LEEs work on BHN and its relationship with PSI

[andym79]

Hi guys I didn't really get an answer to this one as a post in a thread, so I am posting it as a thread!

I have been doing a bit more reading. When I got into using cast bullets I bought both the Lyman manual and the Lee modern reloading.

When I read the later it seem to make sense, a cast bullet can only take so much before it gets hammered! I refer to Richard Lee piece on pressure a BHN; the table lists maximum pressures recommended. For the particular alloy I use with a BHN of 16 the recommended maximum pressure of 20433 psi.

Following this advice I have been trying to develop accurate loads without exceeding this pressure.

Anyway giving the problem I have been having with the 6.5, I have return to 'cast boolit' school and tried to expand my understanding. Firstly thanks to Larry for pointing out the RPM threshold (my other rifles have more forgiving twists) secondly and I hope this is right, thanks to http://www.lasc.us .

So the question I am alluding to is this after reading through their website, it is clear that there is some contradictory evidence out there.

"Bullet BHN / "Minimum" Chamber Pressure For Lead Alloys (PSI)
If the bullet is too hard to obturate, gas cutting usually occurs on the base band on the non-driving side of the rifling and barrel leading is likely. Simply multiply the alloy BHN by 1,422".

So for my alloy 1422 X 16 = 22752 PSI. Hello this is in complete contrast to my interpretation of LEEs work; suggesting that in terms of pressure loads should start where I was stopping.

Does Lees work apply only to plain base bullets and not GC ones?

Can someone confirm for me if this is correct? If so in my 6.5 X 55 I should be aiming for a 1400-1500 fps load that develops a peak pressure of between 23,000 and 35,000 psi, correct?

I know some of you guys have suggested loads of fast powders that would get me there!

So am I right in thinking that I probably need to either move to a softer alloy 9-10 BHN or use a faster powder e.g Unique or Red Dot to achieve my goal.

Using Quickload (I know I can't take it as gospel) in order to get up to that PSI using H4227, I would need to push the projectile at 1900 fps, which is too fast!

Is this a moment of minor revelation or am I just confusing myself!

I suspect that this is just one of many factors, and the is no definite rule relating to PSI and BHN predict accuracy.

[Harry O]

Anyone who brings up this formula gets jumped on by some of the people here. From my experience, it is a good place to start. I have never had a bad experience (bad accuracy) when using that formula. Also, as long as the bullet is reasonably sized, I have never had any leading with it. However, you can often (but not always) exceed the formula. Eventually though, you will have problems with accuracy and leading (even with a properly sized bullet) when you exceed it. It is a good guideline, not gospel.

PS. It only applies to plain base bullets. With gascheck, you can go higher. I easily go 1-1/2 to 1-3/4 times more with GC.

[Duckdog]

I use Lee formula relating to pressure vs BHN and I find it to work pretty good. It is correct that a gas checked bullet can be pushed harder than the formula, but it does work good.

[joesig]

"Bullet BHN / "Minimum" Chamber Pressure For Lead Alloys (PSI)
If the bullet is too hard to obturate, gas cutting usually occurs on the base band on the non-driving side of the rifling and barrel leading is likely. Simply multiply the alloy BHN by 1,422".

So for my alloy 1422 X 16 = 22752 PSI. Hello this is in complete contrast to my interpretation of LEEs work; suggesting that in terms of pressure loads should start where I was stopping.

I pay attention to it when looking for a load. I try not to exceed but don't mind going below, in a rifle. In a revolver, yes too low pressure will cause leading.

If you fill the throat then there is no obturation and with a full throat there is no gas cutting. Going too light is not an issue. As with Cat Sneeze loads, there is low pressure but they shoot well.

I would suggest you check the throat fit. I have grown fond of the pound swage method. Fill a fired case up to the base of the neck with wheel weight. let it cool, insert in chamber and close bolt. drop soft lead wire down the barrel and follow up with a close fitting rod. I will use a gas check one size smaller as a cap before the rod. Rod should be taped at intervals and near tip to keep off the rifling. You will will know to stop hitting with a hammer when the hammer bounces back. You now have a 3D representation of your throat and know exactly how big and long the boolit should be.

My best load has been with an unsized and "oversized" boolit but it fit the throat as the rifle wanted.

[pdawg_shooter]

I find the Lee formula to be a good starting place, however I have a number of loads that shoot great and exceed the pressure limits called for, by a wide margin. There are too many variables for ANY hard and fast rules to apply.

[runfiverun]

if you look at the real pressures of the fast powders you then contradict the pressure thingy Lee predicts.
"the load' of red dot is 13 grs and is over 40-k psi, the same velocity is achievable from a load of 4895 or even 2400 but with a much greater weight of powder [but the pressure is much lower]
gas volume is what gets you there.
14-1500 fps in your 6.5 should be achievable with slower powders such as 2400-4064 without any undue drama, the drama comes from boolit skid [launch], crooked starts, and banging the base of the boolit into the front of the boolit from the start [slump, especially uneven slump will kill you right off]

[MtGun44]

This load is impossible by that measure. This is a max load (somebody says over
max, but not in my guns) so maybe 35,000 PSI or CUP or whatever.



Doesn't seem to match reality for me.

Bill

[dromia]

Don't know if you've seen this thread below but post number 10 is my view on it as well.

http://castboolits.gunloads.com/show...metal-pressure

[zuke]

Just yesterday I fired some LEE TL314-90-SWC that were double tumbled, Not sized out of my Schmidt Ruben's in 308 and 7.5x55. The boolit's miked out at 314/315, and i used 5gr of 700X. No recoil and no tumbling

[andym79]

Yes Lees work seems to have a valid premise, but fast powders sure present a lot of evidence to suggest this is only part of the story!

"The formula of multiplying the BHN by 1422 isn't Richard LEE's, it was published in Hand Loader a few decades ago and it's intended purpose is as a guide, not gospel but a guide to "minimum" pressure needed to obturate the alloy and seal the bore. Somehow over the years some people have misinterpreted it to mean a maximum pressure which is incorrect."

Well if the above is the case that clears things up a bit, I need softer alloys or faster powders to achieve obuturation successfully, plus of course a properly sized bullet!