Velocity or pressure? Confused now!

[7.62Man]

Hi All,

I have been busy making up some water dropped wheel weight bullets for my 44mag and some air cooled wheel weights for my 30-06. I was wondering what the BHN was for my straight wheel weights, so I went out and purchased the lee hardness tester and re-read the lee manual about matching your cast bullets to pressure not velocity. I can match my 30-06 bullet by pressure no problem with the data in Richard Lee's book. But then open the lyman cast bullet manual and read the section on cast bullets for hunting and it tells me to limit my bullet velocity 30-30 levels.

So am I better off using pressure as my guide than the old rule of keeping velocity down to 2100fps or less for hunting with CB. Or am I just confusing myself about the whole thing?

[shooter93]

Actuelly it's pressure you're concerned about...particularly in larger cases. In rifle cases you can have a small amount of powder generating 50,000 psi and it will shoot as much as 4 to 600 fps slower than a case full of slow burning powder generating 50,000 psi, it's the length of the pressure curve. that being said...the "old standard" velocity charts work pretty well and were worked out because your average reloader doesn't have acess to pressure equipment.

[leftiye]

There's more. If you drive those lead boolits too fast, they become varmint boolits, exploding during impact. It probably also is true that if very hard this doesn't happen, but some expansion would be nice? So match the boolit hardness to pressure for accuracy, and then match it also to velocity for controlled expansion (ie. don't shoot it fast enough that it explodes). Air cooled wheel weights are said to expand, and can probably be driven to the old upper limit for accuracy- about 1800 fps and deliver good terminal performance too. There are quite a few threads on this here, even some on making linotype boolits with pure lead noses. It's worth the research effort! Slower powders allow lower pressures with higher velocites, which in turn allow the use of softer boolits.

[Bass Ackward]

Naaaaaaa. Can't confuse the confused.

The problem is simplifying either variable. You have to factor in a lot to get the true answer. Like gun conditions (and this is a long list too), lube quality, bullet weight and bullet design to get your true limits for either pressure OR velocity. And as the guys said above, understanding powder speed to make the proper selection is paramount. But before you establish your own pressure limits for that gun, you have to set an accuracy standard you will accept. My limits quoted here are for MOA or less that can be achieved in my rifle.

Peek velocity and pressure levels occur in 22 caliber. Launch a 50 grain bullet out of a 22-250 at 45,000 psi with ACWW and you can get 3200 fps and 2" groups. (Lead, with any meplat, starts to vaporize at 3200 fps in humid air)(dry or thin air and .... you might get a little more.)

In 30 caliber, with ACWW (14 BHN) and a standard of MOA, I can get a peek of 35,000 psi. This occurs with a 150 grain, one diameter, LBT spitzer bullet. There, 80% of the bullet is bearing area and driving. This allows 2600 fps with RL19 or IMR 4831. The 210 grain limit is 29,000 psi in the 06 case that is limited by bullet design. With that limit I can get 2050 - 2100 fps. With my current throat length, that weight requires a bore ride bullet that is only about 50% bearing area and it fails because of rotational forces before a pressure limit is reached.

The larger the bore, the lower the pressure limits unless you jump up in case capacity. With a 460 Wby, you can approximate 30-06 pressure and ballistics with ACWW as long as you stay below 400 grains and have a 26" barrel to burn the slower powders necessary to achieve them. 28" would be even better.

Just so you know, I water drop for everything under 2000 fps. Now you can be confused. :grin:

[7.62Man]

I found a formula that converts the BHN into cup. I found it in a old post.

"Norm Johnson referred to the factor of a constant "480" x 3 x BHN for minimum chamber pressure and "480" x 4 x BHN for maximum chamber pressure in CUP."

I will try loading my cast to 10% less then there maxium pressure and see how that works.

Thanks for the help!

[Bret4207]

There are a zillion variables. A 30 Carbine can be run at factory velocity pretty easily and thats what? 30-35K CUP? Try that with a 270 and you'll have issues. Add in to everything the other guys said case shape, rifling design, powder lot, specific alloy and any heat treatment, moon phase and political leanings. Living a good, clean life and honoring your Mother and Father also helps, but worshiping idols is out.

IOW- The pressure/velocity curve are part of the same event. What you want in laymans terms, 'cuz I ain't no jean-e-us, is the lowest possible pressure over a fairly long time period balanced with the velocity you're aiming for AND all this in the most consistent manner possible shot to shot. With luck this gives your relatively fragile boolit a smooth start with just enough obturation to fit the throat nicely and a consistent trip down the bore. Some guys claim to have the formula to do this but I can only say it takes experimentation for me as each gun seems to be a law unto itself.