To present this question, I am offering this input a published reloading manual that states X gr of powder = Y velocity Starting will produce Z FPS and X gr of Powder = Y Velocity will produce Z FPS Maximum.

For this question there is 10.0 gr spread of powder between starting and Maximum load, and for the sake of argument a load is chosen at the 6th level of charge, how can you calculate the FPS velocity for the chosen load?

Be spiffic as this is important to me to know this Calculation Process.

1) Velocity published in a reloading manual is not calculated. It is measured by chronograph.

2) Reloading manual velocities are specific to their barrel/bullet/powder lot/primer combination, and are often from a longer barrel than your firearm has.

3) Neglecting (1) and (2), if by "6th level of charge" you mean 6 grains up from the starting load you could get a THEORETICAL idea of velocity based on the published velocities as follows:

((Max load velocity) - (Start load velocity))*.6 + (Start load velocity)

This calculation is based on the 10 gr. spread you specified. For other spreads, instead of .6 you would use (# of grains over start load)/(difference of max load and start load).

But that is only a ballpark figure and your velocity is almost guaranteed to be different, perhaps significantly.

Smokeless powders do not burn at a pre-determined linear rate. As the pressure rises, the burning rate increases, thereby increasing pressure, which accelerates the burning rate...etc. There are a whole lot of factors that will alter the burning rate of any given powder. Most of those factors, you will have no control over. Quite often, an increase in powder will show a velocity drop. This is why ballistics laboratories are in business. Some software ballistic programs attempt to calculate this phenomenon, but it just can't be done with any certainty. The only reliable way is to actually measure the velocity of any given powder charge, under a given set of conditions, with a chronograph. There are just too many variables in the reloading process, to reliably predict a result.

mbartel is absolutely correct.
The pressure curve is dependent on quite a few factors , the brissance of the primer, compression of the powder-or lack thereof- the tightness of the bore, the elastic properties of the bullet as it passes through the bore, all play a factor in the development of the pressure curve, therefore the velocity of the projectile is not linear to the pressure.

It sort of like horsepower in cars, the addition of twice as much horsepower does not make a car twice as fast, in fact, the closer you get to maximum, it takes progressively more and more to get less and less.

The responses are all reasonable and thank you for your input, the thing I had been doing to get a rough estimate was to take the max published velocity and subtracting the starting velocity, dividing the answer by a factor of graduations, as in my origional question of ten then using that answer to step the load by each new answer to try to get an estimated Velocity, knowing full well that the answers would not be exact, but bight be at least in the ball park.

Here is a little monkey wrench in your calculation. You can vary your velocity up to 100 or more fps with a clean barrel versus one that has been fired about 4 rounds. The clean barrel will shoot the lower velocities. Don't believe me!! Get a chrony and see for yourself or look around the site here and you can find loads of info on the subject.

There ARE programs that will predict the approximate velocity for a bullet/powder combination for a particular cartridge. But FIRST you must have at least two different velocities measured by a chronograph from a specific firearm using two different charges of the same powder for that particular bullet.

AS other have said, to many variables. There is only one way to know how fast or slow your load is in your firearm, shoot it over a chronograph. Anything else is a WAG. I've shot identical loads in sim. firearms (same manuf., same bbl. length, etc.) & had as much as 100fps diff. from one to the next. So is your high estimate really high or 100fps less? Most guys I let run loads over my Oehler are really dissapointed to find out it's going 100-200fps less than "the book".

quote:

Originally posted by cobra44:
...how can you calculate the FPS velocity for the chosen load?...

Hey cobra44, Your "guesstimate" is as good as anything else because of all the variables.

If you knew the "actual Velocity" rather than the guesstimated Velocity, what benefit is it to you?

I concur, let a chrony do the calculating. I chrony'd one particular rifle that had a shot out bbl and it was as much as 300 fs below other rifles with the same ammo!

I've had two essentially identical rifles (well actually one of them was my brother's) that were NOT identical in performance

They could use neck sided brass interchangably and both had the same lenth, twist, etc...
Both exen showed pressure signs it the same ecact point and shot well with the same loads.

the one fly in the ointment? one rifle consistantly shot 60-70fps faster than the other on the same day, same bench same box of reloads across the same chronograph.

Both rifles had the same number of rounds through them too....

We never found out why one was slow.

My brother still has the "fast" one, the other was eventually sold.

Estimate, calculate or guess all you like, you never know for sure until you measure.

AllanD