3 bullets - 3 varying shapes - same combustion volume

[renaissance]

This is amazing ( to Me at least )
I have a Dillon Toolhead (650) that I set up some time ago to load .38 spl
I think that the bullet I used was a Hard Cast Lead Truncated Cone in 158 grain.

Went to it today because I was out of 38 Special.

I found three partial boxes of .38 Spl bullets, all in 158 Grain Hard Cast Lead
They Were:
National Bullet Truncated Cone
National Round Nose Flat Point
“B&J” Round Nosed

Using the ToolHead as set up I tried building a cartridge with each of the bullets.

The one same Toolhead Setup:
Put the crimp into the crimping groove of ALL THREE of the bullets
( center of the groove in the Truncated Cone
Bottom end of the crimping groove in the RN and RNFP)
I then measured the “height” of each bullet and the OAL of the finished cartridges
Results follow:
OAL Bullet Shape Bullet Height
1.408 RNFP .645
1.452 TC .695
1.470 RN .686
Subtracting the bullet heights from the OAL of each cartridge I can calculate the Combustion Volume ( measured in terms of "length of cylinder") in each cartridge

RNFP 1.408 -.645 = .763
TC 1.452 - .695 = .757
RN 1.470 - .686 = .764
The are for all practical purposes ……THE SAME !

I can use the one same toolhead setup and any of the three bullets, interchangeably………
If the powder charge is the same ( and it will be in my 650 )
All three substantially varying bullet shapes should behave the same.

Is this a coincidence, or did the designers of these bullets know what they were doing?
Placing the crimp groove so as to yield the same combustion volume.

I am curious to know what the shape of my bullet seating stem is ………….
That it will seat all three bullet shapes just right so that the crimp grooves come out at the same height on the case mouth…….but –
I do not want to queer the set up.

 

[ReloaderFred]

There is another variable involved. The bearing surface of the bullets affects the resistance, or friction, of the bullet as it travels down the barrel. This isn't so much a factor in a lower pressure round like .38 Special, but it has to be taken into consideration in higher pressure rounds, such as 9mm, .40 S&W and 10mm, as examples. It's also a major factor when loading most rifle rounds.

It's great that the three different bullet shapes seat the same, and the volume of the case remains the same, since that simplifies setup. The interesting part will be to see if all three bullets strike to the same point of impact. Sometimes they do, and sometimes they don't.

If your dies are recent Dillon dies, you should be able to just pull the pin and drop the seating stem out of the die and look at it. They made them that way so they would be easy to clean, without changing the setup of the die. Most of mine have two different shapes on the stems. One for round nose and one for flat point bullets. You just pull the pin and turn the stem over for the other shape.

Hope this helps.

Fred

 

[renaissance]

Drop Out Seating Die

My Dies ARE Dillon
but
They are the OLD style
No Clip
I tried to get Dillon to Upgrade me
but
No Support there

 

[alumltd]

Hi renaissance,

What do you mean by "upgrade"?

 

[renaissance]

By "Upgrade", I mean

Swap my "Old" for the "New"
(for a "minimal fee")?.

They are guaranteed
If one got "Broke"
They would mostly replace it with the "new" style
Don't think they have many of the "Old" style lying around