Example for Determining Alternate Barrel Length Muzzle Velocities.

[Florida_Cracker]

When I started reloading for my Henry 357, I found very little commercially published MV data for a rifle length barrel. It seems that nearly all powder manufacturers’ data is based on revolver lengths. (Hodgdon being the exception – their LilGun pistol data is based on a 10” barrel. Hodgdon does not list a rifle load for LilGun and 158 projectile.) Somehow I needed to extrapolate this information to a 20” Big Boy barrel.

For those not familiar with the resource “Ballistics By The Inch” (BBI) (http://www.ballisticsbytheinch.com/ ), I recommend that you review their results. These researchers start with a long barrel Thompson Contender Encore and shoot for MV. Using a chop saw, they reduce the barrel length by one inch and repeat the chronograph shots until they have produced a table of ballistics by the inch for each ammo tested. (I salute these guys for their work.)

Using their data, some graphics and curve fitting, I developed formulas for 357 MV for various bullet weights. The first attached graphic shows the results for all 357 bullet weights tabulated by BBI. The second graph is for the two 158 grain rounds BBI reported. The curve represents the average results for the two rounds, and provides a good estimate of how to extrapolate pistol length data into rifle length data.

For my particular application, I wanted to estimate the 20” MV from Lilgun data reported for a 10” gun using a 158 grain Hornady XTP. From the graph, you can either estimate the MV for 10” barrel, or back-solve the equation for a more precise estimate. I got 1,593 fps. I then repeated this for a 20” barrel and got 1,842. The ratio is 1.156, so multiplying the LilGun data by 1.156 gives a calculated estimate of what the MV would be from a 20” barrel.

I have all of the 357 data in an Access database that I would be happy to share.
Florida_Cracker

BBI 158 grain 357 data

357_158_fps.jpg (70.12 KiB) Viewed 1906 times

[Cofisher]

Thank U for crunching the numbers. I shoot the 357 carbine. Your results are very interesting.

[Squatch]

Interesting indeed. I have posted data for 22s and for my 44 mag on here. I chrono'ed a lot of ammo in both my 10" Ruger Super Blackhawk and my Henry Big Boy. I find the velocity difference was normally around 300 fps with factory ammo. Handloads showed more or less depending on powder used and burn rate.

Having a chrono makes loading much more predictable for me.

[JEBar]

Having a chrono makes loading much more predictable for me.

agree .... ours plays a big part in us knowing exactly how our various loads actually perform

[Florida_Cracker]

Well, I finally acquired a chronograph and compared the measured MV with the estimates derived from the procedure I posted last year.

For the Hornady 158 grain XTP my estimate using Ballistics By the Inch data was low by 1.8%. For the Hornady 180 grain XTP it was high by 1.4%. For MV data, I am more than happy with an estimate using free internet data that agrees within 2% of measured chronograph results when you don't own a chronograph. But your mileage may vary.

Florida_Cracker