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Effects of barrel length on shotgun patterning

Discussion in 'Shotguns' started by Geckgo, Jul 4, 2011.

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  1. Geckgo

    Geckgo Member

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    Hello all, haven't been shooting long, but I'm sick of seeing these threads. Maybe if I'm lucky the mods will sticky this, but it needs a writeup.

    As a physicist, this is an attempt to describe how shotgun patterning tends to work, and why all of us are wrong whether we say shotgun barrel length does or doesn't matter.

    The people who say it doesn't matter are generally referring to the fact that it doesn't matter that much, when not accounting for a built in choke, but straight cylinder bores.

    The people who say it does fall into many categories. Some are concentrating on the minute differences in the above scenario, and admit that it isn't much. Others are pointing out that you remove the permanent choke of the shotgun by sawing it off. Others live under some delusion that because the barrel is shorter it is less acurate, thus the shot will spread "faster". Anyways, here are my thoughts, speaking purely as a physicist and not neccessarily as a shotgun expert.


    First, there are a couple things that I'm taking for granted, number one is a perfect shot cup that for the velocites we are discussing, will perform as it should. It will carry the shot to the muzzle and then drop away. I'm also not going to bring up pellet deformation, as in cylinder chokes, it really doesn't have any real effect. Third assumption is for the same ammo from cyclinder chokes of different lengths, as this is the "meat" of this argument.

    For the sake of argument, I will talk about spread at a distance from the muzzle of the shotgun, not from the action, so the little arguments about "it has a foot or two more room to expand" can be ruled out. This is true, but it's academic.

    So, we have two loads, we'll say one from a 36" cylinder bore and one from a 24" cylinder bore. When fired, we'll say the 36" has a velocity of 1500 fps, and the same load in the 24 will be 1100fps, just for the sake of argument, invent whatever kind of powder/primer load you want to fix this. When measured from the muzzle, I argue that the 24" barrel will account for a larger spread (not much) than the 36" barrel, the reason is this:

    At 10 yards, we check the pattern from our 36" barrel and get a 5" diameter, again, for the sake of argument (this is from the muzzle), we can assume that we are pretty close to muzzle velocity to keep the equations from getting too big (no need to post the actual equations, I think everyone will see my point without them). At 1500fps, it takes the shot .02 seconds to reach the 10yrd target, and begins with a diameter of around .75 inches. So the shot is expanding at (5-.75=4.25inches, 4.25/0.02=~212in/sec),, We can hold this as a general rule of thumb for the expansion of the shot pattern. Using the same expansion vs time, the 24" barrel would make a pattern at 1100fps of about 5.78 inches. At the next interval, the velocity drops a bit and the pattern increases a little more...

    36" :~1450fps now form 10 to 20 yards = 9.39" pattern at 20 yrds
    24" :~1070fps now form 10 to 20 yards = 11.72" pattern at 20 yrds

    I'm sure everyone by now can see where this is going. All other things being equal, the shorter barrel is going to produce a larger pattern because while the expansion of the shot remains the same versus time, the shorter barrel releases it at a slower velocity, impating the pattern at range.

    Seems like a lot of difference at the moment, but there is (among the millions of variables out there) another point to be made, that the faster moving shot will be a bit more unstable when it fires from the muzzle, due to higher energy and more "shock" from the transition from tube and cup to open air, and will have a higher rate of spread, maybe 220 instead of 212 or something similar, I haven't measured these directly so it's just a for-instance, but more velocity=more instability=faster rate of spread. This will allow the 36" barrels pattern to be a bit larger than the above calculation perscribes, and thus the actual patterns will be a bit closer to eachother then the calculation predicts.

    This speed difference is also the reason that shotgun patterns tend to "fan out" at larger distances rather than being related linearly as pattern vs distance. As the pellets slow down, it takes them longer to cover the same distance and they have more time to expand.

    I already know there's going to be a billion know-it-alls out there, and this simple model does not cover everything pertaining to shotguns, there are many other variables, but these are the big ones that will have the most influence.

    Hope this clears the issue up for most people.

    lol, told you I would write it up one day, cheers!
     
  2. roadchoad

    roadchoad Member

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    I don't have a dog in this fight, but it looks like you threw some arbitrary numbers in there and called it proof. :confused: You state that the rate of expansion is constant...but how do you know this?
     
  3. Geckgo

    Geckgo Member

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    None of this is "proof", if I used that word anywhere in there then I apologise, but I don't think I did. I'm using arbitrary numbers because the values that I assigned arbitrarily don't mean much and they are simply to illustrate a point. This is just a model to demonstrate to people what happens when they fire a shotgun and how pattern is affected.

    Expansion will not be constant, but without sawing a barrel off (which I won't do) to get some "real" data, I chose some values that would yield realistic results. Again, this is to illustrate a point. Expansion is going to be based on a lot of things, namely the velocity (as it will determine the drag on the front of the shot wad), the weight and size of the shot (as it will determine how the group of shot will squish when it gets in open air), and the number of pelets (more pellets will cause the group to separate a bit quicker because the odds increase for a couple taking a little more than average energy from the group and flying, similar to breaking up a 9-ball rack vs a 8-ball rack).

