Questions about strange load data

[leadmonkey]

I've been loading pistol ammo for a while, but I'm new to loading lead, so maybe someone can point out what I'm missing here.

I bought some 40 S&W 180 gr LRN boolits that appear to be very close to RCBS #40-180 online. They are wax lubed, hard cast, and .401 dia. The closest data I could find for a lead boolit of this size at the time was in Lyman's Reloading Handbook, page 364, for a 40 S&W 175 gr lead flat nose (I think) boolit, Lyman #401043. The charge weights for Universal are 4.8 gr start load, 5.5 gr max load. I derated those numbers a bit for the 5 gr weight difference, and loaded some test loads up at 4.8 gr, 4.9 gr, 5.0 gr, 5.1 gr, and 5.2 gr. They leaded up the barrel of my Shield, and even bulged some cases. Way too much velocity I assume, and too much pressure I know.

I have since got a Lyman's Cast Bullet Handbook, and I found data for the RCBS #40-180 in it. (right next to the above mentioned data for the Lyman #401043) The charge weights for Universal are 4.4 gr start load, 4.9 gr max load. Obviously, I had overloaded my first batch of ammo for this boolit. I now loaded some test loads up at 4.4 gr, 4.5 gr, 4.6 gr, 4.7 gr, and 4.8 gr, and the results were much better. No leading, no over pressure indications. Nice shooting ammo.

Why is there such a drastic difference in the charges for two boolits so similar in weight? What is there about that Lyman 401043 that allows such a significantly larger powder charge?

[cbrick]

The perfect reason to have several loading manuals.

It's not the 5 gr weight difference, it's completely different firearms used for testing, different lots of components, different people doing the testing, different age of the manuals, different alloys used and on and on.

Rick

[waco]

Look at the col
One boolit may be seated deeper in the case raising pressure.

[leadmonkey]

The load data for both bullets was out of the same manual, with the same firearms used for testing, same lots of components, same people doing the testing.
If I knew the bullet length of that 401043, I could figure seating depth. Maybe that would make some difference, but this much???
Yep. The alloys are different. Maybe that would make some difference too, but this much??? This is the place with the knowledge about lead alloys. I don't know, that's why I'm askin'.

That's a lot of difference. Any specific guesses?

[Cherokee]

as waco said, OAL with resulting powder space difference. It can do it.

[JSnover]

The nose profile might force you to load the RCBS boolit deeper.

[scattershot]

If you have any of that first load left, checkthe neck tension. Sometimes.the boolit can be pushed back.inthe case during feeding , causing higher pressures.

[leadmonkey]

I certainly agree that you can't have too many load manuals. I've run into several situations before where the load data for a bullet I want to load is just not available, and then I have to resort to looking at load data for bullets that are similar, and trying to make educated guesses as to things like OAL, and powder charge from that. This is the first time that this has resulted in potentially dangerous ammo. I'm always looking at ways to make better educated guesses.

[Harter66]

My 48th Lyman is loaned out .......... There is an artical in it called ''Why balisticians are gray'' it discusses chamber,bbl,componet changes to a level that makes sense .

[leadman]

The length of the bearing surface of the boolit also matters.

[JSnover]

The length of the bearing surface of the boolit also matters.

It does. Also the bands are wider on the RCBS boolit, for a greater total surface area. LeadMonkey, you said they were "… very close to RCBS #40-180," can we see a pic?

[leadmonkey]

I did notice that the bands seemed to be wider on the RCBS boolit, FWIW. Here's a pic of the boolit I'm using.
94576

[JSnover]

Ok, my theory about the nose doesn't hold much water...

[leadmonkey]

Does anyone here own this Lyman 401043 mold? If so, what is the boolit length?

[Echo]

That boolit was originally designed for the 38-40. I had a mold once, but didn't have a 40, so I sold it. Not much help, am I?