Reloading question - charge weights/pressures

[Chief Rick]

I was looking at my older copy of the Lyman reloading book and noticed the following:

 

Why does the charge weight go down as bullet weight goes up?  I know it has to do with pressure but bullet design can also play a role (as evidence by the first two 120 gr loads shown).  Is it as simple as a heavier bullet causes more pressure with a given powder charge?

[Griff]

Not just bullet weight, but bearing area and shape.  Bearing area being the amount of area in contact with the barrel and shape should be self-explanatory.  But, yes in general, as weight goes up powder charge needs to drop in order to keep pressure within safe limits of the firearm.

[Garrison Joe, SASS #60708]

This happens with rifle loads too.   The light weight bullets cause a smaller percentage of the powder to burn, since they give less resistance to the chamber pressure (due to weight and bearing surface), and since smokeless powder has a progressively faster burn as the pressure increases, less pressure means less efficient burn.  Heavy bullets confine the pressure better and you get higher percentage of the powder burning completely.  

 

Practical proof - light bullet loads leave more unburned powder in the barrel and on the ground in front of muzzle (throw down a white sheet in front of muzzle sometime!).

 

good luck, GJ

[Cypress Sun]

1 hour ago, Garrison Joe, SASS #60708 said:

This happens with rifle loads too.   The light weight bullets cause a smaller percentage of the powder to burn, since they give less resistance to the chamber pressure (due to weight and bearing surface), and since smokeless powder has a progressively faster burn as the pressure increases, less pressure means less efficient burn.  Heavy bullets confine the pressure better and you get higher percentage of the powder burning completely.  

 

Practical proof - light bullet loads leave more unburned powder in the barrel and on the ground in front of muzzle (throw down a white sheet in from of muzzle sometime!).

 

good luck, GJ

 

It gets worse with light powder charges and loose crimps. Seen it hundreds of times timing CAS shooters when the sun is shining in "rays", with the pistol/rifle in the shadows and the light "ray" is in front of the muzzle. Kind of like watching a video of a slow motion sneeze.

[Preacherman]

Yep, it's all about the power factor and heavier bullets are more efficient. Over the years I've gravitated towards heavier bullets. They just work better. More lead surface riding down the barrel, especially in the rifle. I think that the race towards lighter bullets has been a mistake. The only cowboy load that I ever had  a"leading" problem with was the 160g in a 45. They're so short that there is almost nothing contacting the rifling. That's also why I shoot full wadcutters in the revolvers when I can. Good shootin'!!

[Texas Jack Black]

7 hours ago, Chief Rick said:

I was looking at my older copy of the Lyman reloading book and noticed the following:

 

Why does the charge weight go down as bullet weight goes up?  I know it has to do with pressure but bullet design can also play a role (as evidence by the first two 120 gr loads shown).  Is it as simple as a heavier bullet causes more pressure with a given powder charge?

NOT ALWAYS

[The Original Lumpy Gritz]

Temperature plays a big part in pressure readings.

The pressures shown in the manuals come from specifically made pressure barrels. They have no bbl/cyl gap for revolver calibers. 

[Yusta B.]

The two 120 gr bullets are seated to diff. OAL also which may cause pressure change.  .040" is quite a change.

[Smokin Gator SASS #29736]

I have seen a lot of new reloaders or potential reloaders who assume that a heavier bullet means you'll be automatically using more of the same piwder. They just figure a heavier bullet must need more powder without any understanding of pressure.

[Widder, SASS #59054]

Bullet design can (but not always) effect 'seating depth'.   The deeper you seat the bullet, the greater the chance of 

having a 'Compressed Load'.    With some powders and bullets, this might not be drastic...... but with other bullet/powder

combinations, it could be devastating.

 

I normally use 2 or 3 reloading manuals when deciding on my 'recipes' and try to steer clear of contridicting formulas.

 

Happy Holidays.

 

..........Widder

 

[Kid Rich]

Yusta B and Widder both have the answer, case volume is reduced if a heavier bullet is loaded to the same length thereby reducing available case volume.kR

[Bison Bud]

I think the opposite is even more surprising in that lighter bullets require more powder!  However, there is a trade off with that statement in that a smokeless powder burns at the same rate with either projectile, but the lighter bullets can and will leave more unburned powder simply because they don't develop enough pressure.  Therefore, there is a real limit on how much will actually increase the pressure or just be wasted.  While I agree that our game needs lighter, target type loads, rather than hunting or magnum loads, I also think the whole advantage to very light loads has been way over blown.  When I was shooting regularly, I was very successful with a .38 Special load with a 140 grain bullet that clocked about 800 fps. and that is pretty hot compared to many I've seen.  I simply never needed to go any lighter to control recoil or otherwise, but that really is the true beauty of hand loading as it is an individual choice and loads can be tailored to suit you personal needs and/or preferences.  Good luck and good shooting to all. 

[Jefro, SASS#69420]

On 12/15/2022 at 6:06 AM, Kid Rich said:

Yusta B and Widder both have the answer, case volume is reduced if a heavier bullet is loaded to the same length thereby reducing available case volume.kR

Plus it takes more umpf to push a lighter bullet