    Again, Model, Description. If you want me to post Differential equations that will take a considerably longer post and lots of charts and diagrams that I don't have time to draw at the moment. This is not a "proof" of anything, there's nothing to be proven. The proof would come from the field. Maybe I can look up and see if anyone has sawed up one of their shotguns just to see the effect. I'm just trying to give people a better understanding of what the pellets are doing and how minimal the effect of "sawing off" a shotgun could be.
     
  4. Geckgo

    Geckgo Member

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    Terribly sorry, I think I misunderstood your question :what:

    I though you meant consistant expansion profiles between the two barrel. Once the shot starts expanding it will continue to expand at the same rate in time, IE the expansion rate won't change between 10 yards and 20 yards, this is just a fundamental law of projectile motion, derrived directly from "an object in motion will tend to stay in motion and in a straight line..." When talking about projectiles you have the forward component x and the lateral component y to describe the motion. The lateral component on it's own does not encounter enough resistance to "slow down" unless we are talking about ranges where shotgun patterns make no sense, so once the bbs start drifting away from eachother, they will continue drifting at the same rate. 1 second is not enough time for them to change their rate enough to matter, so, at fractions of a second it can be ignored. Hope this helps.
     
  5. Leadhead

    Leadhead Member

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    It's more fun to just pattern your gun with different loads and chokes.
     
  6. Geckgo

    Geckgo Member

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    I agree completely leadhead, just tired of seeing people arguing about this and comming up with all kinds of bizzaro theories on it. One of these days I will test putting a piece of cardboard or felt in front of the pellets (if I ever start reloading shells). I bet that would make the spread immense b/c the low sec density of the material would cause it to push all the way through the the bundle of pellets and in theory would disperse them very fast. You may end up with a "dougnut" of pellets though.
     
  7. Sam Cade

    Sam Cade Member

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    Its been done, some time ago. :D


    lol

    http://www.possibleshop.com/s-s-wads.html

     
  8. o Unforgiven o

    o Unforgiven o Member

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    What I got out of this was as follows;

    Longer barrels do pattern tighter but not for the reasons many think. In other words, the pattern size difference of long vs. shorter barrels is simply a function of velocity. Am I correct?
     
  9. Geckgo

    Geckgo Member

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    Unforgiven, pretty much, barring chokes, different shot sizes, or faulty cups. Also pointing out that the difference is not as much as one would expect, and this doesn't take into account weird things that might happen with really short, say 4-8" barrels, there may be other things to consider.
     
  10. Geckgo

    Geckgo Member

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    Unless I've overlooked something of fundamental importance..... nah, that's how it works :D
     
  11. o Unforgiven o

    o Unforgiven o Member

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    I suspected so. I still think my explanation was a bit more, eloquent. ;)
     
  12. Geckgo

    Geckgo Member

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    Well, you definitely summed it up nicely. I think if I would have just said that then there would be a lot of "no, it's because of <insert cliche'>"
     
  13. Virginian

    Virginian Member

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    Next you can take on the "pump guns shoot harder" theory.

    And if you manage that, I would personally appreciate it if you would explain to me why hand fitting makes some people go all dreamy eyed. (Yes, I am one of those Wingmaster lover lowlifes who NEVER grasped the love of the Model 12).
     
  14. Geckgo

    Geckgo Member

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    hmmm, I'm going to sleep now but I'll look into it :D

    There's nothing wrong with a wingmaster.
     
  15. eastbank

    eastbank Member

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    i went to a 21 inch barrel from a 30 inch barrel on my 3 inch mag rem 1187 to use on turkeys and i can,t tell the difference out to 50 yrds with a .640 choke and federal 2oz copper plated 6,s with shot buffer, other than its lighter and handier in the woods. i,m sure i lose some volocity but not enough to matter to a turkey at 50 yds. i don,t like to shoot beyond 35 yds,but it,s nice to know your gun will get the job done if the bird hangs up. eastbank.
     
  16. RickMD

    RickMD Member

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    The problem with the argument is that modern, smokeless, shotgun ammunition achieves maximum velocity in an 18" to 20" barrel. A longer barrel adds nothing to, and can actually decrease velocity if it's overly long.
     
  17. MCgunner

    MCgunner Member

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    Well, for all PRACTICAL purposes, I've found that a 20" shotgun does just as well on doves as a 28"er. Whether there's 2 percent difference in pattern density with a given choke or not, I wouldn't know and don't really care. All I know is a dove inside 35 yards is dead and 40 is a probable with a modified choke from my 20 gauge coach gun IF I do my part. I can't knock it considering I can shoot better than 50 percent with it inside 35 yards if I ate my cheerios that day. Now, yeah, I have my bad days, too, but that ain't the gun's fault.

