Standard deviation

[jhutto]

(I recently bought a chrony)
What kind of consistancy should I be looking for in my .45ACP loads?
I curently reload with a 550 press. 5.6 Grains win231, Hornady XTP 230gr, and a 1.230 OAL. I am getting between about 780fps, and 830fps. Should my loads vary this much, or is this a sign of overpressure? These numbers are out of my 5'' 1911. Also, I was considering playing with the OAL, can the OAL cause overpressure If I make the OAL longer?
Thanks

[nuparadigm]

As you decrease the OAL, the pressure will increase.

Your variances in velocity are fairly small.

[KBCraig]

There are many variables in reloading. The powder charge and OAL are only two of them.

First: is all your brass identical? Same manufacturer, same weight, same number of previous loadings?

Weigh your brass. That will quickly tell you if the internal volume is the same.

Next: weigh your bullets. I know, you paid good money for them from a known manufacturer. Weigh them anyway.

Finally: weigh your powder charge. It's always hard to get consistency when measuring powder by volume. Weight is absolute, but volume varies, and weight (mass, actually) is the only true measure of the amount of powder.

Benchrest rifle shooters weigh every charge, and use powder tricklers to "sneak up on" the exact charge weight. You can't do that with pistol ammo, at least not if you hope to shoot another IDPA match in this lifetime. But you can check your volume measure by throwing every fifth charge onto the scale, and taking careful notes about consistency.

Kevin

[kmitchl]

I rely on standard of deviation to evaluate pistol loads. If you have a SD of 10 this means that statistically 68% of the velocities for that load will be within +/- 10 fps of the average. If you expand the range to 2X the SD (+/-20 fps) 95% of the velocities will be within that range. Although not the only criteria, I generally look for a SD of less than 10 when evaluating pistol loads.

The other way is to shoot from a Ransom rest so the accuracy can be accurately measured.

Keith

[jhutto]

thanks for your replies, I am learning. Hopefully i will not have to learn the hard way. Tomorrow I am going to buy a good set of checkweights for my scale. i want to make sure i am getting things right from load to load. less than 10 is about what I am looking for. Now I have somthing to strive for. Thanks!