changing accuracy load per bhn change?

[BJung]

After I determine an accuracy load with a specific lot of lead, how much of a change in the bhn is allowed before I lose accuracy and have to readjust my charge?

[Winger Ed.]

You'd pretty much have to do that on a case by case basis.
Depending on your alloys, boolits will drop at slightly different weights,
and maybe have different amounts of resistance going down the barrel.
Either one can change their velocity and point of impact to one degree or another.

[charlie b]

Also depends on what kinds of loads, light pistol, heavy pistol, light rifle, high vel rifle, BPCR, etc. Also depends somewhat on the lube or if you are powder coating. Gas checks or plain base.

I'd not be concerned unless you are changing by a lot, ie, 10 to 18.

[stubshaft]

It would depend on your gun, load and what you consider accurate. I have found marked differences in accuracy (over 1/2") by just changing primers and/or lube!

[BJung]

In that case, the next time I cast I'll cast a lot of the same bullet with the same alloy. Then, cull out the outliers.

[Shopdog]

As posted above;

It depends on what your acc metric is? It's right much more "involved" than just a BHN #. Reason for post though,this is one more reason for precise notes in your rigs data log book. Testing is the path to righteousness (haha)....but,if you don't write it down? Well,good luck with your memory.

[mnewcomb59]

I have seen BHN move POI many times. There will usually be a certain BHN that is most accurate, then if you move 3-4 points either way accuracy will still be acceptable but POI might move 2 inches at 50 yards.

[MUSTANG]

I would recommend that you consider the alloy BHN; take the hardness and look for the Max PSI in the Chart Below. Look at your Load reference manual/s and determine what the approximate PSI if for your load. Compare the two - and if you are above Max PSI or going there; the chances are accuracy will shortly fall off (Sometimes Quite Rapidly).


(Double click on the pic; it will open it in a new window. Then Enlarge by "Zoom In" under files on your computer control bar to expand the size of the information displayed).

Attachment 301548

[GregLaROCHE]

If you have an accurate load worked up, why would you have to change your BHN. Maybe your supply of lead has changed. Then maybe you will have to work up a new load. Otherwise, try and stay with the BHN you were using and fine tune a new alloy to be as similar as possible.

[Larry Gibson]

The major problem with the chart above is the "constant" in the formula to get the "working pressure" is based on the elasticity of pure lead. If you are using a binary alloy the chart is close. However, if using a ternary or quadra alloy then the chart is misleading/wrong.

For example, I have measured the psi of my HV 30x60 XCB loads pushing the 164 gr XCB bullet (cast of #2 alloy, a ternary alloy) at 2900 fps with a measured 50,000 psi +/-. The measured BHN (with a Lee tool) is 22 - 24. By the above chart the bullets should have "failed" at 34-35,000 psi yet they did not as accuracy was >moa to 1 1/4 moa at 600 yards with 10 shot groups.

[mnewcomb59]

For my uses I like a harder bullet for deer driving that has good penetration for whatever shot angle I am presented at close range and high impact velocity, then a softer bullet for ambush hunting where I can pick my shot and angle and give maximum damage with a moderate chance of exit.

Using the same load and bullet but changing BHN usually I see left to right change of 2-4 MOA. I usually end up sighting in with my soft (2-2-96 AC 11 BHN) bullet for long shots and settle for my harder deer drive bullet (2-2-96 WD BHN 15) hitting 1 inch left or right at 50 yards. I shoot most of my deer on drives between 20 and 30 yards where the change in POI doesn't matter.

I have seen the left to right change in POI in 5 different Rossi 357 rifles and my 45 Colt Marlin does it as well. It seems to never be vertical POI change but always horizontal.