Effect of Shortened Barrel Length on Muzzle Velocity

Effect of Shortened Barrel Length on Muzzle Velocity

This is a discussion on Effect of Shortened Barrel Length on Muzzle Velocity within the Defensive Ammunition & Ballistics forums, part of the Defensive Carry Discussions category; I discovered a very interesting article here explaining Homer S. Powley's simplified model for estimating the muzzle velocity of a bullet in a shortened or ...

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  1. #1
    Member Array sentioch's Avatar
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    Effect of Shortened Barrel Length on Muzzle Velocity

    I discovered a very interesting article here explaining Homer S. Powley's simplified model for estimating the muzzle velocity of a bullet in a shortened or lengthened barrel,

    Topic of the Month

    I haven't bothered to track down any of the actual writings of Mr. Powley, so I am unsure of the reliability of these equations. However, I created a chart based off of these equations which will allow anyone to easily estimate the muzzle velocity of any typical handgun round out of a shorter or longer barrel for typical barrel lengths.



    In order to use the chart, use the table that corresponds to the caliber in question. Locate the column that corresponds to the barrel length which the muzzle velocity was measured at. Then multiply by the factor provided in any given row to get the expected muzzle velocity out of a different length barrel. This gives you the expected muzzle velocity of THE EXACT SAME AMMUNITION fired out of a shorter barrel.

    In general, the model predicts somewhere between 43 fps - 94 fps loss in muzzle velocity per 1 inch shortening of the barrel.

    Keep in mind that muzzle velocity depends on a great many factors such as the barrel rifling, load, and shot-to-shot variability.

    However, it is my hope that this chart might prove accurate in comparing the same ammunition fired from different models of gun produced by the same company. For example, in order to assess the relative loss of muzzle velocity between the standard, compact, and sub-compact Glock varieties using the same ammunition.

    I would welcome any chronograph evidence that you may have which can support or refute the accuracy of this chart. Thank you!
    Last edited by sentioch; June 2nd, 2010 at 02:08 AM.


  2. #2
    Member Array UncleDannie's Avatar
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    I have often wondered about sawing off the barrel of a 4" .38 revolver by 2", making it into a snub-nosed version. I can't understand if the muzzle velocity would be impacted by the barrel rifling.

    Decisions, decisions, decisions.
    Uncle Dannie
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    Member Array sentioch's Avatar
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    Quote Originally Posted by UncleDannie View Post
    I have often wondered about sawing off the barrel of a 4" .38 revolver by 2", making it into a snub-nosed version. I can't understand if the muzzle velocity would be impacted by the barrel rifling.

    Decisions, decisions, decisions.
    The more significant concern is expansion ratio. Because the .38 special has such a large case, that means it's going to have a very small expansion ratio so shortening the barrel will have a much more dramatic effect than on, say, a 9mm.

    I am unsure of the case volume of the .38 special, but I would be curious myself to tabulate the values...as well as for some other rounds that I missed such as .357 magnum, .45 GAP, .44 magnum. However I do not know what the case capacity is for these rounds and I cannot calculate it without that.

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    JD
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    OK, I'm going to throw a couple things out there...

    1: For velocity specs by barrel length check out Ballistics by the Inch

    2: Regarding the rifling effecting velocity...I'm trying to word it right but it's coming out like gibberish....

    3: Expansion is not relevant to case size, but the powder charge in that case. I don't care how big or small a case is, it's the charge pushing the bullet and the velocity of it being forced out the barrel that will effect expansion...

  5. #5
    Member Array sentioch's Avatar
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    Quote Originally Posted by JD View Post
    OK, I'm going to throw a couple things out there...

    1: For velocity specs by barrel length check out Ballistics by the Inch
    Nice find!!

    2: Regarding the rifling effecting velocity...I'm trying to word it right but it's coming out like gibberish....
    I would expect the rifling grooves to ever-so-slightly increase friction of the bullet as it travels through the barrel, meaning that a higher twist rate should ever-so-slightly reduce muzzle velocity.

    A higher twist rate is necessary on faster moving projectiles in order to keep them stabilized, but a slightly higher twist rate isn't going to have any negative impact on the external ballistics of the bullet. Really all it does is improve accuracy and stability of the flight path.

    I don't think twist rate should be an issue for UncleDannie, as shortening the barrel reduces the expansion ratio which will reduce the velocity of the bullet, meaning that it now has a slightly higher spin rate than necessary. In fact, since the gasses would not be able to fully expand in the shortened barrel, the irregular expansion that occurs just outside of the barrel might contribute to some loss of accuracy, which would justify the need for a slightly higher twist rate.