    I think perhaps a few percentage points might matter to a skeet or trap shooter, but it don't matter to me. The fact that this little gun is light and fast, yet I can swing it smoothly, and it packs in the saddle bags of a motorcycle on a trip is all that matters to me.
     
  18. MCgunner

    MCgunner Member

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    Oh, and with my 24" 10 gauge H&R, I've knocked geese down at 60 yards with a 3.5" load of steel T shot shooting a modified choke. I patterned that gun at 0ver 90 percent 30" circle at 40 yards. It's the best patterning gun I've ever shot.
     
  19. SeekHer

    SeekHer Member

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    First, I don't really know what you're trying to prove?

    Second, if you are using the same load--and you should be, NO you must be--, otherwise you're comparing apples to salmon, the difference in velocity between a 36" and 24" barrel is not going to be 400 fps--probably 50 to 80 fps--36" has a velocity of 1500 fps, and the same load in the 24" will be 1100fps is that because of in\decrease of velocity to barrel length or that you're using different loads.

    If I take the same guns--configured as you wrote with the same choke constriction at the barrel end and set up two(2) sets of paper patterning targets (60" x 60") with a bird aiming point in the exact centre at 10 yds, 20 yds, 30 yds, 40 yds and 60 yds, then take the exact same load of shot (with or without a shot cup) and over the same powder charge and fired one shot from each gun at each of the targets and the other gun at the other set;, this is what you'll observe and I've done this exact experiment numerous times.

    The pattern from the shorter barrel will have expanded slightly more at each range and a little bit wider (not proportionately) at each increase of range!.

    Why, because the shot will not have had that extra 12" of steel tube to keep the pellets together, that minuscule amount of time longer, so the forces will have begun to act upon the shot column sooner beginning with flyers.
     
  20. SeekHer

    SeekHer Member

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    What a longer barrel will give you is a) a smoother swing and b) follow through, c) a longer sighting plane, d) slightly lessened recoil due to the additional weight of the barrel(s) e) a noticeable decrease in muzzle flash and f) "possibly" a better balance point
     
  21. Double Naught Spy

    Double Naught Spy Sus Venator

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    This was our take, though we didn't do percentage. We just measured group sizes.

    We have done this test with bird shot and buckshot using a 10" barrel, 18" barrel, and 24" barrel, all unmodified. At 7, 10, and 20 yards, we didn't see a lot of difference. Basically, once the shot left the barrel, expansion was at approximately the same rate for a given shot and load.

    The one big difference was how particular ammo performed in a given gun. Different brands of ammo produced more differences than barrel length.

    A bigger concern, however, was flyers especially with buckshot. Some of the ammo did not like the 10" shotgun and while it mostly patterned great, there would be flyers. That wasn't the same for all brands, however. The longer guns tended to be less prone to individual buckshot flyers.

    All three guns were smooth bore, unmodified, pump action, but were not the same brand.
     
  22. Bud Tugly

    Bud Tugly Member

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    I may be missing something, but I'm also very skeptical that changing barrel length from 24" to 36" would increase muzzle velocity from 1100fps to 1500. The studies I've seen all indicate that barrel lengths much beyond 20-22" give little or no actual increase in muzzle velocity.

    If anything, I'd think a longer barrel might give wider patterns since there would be more friction with the walls causing more deformed shot and flyers.
     
  23. oneounceload

    oneounceload member

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    Muzzle constriction determines shot pattern density
    I'd like to see proof of 400fps increase with barrel length - not happening - if anything, due to the low pressures of shotgun ammo, it would start slowing down, not speeding up, with a longer barrel - this isn't a high pressure rifle round

    Sorry, I'm disagreeing with most of this

    As for a 20" doing as well as a 28" on dove, you must not hunt, as the swing dynamics play an important art in actual field success
     
  24. Dave McCracken

    Dave McCracken Moderator In Memoriam

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    A couple things.....

    First, in the real world, apples to apples comparisons are seldom achieved. Each barrel has its own pecularities and peccadiloes, so its darn nigh impossible to do more than indicate trends.

    Second,even without a choke, pellet deformation will happen, assuming lead or alloy pellets. Setback at launch will deform the bottom pellets due to the 60G or so of acceleration. Bouncing off the forcing cone will also ding up lead pellets.

    When I used Frankenstein with its 21" barrel for geese, I had little trouble keeping a tight pattern with a Mod choke WOD. That was with Winchester's 3" BBB steel loads, then the best of the non toxics for me.BTW, blast was so bad with that combo it ended a friendship.

    IMO, barrel length MAY affect the pattern,but so do many other factors. Best to pattern and test until the results you want are achieved.

    And guys, let's keep this civil.....
     
  25. Pyro

    Pyro Member

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    I just cut my 30 inch single shot w/ cylinder choke to 18.5 inches.
    Should be interesting.
     
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