Home Forums Reloading Big Game Rifles What produces higher pressure - New brass or fire formed?

OP, please talk to us about the load you're working up.

I am a novice reloader but, first principles lead me to expect the once fired to have higher pressure. I am saying this under expectation that the power that once went into making the brass conform to the chamber is now running into the boundary conditions of the chamber wall much sooner, thus pressure builds. There are some very experienced folks on this board... maybe find one and DM them if the above answers don't already provide a definite answer (preferably with peer-reviewed reference).

Not exactly the same , but interesting.
I have a sub moa load in my model 7 7mm-08 using Starline brass , 120 bt’s , WLR , and Varget.

Recently picked up some new Starline brass just because I could.
I loaded some of them up with the same load. They still shot sub moa , but consistently printed groups 1 1/2 moa lower than the same load in multi fired brass.

What the heck ? Went and grabbed the chrono. Both loads shot the same velocity , ES, and SD , but new brass still printed 1 1/2 moa low .

???? Don’t know other than anytime ANYTHING is changed in a load all bets are off!

Reloading can be interesting, can't it? I think I might be able to help with some concerns. Excuse the length.

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You bought some new 308 Win. Lapua cases and determined the unfired case volume is 54.7 grains (water weight method). You want to load Hornady 165 gr Interlocks. The Hornady manual says 44.3 grains of Re 15 is the maximum load.

For the purposes of this explanation, I will forego working up the load. This is purely a mathematical exercise.

You load the cases with 44.3 grain of re 15 and seat the bullet so that the COAL is 2.800 inches. The computer simulation says that the case is 100.2 percent full and will generate a chamber pressure of 53995 PSI.

You take five loaded cases to the range and fire them over a chrono. The average of the five shots is 2670 fps. The group looks good!

You return to the house and re-measure the case volume using the water weight method. The volume has increased to 55.7 grains (1 grain) because they are once fired. You reload the once fired cases with the same load as before - 44.3 grain of re 15 and seat the bullet so that the COAL is 2.800 inches. The computer simulation has changed! The once fired cases now have 2% more volume. It shows that the case is now 98.1 percent full and will generate a chamber pressure of 51770 PSI. This is a drop of 2225 PSI.

The pressure drops because there is more internal capacity in the case. You are burning the same amount of fuel, but in a larger space. What happens next?

You take five loaded cases to the range and fire them over a chrono. The average of the five shots is 2640 fps - a drop of approximately 30 fps.

The pressure dropped because there was more room in the case for the powder to burn. The lower pressure that results means that the bullet will be pushed out the barrel at a lower velocity.

Did the computer simulations account for the fact that the volume of the brass changed more during the first firing than the second as it expanded to fill the chamber? Or did it act as if there were two chambers of different but fixed volumes?

You have to continually update the data. IOW, measure the volume after the second and subsequent firings and use any volume changes to determine velocity and pressure.

According to the simulation, you would have to add 0.4 grains of Re 15 to duplicate the velocity of the unfired brass. That first use will be the largest change and the only adjustment you should need to maintain the original velocity. That does not take into account the changing throat or different lots of powder. These changes will be gradual however, and for the purposes of the discussion should not be considered.

I mean literally, during the first firing only, as the brass expands its internal volume changes. Does the computer simulation account for the fact the combustion volume isn't constant?

No, it only determines results for a given period in time. A snapshot, if you will.

This is one of the problems with software. As the case expands to conform to the chamber, and the volume changes, there is no adjustment or constant that takes the change into account. Hence the need to repeat the measurements.

I do not believe that this helps the discussion because it adds complexity, but for mathman, this is a Quickload prediction of the bullet's travel time vs pressure.

You can influence the curve in so many ways. A partial list would be -

throat length
condition of rifling
barrel diameter
propellant used versus control lot (quickness)
volume of propellant
case volume

Maybe this is an oversimplification, but it seems to me that new brass would have to be higher pressure, all other things being equal. Fire forming means the brass expands to fit that particular chamber, no? Larger brass (all other things being equal) means increased interior volume which means lower pressure. Fire forming sure doesn't shrink the case, which would result in increased pressure if everything else is held constant. Is it significant or meaningfull? Who knows unless you have a way to measure pressure. I do not. Just my 0.02.

Regards,

Kris

Everything said above about case volume and pressure can be taken to the bank!
I would stop, take a big breath and re-evaluate everything if you are blowing primers and experiencing hard bolt lift and extractor marks on the case head. There is definitely dangerous pressure and something is not right.

Measure your case length and trim to the trim to length measurement. Reduce your load maybe even down to the starting load and seat your bullet to the suggested COAL length. Then and only then change things one at a time.

Be safe. Something is wrong. Has your powder gone bad by exposure to heat or improper storage? Maybe try another powder?
KaBooms are a real danger. Be careful and let us know how things go

QM