    If you have any reason to disagree with me on these points, please do say so!

    3: Expansion is not relevant to case size, but the powder charge in that case. I don't care how big or small a case is, it's the charge pushing the bullet and the velocity of it being forced out the barrel that will effect expansion...
    Expansion is relevant to case size because it is directly linked to the amount of powder charge. "Expansion ratio" is a commonly used heuristic that is calculated as case volume + bore volume divided by case volume. Assuming that the case volume is filled with charge, it is essentially tells you the relative difference in the size between the initial powder and the room which it has to expand in the chamber.

    When you change only barrel length, the affects due to various cartridge pressure levels and burn rates is largely negated, which is why expansion ratio becomes a good indicator. Powley's approximation formula depends on expansion ratio which is why I needed to know the case volume in order to apply the equation.

    The table I have calculated is not intended as an exact calculation, as it does not take into account the variability in case volume as a function of bullet grain, and Powley's approximation also does not take into account the specifics of cartridge pressure or powder burn rate. It is merely designed as an easy to use approximating formula.

    Because it is just an approximation, it is clearly much better to use real data whenever possible. Thank you again for this great resource! This will also allow me to test the usefulness of Powley's approximation.

  6. #6
    Member Array Gunsmoke16's Avatar
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    Chopping off your barrel?

    Quote Originally Posted by UncleDannie View Post
    I have often wondered about sawing off the barrel of a 4" .38 revolver by 2", making it into a snub-nosed version. I can't understand if the muzzle velocity would be impacted by the barrel rifling.

    Decisions, decisions, decisions.
    If a competent gunsmith does the job, it has to be re-crowned
    (the rifling grooves in the barrel, at point of where the bullet leaves
    barrel will have burrs and re-crowning, especially with a slight inset,
    will protect them and allow the bullet to have true trajectory).

  7. #7
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    "...Powley's approximation also does not take into account the specifics of cartridge pressure or powder burn rate."


    This hits the nail on the head.


    The expansion characteristics of a cartridge's powder charge may also be changed by barrel friction or bore diameter.



    Caution: non-fact ahead.

    Here's an unscientific theory about how rate of twist could affect velocity as mentioned in posts above. I've pondered the possible reasons but have never before seen anyone else speculate on this aspect of interior ballistics and how rate of twist might affect bullet velocity.

    I have a number of Colt and Smith & Wesson .38 Special revolvers and also Colt and Smith & Wesson .357 Magnum revolvers.

    In chronograph tests conducted through the years, I've observed that the Colt revolvers generally yield slightly higher velocities with a given load than do the Smith & Wesson revolvers, assuming barrels of equal length. This is only the observation of one person and with a small sampling of revolvers.

    Colt rifling is 1 in 14 inches. Smith & Wesson rifling is 1 in 18 3/4 inches.

    I've long wondered if the quicker Colt rifling slightly elevates pressure behind the bullet which more than overcomes the impediment of any increased friction that could be generated due to the faster twist. This increased pressure might be the cause for higher muzzle velocities in the Colt guns.

    Colt bores appear to be slightly tighter. The Colt barrels I've slugged measured .356/.357 whereas the Smith & Wesson barrels measured .358. Perhaps the higher velocity is due to the tighter bore rather than the differences in rifling rates.

    I've noticed this in 2, 4, and 6 inch guns manufactured by both companies and with both the .38 Special and the .357 Magnum cartridges.

    It happens that the mildest hollow base wadcutter handload will generally give slightly higher velocities from the Smith & Wesson revolver than it will from a Colt with the same barrel length. Such loads are extremely low pressure though.

    As the velocity differences are small, debating such a matter is akin to arguing over how many angels can dance on the head of a pin. Besides, my theory may be gibberish too.

    It is possible for some short barrels to consistently give higher velocities than a particular longer barrel will. I once experienced this with a .357 Magnum revolver. A 6-inch .357 revolver I have gave higher velocities than an 8 3/8-inch version of the same model revolver with every different load that was tested in both guns. I considered that the 8 3/8-inch revolver just had a "slow" barrel and later sold it.
    Charter Member of the DC .41 LC Society

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  8. #8
    Member Array Jim Downey's Avatar
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    I mostly just wanted to pop in and say Hi! and thank JD for citing BBTI.

    JD - central Iowa? I did college at Grinnell (yes, there are hard-core gun nuts like me who went there . . . though it was a few years ago) lived in Montezuma for a few years after. Loved it.

    Anyway, back to your discussion . . .

