BHN vs PSI vs Bullet Obturation

[Rizzo]

New to casting and am confused on this issue.

My understanding is that we find from our load data the pressure (psi) generated for that load and match the BHN of our alloy to that psi value so that bullet obturation can happen.
A bullet too hard would not obturate and gas cutting, leading, etc. could happen.

I plan on casting some 125 grain 9mm boolits using Win231 powder and according to my Lee Modern Reloading manual this would produce about 33,000 psi.
Looking at my Lee Hardness Tester Chart I see that for 33,000 psi I would be needing a BHN of about 25-26.
25-26 seems a bit hard to me from what I've read elsewhere.

I have some Monotype that is BHN 26 but with supposedly 19% Antimony in it this seems very brittle to me.

In theory this BHN vs PSI makes sense but the end result gives me doubts since the boolit would be so brittle.

What should I do?
Thanks

[Outpost75]

Hard doesn't necessarily mean brittle. It depends upon the structure and chemistry of the alloy.

For 26 BHN you can blend 50-50 wheelweights and plumber's lead, cast bullets so that they are uniformly frosted, then quench immediately from the mold in room temperature water, then take the wet bullets from the bucket, put them in a Ziplok bag and cold soak them in the freezer for 14 days.

You will get the hardness you want, and the bullets will also be ductile.

[John Boy]

Here's some numbers you can play with ...
Alloy Bhn multiplied by 1422 = Minimum Pressure

[Wayne S]

That 33000 psi seams a bit high, how much 231 are you using??
Maybe someone with the "quik Load" or other programs will offer their thought on the psi generated. from there just divide the psi # by 1422 and you will have your BHN

[runfiverun]

or just do what I'd do and ignore the good idea math bhn fairy, cast some ww alloy boolits, size them to 358, and start my load process from the low end, and work up till I'm happy.

I start my process by making a couple of dummy rounds and insuring they will feed 100% from my magazine.
then I figure the case volume left over and compare that to a known book load.
reduce that load slightly and start working up.
you'll probably end up starting at about 3.7 grs of 231.

[Rizzo]

Hard doesn't necessarily mean brittle. It depends upon the structure and chemistry of the alloy.

For 26 BHN you can blend 50-50 wheelweights and plumber's lead, cast bullets so that they are uniformly frosted, then quench immediately from the mold in room temperature water, then take the wet bullets from the bucket, put them in a Ziplok bag and cold soak them in the freezer for 14 days.

You will get the hardness you want, and the bullets will also be ductile.

Good point about hardness doesn't mean brittle.
I like the idea of water quenching, or even heat treating an alloy to get the BHN up.
I hadn't thought of that.
Thanks



Here's some numbers you can play with ...
Alloy Bhn multiplied by 1422 = Minimum Pressure

Thanks, I'll put that into my notes.


or just do what I'd do and ignore the good idea math bhn fairy, cast some ww alloy boolits, size them to 358, and start my load process from the low end, and work up till I'm happy.

I start my process by making a couple of dummy rounds and insuring they will feed 100% from my magazine.
then I figure the case volume left over and compare that to a known book load.
reduce that load slightly and start working up.
you'll probably end up starting at about 3.7 grs of 231.

Thanks for the input.
Can't argue with success, assuming you're not getting any leading.

[Larry Gibson]

That formula is over simplified.

The problem with the "psi" in that formula is that it is based only on the peak psi. How long it takes to get to the "peak psi" is more relevant. That means the time pressure curve needs to be slower or faster depending on the desired outcome.

Larry Gibson

[osteodoc08]

Just load some up and go shoot. I find these formulas to be lacking in the real world. I shoot mostly magnum revolver and shoot COWW with some added tin for BHN around 11 using a SAECO tester at velocities up to 1500 FPS without much if any leading. Proper sizing and good lube goes a long way. Insert plug for RandyRats Tac1 and Tac2 here. Love that stuff.

[mfraser264]

Read the latest loading manual from Richard Lee of Lee Precision - he has an entire section on this subject.
Exact title "Modern Reloading 2nd Edition, Revised" Reloading Manual Richard Lee

[dubber123]

I have been getting by just fine with air cooled WW's. My load runs a 150+ gr. boolit to about 910 fps. from a 3.7" barrel with no leading whatsoever. Bhn is right about 14. Accuracy runs in the 1-1.5" range at 20 yds. I think depending on obturation is a bad idea. I'd much rather make the boolit fit to start with.

[KYCaster]

I think depending on obturation is a bad idea. I'd much rather make the boolit fit to start with.


Sounds like a winner to me.

