Load data extrapolation

[MoreIsLess]

If you know the OAL, bullet type, powder mfg, powder charge wgt for a particular load to produce a certain FPS velocity, is there a way to extrapolate that so you can determine what powder wt to use to obtain the same FPS using a different OAL for the same bullet type, powder mfg.

Example:
If 4.2 gr of Bullseye with an OAL of 1.065 with 120gr lead RN will produce 1175fps

how many grains of Bullseye with an OAL of 1.142 with 120gr lead RN does it take to also produce 1175fps

 

[rcmodel]

No there isn't.

You can't even depend on the same load giving within 50-75 FPS of each other in two identical guns.
Let alone a change of barely more then 1/16" in seating depth.

Thats what chronographs are for.

rc

 

[Rule3]

Maybe buy Quickload Software,

http://www.neconos.com/details3.htm

 

[rsrocket1]

+1 on the Quickload. It is the ideal software to see the effects of "what if" circumstances. It is amazingly accurate most of the time, but just like load books, should never been taken at face value for your own loads. You must do that yourself by working up loads in your own gun.

For the OP's question, download the demo version of QL. Once you learn how to work it, you can adjust the load, seat depth and bullet weight using the sample bullets and powders to see how they all interact.

So yes it can be done, but not with a "rule of thumb" like minimum twist rate vs. bullet weight.

 

[Shadow 7D]

every load I have seen states something to this effect
"do not load the maximum, start with a medium load and work up to what performs best in your gun."

There are quite a number of reason, safety being a big one, also many gun perform best with loads other than the maximum.

 

[1SOW]

FWIW:

If you are not loading at the extremes of the data ranges, YES--sort of.

Case volume and powder load are NOT proportional above or below a certain point depending on the powder. A chrono is essential to determine the actual bullet speed

A simple ratio relationship of powder to oal will work to find a "NEW STARTING POINT FOR YOUR LOAD" up to an unknown point. NOT for extreme loads.
EG: 4.2/1.065" X L/1.142" = 5.1grs at 1.142 START low and work up.
"L" = Powder Load
Sanity checks are also needed. Does that load compress the powder? Measure the powder depth and compare it to the new seating depth. There should be empty space between the bullet base and the powder level.

A chrono is necessary to determine speed, and advisable to use for checking to your load build results.

I make NO guarantees. This works for me with light loads to find a starting point at a different oal using the "same" bullet.
If it's a different bullet of the same weight, I use the old bullet's "seating depth" to determine the new oal in the ratio above.

Hope this makes sense.

 

[Lost Sheep]

Never extrapolate. Interpolate all you want.

If you know the OAL, bullet type, powder mfg, powder charge wgt for a particular load to produce a certain FPS velocity, is there a way to extrapolate that so you can determine what powder wt to use to obtain the same FPS using a different OAL for the same bullet type, powder mfg.

Example:
If 4.2 gr of Bullseye with an OAL of 1.065 with 120gr lead RN will produce 1175fps

how many grains of Bullseye with an OAL of 1.142 with 120gr lead RN does it take to also produce 1175fps

Never extrapolate outside of a ballistics lab. Interpolate all you want. Staying INSIDE performance parameters is pretty safe. Extrapolating can put you outside safe performance parameters without you knowing.

1SOW is right on.

Having said that, remember, it is not the length of the finished cartridge that is the parameter of importance. It is the free volume UNDER the bullet that is the main determinant of the pressure, and velocity. Heavier bullet with the same volume under it will have a longer C.O.A.L. Also, a longer COAL will be closer to the rifling, which USUALLY tends to produce lower velocities.

Invest $100 in a simple Chronograph. You will be miles ahead of where you would be if you try to get a specific velocity by calculating. Calculating will promise. Measuring will tell. Who you gonna believe? A promise or real evidence.

Good luck. Thanks for asking our advice. Your head is in the right place. Small tweaks to get to the results you want is smart. But without some way to measure the results with certainty, the calculations are just a guess, really.

Lost Sheep