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Velocity, Barrel Length and the "Book"

Discussion in 'Handloading and Reloading' started by Rockfish61, Oct 16, 2011.

  1. Rockfish61

    Rockfish61 Well-Known Member

    Hello all,

    Does anyone know of any Written source for expected velocity change vs. barrel length? The data in the books are based on the "Test rifle" which is always longer than mine. I expect the velocity to be less but in the case it is the same Are my pressures getting to high?
  2. 1911Tuner

    1911Tuner Moderator Emeritus

    The simple answer is...it depends. Velocity gain or loss per inch of barrel gained or lost depends on the powder burn rate...bullet mass...and even relative humidity it seems. <---insert joking smiley here.

    A pretty good rule of thumb for autopistols and non-magnum revolver cartridges is 35 fps per inch. For magnum-class loads with the typical slow powders used, you can figure on 50 fps and be pretty close.
  3. steve4102

    steve4102 Well-Known Member

    Maybe, maybe not, it depends, it depends on a lot of things. Velocity is only one of the factors when trying to read mysterious pressure signs.

    You might find this interesting.
  4. 1911Tuner

    1911Tuner Moderator Emeritus

    Oh, yeah. It's possible to have higher pressure and lower velocity with one load than with another. Powder burn rate is a major factor. In...say a .41 magnum with a 210-grain cast bullet...1200 fps with Unique is a much different animal than 1200 fps with 2400.
  5. Rockfish61

    Rockfish61 Well-Known Member

    Mystery is right!

    I made up a mid-range (by the book) load using Varget. My velocity was 150 fsp less than the book but I noticed the Cases were flowing into the ejector hole.

    With my 4320 loads I am at the top end of the range, my velocity is close to the book value and no signs of pressure issues at all.

    Fortunately for me the FUN is getting there:D
  6. 1911Tuner

    1911Tuner Moderator Emeritus

    I learned a long time ago not to pay much attention to the accepted signs of pressure. Primers especially, but even slightly sticky cases...or the lack of same...can be misleading.

    I place more store in how the cases behave. If you have to trim every firing...your pressures are either too high, or you've got a headspace problem. If your primer pockets get loose after two or three firings...your pressures are probably over the peg.

    I knew a guy once who was a varmint shooter who took an annual trip to Montana just to make long shots at Prarie Dogs with a .220 Swift, and shorter shots...out to 500 yards or so...with a .22-250.
    He talked about velocity as though it was the Holy Grail...and he fought hard to wring every fps out of his rifles. His yardstick for too much pressure was blowing a primer pocket. He bragged about loose primer pockets within three rounds. I made it a practice to stand well away from him when he sat at the bench with one of his rifles.
  7. rsrocket1

    rsrocket1 Well-Known Member

    Get yourself the demo version of Quickload. Although you are limited to a couple of calibers and a few generic types of powders, you can punch in different barrel lengths and see the effect of muzzle velocity vs. barrel length.

    Their disclaimer says it is not perfectly accurate and that it should not be a substitute for personal or professional load data, but most calculations are very close to reality and more importantly, relative differences (powder charge/barrel length/seating depth) are represented very well.
  8. rcmodel

    rcmodel Member in memoriam

  9. Canazes9

    Canazes9 Well-Known Member

    To further muddy the water, expansion ratio can have a significant effect on barrel length vs velocity. The expansion ratio - Expansion ratio is the case volume plus bore volume, divided by case volume. The smaller the number the more critical barrel length is.

    A 358 win and a 243win both are based on a 308 cartridge and have similar case capacities. The 358win will be affected far less by chopping the barrel length than chopping the barrel on a 243win. I have seen chrony results from a 358win w/ a 16.5" barrel that was only about 50fps slower than published data for a 24" barrel. Conversely my sons 243 Ruger compact (16.5" barrel) would only shoot about 2450fps w/ 100grn loads, a loss of 50+fps per inch of barrel loss.

  10. suemarkp

    suemarkp Well-Known Member

    There's also the fact that no two barrels and chambers are the same. You can take two guns with 6" barrels and have 100 fps difference between them with the same load.
  11. USSR

    USSR Well-Known Member

    Bingo! I have 2 Match rifles, both with 26" barrels. With the exact same load, there is 75fps difference in velocity. There is no substitute for a chronograph.

  12. gamestalker

    gamestalker member

    Velocities will vary from one firearm to another for a number of reasons, not just barrel length. I have two 7mm RM's with the same barrel lengths and one shoots about 150 fps slower than the other. It can be the effect of barrel wear, different barrel chacteristics, chamber dimensions, seating depth, and certainly some other inadvertant elements of firearm or cartridge form.

    Now regarding pressures, this is something that goes way beyond a simple calculation. It is common place in reloading to see loads that produce higher velocity while producing over all lower pressures and visa versa. Pressure pattern or the pressure curve as termed, is a major factor in how much velocity is obtained. This is where the powder burn rate and seating depth of the bullet come into play. Fast burning powders produce a sharper pressure curve, and that even though pressures can often be higher than that of slow burning powders, velocities are commonly slower.
  13. Rockfish61

    Rockfish61 Well-Known Member

    Thanks for the information all, This is why I love this sport, Always learning and something new to try.

    I have attached my data for a load workout I did on the 155 smk. The projected velocity is from the book which noted a 26" test rifle length. Mine is 20". Load # 2 and 4 showed definite signs of over pressure and my measured velocity was where i expected it to be with a shorter barrel. Load 14 exceeded the book estimate and the cases showed no signs of over pressure ( though the accuracy suffered) but are the pressures to high because the velocity exceeded the book data? That is what started this thread.

    Attached Files:

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