is why SAAMI chose the shorter leade.
Bill Wylde designed the chamber as a compromise between the accuracy of the the SAAMI chamber and the long leade military chamber so you could get the best of both, accuracy comparable to the .223 Remington chamber and still shoot military loads without excessive pressure. As a fortunate side effect was the ability to used heavy bullets with a long seat.
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