Jerry

[wordsmith]

I defer to the collective knowledge of folks here that have been casting longer than I've been alive. However, to give you the perspective of another relative newbie to casting, there's a lot of conflicting information regarding this subject. My conclusion is that unlike j-word loading which is fairly linear and predictable, cast boolits present a number of variables such as bore diameter, bore condition, alloy composition, cast bullet diameter / sizing (and corresponding "fit" to the bore), BHN / Heat Treating, casting technique, molds, lube / coating, powder burn rate, case sizing / bullet seating technique, ad nauseum... all of which you have to control to some extent to find success.

This hobby is not one you just jump into casually and find success over the long term. I read a lot of books and articles before casting my first bullet, and I would suggest the same for you. If you don't have a good comprehensive understanding of what you're doing, it will be a tough row to hoe. Places to start...

Lee Reloading Manual
Lyman Cast Bullet Manual
http://www.lasc.us/CastBulletNotes.htm
NRA Cast Bullets
Jacketed Performance with Cast Bullets
This Forum
...Plus many more out there

Knowledge is truly power here. Good luck and keep posting.

[Rizzo]

Correct (accepted) definition of obturation is boolit deformation while in the case that causes the case to press against the chamber, sealing the chamber. Closely related to 'fit'. What we call 'fit' is making oversized so the bore sizes the boolit for sealing. A softer alloy will allow the boolit to maintain 'fit' all the way through the bore, assuming the pressure stays high enough. The OP says he is confused so lets not add more confusion.

The definition I read was: "Obturation is the plastic deformation of the bullet metal in response to the applied pressure (from the burning powder)."
...also..."It is important to recognize that obturation is not simply an increase in bullet diameter, it is also a backfilling of defects obtained in the engraving process, and therefore plays a role in every shot fired with a cast bullet, even those that are properly (or over-) sized for the bore."
From the article posted- From Ingot to Target: A Cast Bullet Guide for Handgunners

Regardless, being new to casting (I have been reloading for two years now in 9mm, 38 spcl, .223) I have done a bit of reading and started purchasing equipment.
I did slug my barrels and have determined which molds and sizers I need.

The issue of bullet obturation and "fit" seem to be somewhat similar. That is, we want a good bullet 'fit" as it travels down the barrel. This would prevent any leading.
Lube also aids in this function. If the bullets cast give a good "fit" due to slugging the barrel first and casting/resizing accordingly, then bullet obturation would seem needed only to aid in that "fit" by the lead being "plastic" enough to add some outward pressure of the lead to give a tighter fit. If too hard, that would not occur. If too soft, when shooting higher pressure loads (such as 33,000 psi for 9mm) then the way I understand it is that the soft lead could melt and streak lead down your barrel.

Well, at this point this is what this newbie caster has read/learned so far.
Interesting topic and several different opinions have been given.
I'm trying to do things correctly so I do appreciate the input from you experienced casters.
Thanks.

[bobthenailer]

In some old handloader magazine , there was a article about cast boolet alloys and the relationship of the BHN hardness needed for a given pressure and velocity !

I wrote it down on a index card and looked for it ! but i cant find it at the moment , I presently dont have time to search my libary of HL

[Petrol & Powder]

OK, I have the opposite question:
How soft can you make a solid wadcutter for a 38 Special fired at 700fps, assuming it is sized correctly?

[MT Chambers]

OK, I have the opposite question:
How soft can you make a solid wadcutter for a 38 Special fired at 700fps, assuming it is sized correctly?Pure lead is fine, I prefer to add a bit of tin.....some folks use bought w/c that are swaged and they use pure lead to swage them. AS to the orig. poster....I think books could be written on this and they all would differ, i don't load any boolits to 33K and end up being more concerned about the terminal results of cast boolits.

[John Boy]

Obturation:
* In the chamber is the thin wall case sealing against the chamber walls at the moment of ignition to prevent gas leakage (blow back)
* Next is the bullet base bump up (slumping) from the ignition gases
* Optimum obturation of the bullet in the bore is the bullet diameter at the driving band and down to the base sealing in the bore grooves

[Petrol & Powder]

MT Chambers, Thanks !

[bobthenailer]

In some old handloader magazine , there was a article about cast boolet alloys and the relationship of the BHN hardness needed for a given pressure and velocity !

I wrote it down on a index card and looked for it ! but i cant find it at the moment , I presently dont have time to search my libary of HL

OK i found it !
lead---- tin ----antimony
98------0-----2-------lead 8 to 9 K psi (lead)
90------5-----0-------10 to 12 K psi
90------3-----7-------11 to 14 K psi (WW)
90------5-----5-------13 to 15 K psi ( lyman#2)

5lb of ww to 2 lb LT good for heavy doses of slow powder (accurate

6lb of WW to 4 lb LT good for full loads 25 to 30 K psi