    Jim D.
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  9. #9
    Member Array sentioch's Avatar
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    Quote Originally Posted by bmcgilvray View Post
    I've long wondered if the quicker Colt rifling slightly elevates pressure behind the bullet which more than overcomes the impediment of any increased friction that could be generated due to the faster twist. This increased pressure might be the cause for higher muzzle velocities in the Colt guns.

    Colt bores appear to be slightly tighter. The Colt barrels I've slugged measured .356/.357 whereas the Smith & Wesson barrels measured .358. Perhaps the higher velocity is due to the tighter bore rather than the differences in rifling rates.
    Hi bmcgilvray, and thanks for joining the discussion!

    The twist rate should not affect pressure in any way. However, having a slightly tighter bore can increase velocity (to a point). I believe the reason is because it causes the bullet to deform slightly more, creating a better seal. I also modify airguns and one of my favorite modifications is to use a barrel which is slightly tighter than stock and usually increases FPS by about 15%. I think this is the most likely explanation for the differences in muzzle velocity you noticed.
    "In a world of compromise, some don't." -HK

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    Distinguished Member Array razor02097's Avatar
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    Quote Originally Posted by sentioch View Post
    I would expect the rifling grooves to ever-so-slightly increase friction of the bullet as it travels through the barrel, meaning that a higher twist rate should ever-so-slightly reduce muzzle velocity.

    A higher twist rate is necessary on faster moving projectiles in order to keep them stabilized, but a slightly higher twist rate isn't going to have any negative impact on the external ballistics of the bullet. Really all it does is improve accuracy and stability of the flight path.
    Keep in mind however if the round is designed correctly it will continue to accelerate the bullet down the barrel. Friction only would come into play if the bullet is decelerating. The instant the bullet exits the barrel and gasses flow around it is when it starts to decelerate.

    Case in point

    Caliber = .44
    powder = 24.6g H2400
    primer = magnum

    Velocity
    8 3/8" barrel 1,590fps
    3" barrel 1,136fps

    There was a lot more unburnt powder and residue in and around the 3" barrel gun. The rounds I hand load for my 3"bbl will have a different powder because it does not have the length to accelerate the bullet to a decent speed. Most loads I can squeeze between 1,100 and 1,300fps. most times I go with slower speeds below 1,000fps as this increases accuracy dramatically!

    YMMV
    There is something about firing 4,200 thirty millimeter rounds/min that makes me feel all warm and fuzzy inside.

  11. #11
    Member Array sentioch's Avatar
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    Hi Razor, yes absolutely the bullet will continue to accelerate (as long as the gas has not had room to fully expand behind it in the barrel).

    You probably already know everything I'm going to say here already but I am just going to say it anyway in order to clarify my previous post for anyone else who is reading it.

    Without any friction in the barrel the expanding gasses would slip past the edges of the bullet and it would not go anywhere, so increasing friction up to a point where an excellent seal is made will increase velocity. After that point, increasing friction will cause the powder energy to be wasted in needlessly deforming the bullet rather than accelerating the bullet.

    This is why polygonal rifling in the Glock pistols gives them a slight advantage in terms of muzzle velocity in comparison to traditional land and groove rifling; the bullet does not need to be deformed as much in order to make a tight seal, because the shape of the barrel is closer to the original shape of the bullet. This means that more of the energy can be spent on accelerating the bullet.
    "In a world of compromise, some don't." -HK

  12. #12
    Distinguished Member Array razor02097's Avatar
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    While polygonal rifling may allow a slight advantage it isn't without it's disadvantages. It doesn't do well with lead for one thing. Another thing to think about is bullet design. Many FMJ aren't. some have an open back with exposed lead. That leaves deposits and residue. TMJ typically will make a better choice.

    Despite what people think even going to match grade machined and polished barrels new barrels are going to typically be tighter anyway so there isn't a point in trying to maximize a hand load out of a brand new barrel. Once your ready to start maximizing a load research will go a long way in saving you time and money. A good, cheap test is to fire a shot into media that you can recover a bullet from. Save the spent brass and bullet for analysis. Checking the rifle pattern on the bullet as well as the change in diameter can help determine if you have a tight barrel. Check the spent brass and primer for any signs of over pressure. For autos check the lip of the brass for head spacing issues (blow by and lip damage)


    To be honest I don't see much potential in trying to perfect a hand load in a pistol. Sure it may be fun but talking about stuff like friction in a barrel VS. velocity is typically something you would find in a rifle debate. A pistol that shoots well with a certain load is what I will shoot. It may not be the fastest or loudest but if it is accurate enough to blow up a squirrel at 15 yards I am happy.
    There is something about firing 4,200 thirty millimeter rounds/min that makes me feel all warm and fuzzy inside.

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