Hardness, pressure, and time in barrel

[easilydistrac...]

This question is mainly directed at rifle loads relative to pressure levels for cycling ARs, especially with short barrels, but also a consideration for accuracy relative to optimal barrel time.

So I know that a boolit needs to be soft enough to obdurate but hard enough to stick together at a given velocity/twist rate. To what extent does hardness effect friction between the boolit and rifling. I seem to remember reading that jacketed loads generate higher pressure for a given powder because they are harder. Is this correct?

I guess I'm not clear on whether a jacketed boolit would be slicker despite being harder (meaning peak pressure is reached sooner?) While the cast boolit would take the rifling more easily but be "stickier" the length of the barrel, so a more even pressure curve?

Short version: I know a harder boolit requires higher pressure to obdurate, but can a harder alloy also generate higher pressures?

[HangFireW8]

The range of hardness of cast matters for accuracy, but hardly matters for pressure. Compared to gilding metal jackets, all cast boolits are soft.

If a boolit is big enough to fill the grooves, then it obturates, even at moderate pressure.

[Rcmaveric]

The copper jackets of bullets are harder. It takes more pressure to swag that bullet into the throat of the fire arm. That jacket also protects the lead core. The coper jacket has more spring back and creates high pressures to push the bullet down the barrel

Cast bullets generate lower pressures in part due not having that hard jacket. Also once they get going down the barrel their dwell time is lower and pressures are much lower. Slug a barrel and it will show you. Takes a few good wacks to get it going then taps ro keep it going.

Pressures are important to cast bullets because if you obturate the bullet to much you deform it. Not enough and you dont seal the barrel. Peaking pressures too quickly gives the bullet a hard kick in the the rear that can cock it, slump it, or deform the projectile. Hardness of bullet affects the minimum pressure required to obturate the bellet and extends its upper pressure limits. I do beleive that a hard cast bullet mask poor fit that can be correct and much softer bullet used. Then there is the ductility of the lead. Making the lead too hard and you loose that ductility.

For bolt guns you can in theory use what ever gun powder you want as long as you dont excede the pressure that the bullet can handle. Even with red dot, pressures spike quickly but by the end of the barrel you barrel hav very little psi. Gas guns need enough pressure by the time the bullet passes the gas port to operate the action. I cant remember what the pressure is. So a slower powder is needed to have the pressures maintained. Also a bullet that is more gently launched and then a high pressure maintain as it dwells down the barrel will be more accurate and have a high velocity.

Quick Loads is a great tool to help estimate pressure and see what different things will do to your pressures. Seating depths, being jammed into the lands ext. will all change the pressure. Pressures effect burn characteristics of the gun poeder. It will also show you the pressure down the barrel. So you can see if thay load will cycle your gun.

OTB is interesting concept that i do beleive in. With cast bullets it a secondary property that i tune for. You first have to match a powder with burn/pressure charastic that suits the bullet and cartridge that can produce the velocity you want. Then i tune that load further to find the middle ground.

Most often I have noted have that a powder will produce several nodes of accuracy at different velocities with one node being the most accurate. I do large test in a wide swing of the velocities and most often the groups go in waves tight, tighter, bigger as they progressively get smaller. Its normaly the load at the end of string, a few groups before the pressures get to high and the groups suddenly open, that is the most accurate.

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