Question on C.U.P. and brass stress

[Rac97crawford]

I'm working on starting out a batch of 40cal on my Dillion 650. I was reading the load data in my Lyman 49th edition. I have a few of the listed powder options for the 180gr RN that I want to load.
I see that the C.U.P isn't proportionate to bullet velocity so it brought up a question in my mind.
Example:
A 180gr bullet with clays at 3.4gn will do 551fps and have a CUP of 17,900
Using n350 at 5.4 will do 847fps and have a CUP of 17,200

Assuming that less case pressure would be easier on your brass and allow for more reloads and powder options were abundant..

In choosing a recipe, would you prefer a powder that will produce a higher bullet velocity with a lower CUP?

[mark501w]

High velocity & low pressure is a good place to start. But, ultimately your held hostage to what your gun likes.

[Rac97crawford]

That's kinda what I thought. So along those lines, I'm guessing that a lower pressure won't produce as much force to cycle the action on my xd40 and if that was the case then I'd have to change the load or install a lighter spring in the gun. Correct?

[mark501w]

I think velocity is more what you should worry about in terms of functioning your pistol. I wouldn't worry about springs unless I was shooting a low speed lead load.

[chknlyps2]

If your worried about case life verses pressure I would not be to concerned. I would find a load that functioned well and was accurate. My load with Bullseye in .40 is not far below max, about 3/4 between min and max and I have at least 10 loads on my batch of brass and they still look good. If the pressure curve is even between min and max I should be above 20,000, closer to 21,000.

[rsrocket1]

3.4g of Clays is closer to the SAAMI max pressure of 35,000 psi. Be careful. For most powders, you want at least 10,000 psi for a clean burn. Slower powders need either more powder or a heavier bullet up to that pressure when working up from low charges. Fast powders need less to jump up to pressure.



On the other side of the spectrum, as you increase the powder charge, fast powders (like Clays) get up to max pressure before driving the bullet to any decent speed. Slower powders will not max out your pressure until you get up to higher charges. That lets you get the bullet going to a higher speed.

In my M&P40, 3.0g Clays gets me 770 fps, a very light recoil (similar to a 9mm gun). Quickload predicted 787 fps, so its predicted max pressure of 22,000 psi is probably close.

Don't worry about brass stress wearing out your cases. Unless you are shooting an early Glock with an unsupported barrel, just make sure you are below max pressures and the brass will last a long time. Quite frankly, except for the base of the case, you can even shoot a case with a cracked mouth or a crack in the body and still survive. The barrel holds all the pressure in all parts of the case except where it's unsupported near the ramp.