I have always believed that a decrease in case volume would increase pressure, therefore a decrease in powder would be needed.
I picked up some solid copper 224 bullets, 55 grain but they are longer than a FMJ 55 grain.  I couldn't find any load data so I started
my load about 2 grains short of my normal load (24.8 of 2230).  A well respected friend of mine told me that the load should be the
same, as the weight of the bullet is the same.  I argued that the decrease in volume would increase the pressure but now I'm not
convinced of that.  What say you?

33 views and no opinions?  This isn't GD is it?

I agree that the longer projectile, when seated to the same depth, will decrease case capacity.  But solid copper projectiles do and will behave differently from jacketed ones.  Their bearing surface and lubricity, and thus the pressure curve, are different,.  Your best bet is to do a fresh work-up with the new projectiles.

Thanks for the reply but that doesn't answer my question.

Bottle necked rifle rounds.

Case volume affects pressure, we agree.

Reducing case volume by seating a bullet deeper (reducing volume) does not.

John Barness of Handloader magazine explained it something like this. ( I will find his actual words later).

Max Pressure is achieved after the bullet has exited the case and entered the Rifling of the barrel.  In some case with slower powders as much as 3-4 inches down the bore.

So, if Max pressure is not achieved until the case is fully expanded and the bullet has left the case, seating deeper will not actually "Decrease" case volume.  It will decrease the case volume of a loaded round, but not of a fired round.  

Barnes has load data and so does Hornady, at least look at one of these sources before proceeding.

Longer bullets and longer bearing surfaces equals higher pressures.

Ramshot and AA list some data for Barnes bullets as well. I think mostly TSXs which have the grooves which change things.

If you don't know your exact bullet perhaps you could post a pic and people could point you towards something similar.

Copper bullets are odd, some of the Barnes despite their greater length have data indicating significantly higher charges than similar weight lead. IIRC their match burners are to be loaded using traditional match bullet data.

I would venture traditional data for the weight class would be fine starting low and working up, possibly beyond max for a traditional jacketed bullet.

Solid copper has its own set of loading data.

 
I'm far from an expert, especially when it comes to all copper bullets. I do load TTSXs, but I use Barnes or Hodgdon data for them.


That being said, the post I quoted is the way I look at it too. A shorter bearing surface will decrease pressure, while a longer bearing surface will increase it.


all copper bullets (as in completely copper) don't have the heavier lead to increase weight so they do it with a longer bullet. The ones I have seen also have a longer bearing surface.


If in doubt go look at Nosler reloading manual #7 with the e-tips in it. It specifically states to reduce starting loads to the minimum listed load or below when loading e-tip bullets.

I agree, but.

I took the Original post as a reduction in Case Volume as that is what the OP heading was about.

If the OP is talking bullet design and bullet composition, bearing surface, Then my first post is out of place and in error.

Well at least someone can read.  Yes I was talking about "what" specifically creates the greater pressure?
"Decreased" volume in the case, or the weight of the bullet?  If we increase the weight of the bullet we decrease the case volume but is it specifically the reduced volume that is causing the pressure spike?

Certainly a heavier bullet is going to be longer and hence we add another component of surface tension.  I'm no physicist but I wouldn't think surface tension is going to matter if the mass is equal.

In summary, I'm not looking for load data (I can safely work that up).  The process just got me thinking of the physics behind pressures.

1. Burn rate
2. Powder quantity
3. Case volume?
3. Bullet weight
4. Surface tension?

One last thought: Full pressure develops even after the bullet leaves the case.  Is it not possible for the pressure to peak
just a little further down the barrel, with a reduced volume in a case,hence making up for that lost air in the case?

Solid copper bullets are normally harder to engrave on the rifling.

The grooves turned around the bullet are to try and bring the force down.

Op. steve4102 post already answered your case volume question.

A heavier bullet "can" generate more pressure but nearly all of it is do to the increased weight and nothing more.

We are mixing to many variables here. Plus there are no set in stone rules that cover this. One example is bearing surface. A shape of a bullet has as much to do with its bearing surface as the weight. I have flat base 150gr 30 caliber bullets that have more bearing surface than 185gr 30 caliber boat tail bullets.

Then you start adding in jacket material to the mix and it changes again.

Motor

With all due respect, yes their are many variables and the more of them we understand completely the better we will be
and yes these rules are set in stone...it's called physics.  Understanding them and how they interact may be a different story.

All I'm saying is there are too many variables for even an experienced handloader to just look at a bullet and decide what data to use.

It's always best to use published data. Preferably for the specific bullet you are using.

Motor

Read a couple posts back of mine.  I specifically said I am not looking for load data.

Popnfresh did some research on this, though IIRC it was to answer how jamming or jumping affects pressure.





This looks like it.

http://www.ar15.com/forums/t_6_42/428271_Seating_Depth_COAL__and_the_Relationship_to_Chamber_Pressure__An_experiment_UPDATE_IN_OP.html

Ok. I would agree with your friend, if the bullet was not a solid copper bullet. That changes everything.

You will see a much greater difference in pressure because of bullet jump than you will from a change in case volume because of a change in seating depth.

This is what say I.

Motor

 lnl:  I haven't conducted a controlled experiment, but as near as I can tell from my casual observations, as long as the bullet is not close to the lands (as is the case with most AR-15 rifles) the more airspace left inside the loaded cartridge, the lower the pressure.  For a given load, longer bullets (e.g., those with plastic tips) will produce higher pressure and velocity than a shorter bullet of the same weight loaded to the same OAL.  

Once you start getting close to the rifling, then another factor takes over.  Jacketed bullets jammed into the lands produce more pressure than bullets with some freebore going for them, regardless of the amount of airspace inside the case. - CW