Figuring Cast starting loads from jacketed data

[No_1]

There is plenty of data for the popular cast boolit cartridges but little for the odd ones. Where would one start when trying to use know jacketed load data for those odd ones?

R.

[jhrosier]

I generally look at the starting loads for jacketed bullets in the Lyman manual and try to find one with reasonable velocities for cast boolits and pressures in the 20,000 to 30,000 psi range, assuming that your gun would handle these pressures.

Another possibility is to find another cartridge, close in caliber and with the same or slightly less case capacity, for which cast boolit data for the desired boolit weight is available.

As an example, I used 30-06 cast boolit data when developing loads for my 8x58RD. I was careful to select loads that gave pressures below 28Kpsi in the 30-06, as I am working with a "vintage" rolling block.

Jack

[bruce drake]

In Richard Lee's Modern Reloading Vol 1, he puts out a simple formula to drop a jacketed charge down to a cast bullet level. The better part of that chapter is that he explains the why behind the process so that you can carry it over to other loads. I'e been using that formula for the last ten years. I've been told that his Vol II actually has cast loads alongside the jacketed like the older manuals once did.

Bruce

[No_1]

Hi Bruce,

Can you quote that fomula here? I do not have a copy of that manual.

Robert


In Richard Lee's Modern Reloading Vol 1, he puts out a simple formula to drop a jacketed charge down to a cast bullet level. The better part of that chapter is that he explains the why behind the process so that you can carry it over to other loads. I'e been using that formula for the last ten years. I've been told that his Vol II actually has cast loads alongside the jacketed like the older manuals once did.

Bruce

[LeadThrower]

This is about estimating cast boolit charges from cast boolit data, rather than cast from jacketed:

In two cases I've used a linear regression fit for starting charge weight of a specific powder vs. lead bullet weight to go where I've found no data. Common sense must weigh in more than the numbers, to be sure, but the practice has gotten me to good starting points from which I can work up.

Excel or any spreadsheet software (I recommend OpenOffice -- go to http://www.OpenOffice.org) will do this, as will any quality scientific or engineering calculator.

[bcp477]

Formula or no, there is no exact, direct way to mathematically convert jacketed loads to cast ones (I've tried, even using the aforementioned formula....the results are not consistent). I think that the best one can do is to first consider the pressure limit of the cast bullet, then extrapolate the charge weight (with a given powder) down from a maximum jacketed load, to the level which the cast bullet can take. If the firearm in question cannot take a maximum pressures (such as an old milsurp rifle), then that must be considered also. The numbers are all over the place, varying widely, based on the powder used. It also MUST be stressed that some powders are not safe in heavily reduced charges - those being the slow rifle powders. Every powder load can be reduced somewhat, but only so far. The general rule is that, for every 1 % reduction in charge weight for a given powder....the velocity will reduce by 0.7 - 1 %....and the PRESSURE will be reduced by about 2.5 %. Maximum safe charge reduction percentages vary from no more than about 5 - 10 % for the slowest powders....to as much as 50 % or more, for the medium-fast rifle / large-calibre handgun powders. The fastest powders, such as Bullseye or Unique can be reduced at will, with no danger.

For example, for my milsurp Yugo Mauser, which is of the "98" design and thus safe to about 50,000 psi loads....I first consider the cast bullet. I am using a bullet with a BHN of 15. 15 x 1422 gives 21,330 psi... the approximate maximum pressure that such a plain-base bullet can stand. Obviously, if using a gas-checked bullet, or a paper-patched one...the maximum pressure number will be alot higher. But, for this example, I'll only consider a plain-base one. So, I've got to stay below 21,000 psi...regardless of any jacketed bullet data I may have on hand. Bear in mind that the faster the powder - the quicker that pressure builds, as the charge weight is increased. For my cast loads, I like IMR-3031, because it is slower than powders such as Unique, 2400, SR4759, etc.....and seems to work well in my rifle (a slower powder will give a gentler "push" to the bullet than a faster one). As it happens, my rifle prefers slower powders to faster ones, anyway. Using the 1 % charge reduction = 2.5 % pressure reduction "formula"...and working down from max. jacketed load data for 170 grain bullets (the cast bullets I am using are also of 170 grains)....the math indicates that I need a charge of about 15 - 16 grains of this powder. That is, as it happens, just about the minimum safe (reduced) charge weight for 3031. Nevertheless, I add cotton wool filler, just to be safe. So, with this, I can get a usable load with the IMR-3031, for this particular bullet. Alas, such a load is not acceptably accurate in my rifle. Nevertheless, this is the best method or "formula" for working out a starting point for cast loads, that I have found. Unfortunately, there is no easy, packaged formula that will work every time.

In summary, I would suggest that you start with a bullet that is known to work well in rifles of the type you have....and start with a fast powder, such as Unique or 2400, with a charge weight also known to work well in most rifles of your type. Then, depending on your results, you can try slower powders in increments, keeping in mind the limitations and safety factors...until you find an acceptable load. It isn't easy - nor is it quick.