Effects of barrel length on shotgun patterning


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Geckgo
July 4, 2011, 10:21 PM
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!

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roadchoad
July 4, 2011, 10:34 PM
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?

Geckgo
July 4, 2011, 10:50 PM
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.

Geckgo
July 4, 2011, 11:04 PM
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.

Leadhead
July 5, 2011, 12:50 AM
It's more fun to just pattern your gun with different loads and chokes.

Geckgo
July 5, 2011, 01:02 AM
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.

Sam Cade
July 5, 2011, 02:02 AM
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).


Its been done, some time ago. :D


lol

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

In independent testing Felt Wads have drastically improved pattern density over conventional wads by as much as 100%.

o Unforgiven o
July 5, 2011, 03:09 AM
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?

Geckgo
July 5, 2011, 03:18 AM
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.

Geckgo
July 5, 2011, 03:23 AM
Unless I've overlooked something of fundamental importance..... nah, that's how it works :D

o Unforgiven o
July 5, 2011, 04:58 AM
I suspected so. I still think my explanation was a bit more, eloquent. ;)

Geckgo
July 5, 2011, 06:37 AM
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'>"

Virginian
July 5, 2011, 07:36 AM
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).

Geckgo
July 5, 2011, 07:58 AM
hmmm, I'm going to sleep now but I'll look into it :D

There's nothing wrong with a wingmaster.

eastbank
July 5, 2011, 09:35 AM
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.

RickMD
July 5, 2011, 11:40 AM
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.

MCgunner
July 5, 2011, 11:57 AM
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.

MCgunner
July 5, 2011, 12:03 PM
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.

SeekHer
July 5, 2011, 12:10 PM
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.

SeekHer
July 5, 2011, 12:20 PM
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

Double Naught Spy
July 5, 2011, 12:37 PM
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.

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.

Bud Tugly
July 5, 2011, 01:34 PM
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.

oneounceload
July 5, 2011, 01:40 PM
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

Dave McCracken
July 5, 2011, 05:59 PM
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.....

Pyro
July 5, 2011, 07:20 PM
I just cut my 30 inch single shot w/ cylinder choke to 18.5 inches.
Should be interesting.

Geckgo
July 5, 2011, 07:22 PM
my fps difference in the barrels could be wrong, again, to illustrate a point. Several people here have already noted about patterns not changing from just barrel lenth. A 400 fps difference in the example (pretty much just pulled that number out of my butt) only made a difference of inches at 20yrds. Make it 100fps difference, and you will see very little difference with the pattern.

What I'm going here for guys is that barrel length can affect the pattern very slightly (took a 400fps drop to really see the difference), but not enough to worry about when choosing a shotgun for different purposes. Something a lot of the posters on this thread seem to already know so far.

Longer barrels do nothing magical to "stabilize" the shot cup. I would venture to say that the shot cup is pretty much done with all of it's changes due to acceleration within the first 4 inches of travel, unless you are talking about a tapered barrel.

Geckgo
July 5, 2011, 07:26 PM
haha, pyro, let me know what happens!! :D

kozak6
July 6, 2011, 06:03 AM
My reloading manual says that you lose/gain about 5 fps per inch of barrel length, so your velocity figures are somewhat unrealistic.

Geckgo
July 6, 2011, 06:14 AM
^- just more fuel to debunk the sbs scattergun myth

Double Naught Spy
July 6, 2011, 09:45 AM
What I'm going here for guys is that barrel length can affect the pattern very slightly (took a 400fps drop to really see the difference), but not enough to worry about when choosing a shotgun for different purposes.

Right. I would agree with this.

I was really disappointed to find a 10" barrel shotgun (I wanted a Serbu Shorty) didn't have a significantly more spread pattern at defensive distances.

oneounceload
July 6, 2011, 10:37 AM
That's because it is the choke that determines pattern and spread, not barrel length. A short barrel like a Serbu WILL lose velocity, but once your shotgun barrel is past the 14" range (give or take an inch or so), the charts I have seen show a consistent velocity except at 20" where it drops a little (and no one has an explanation). The barrel length with today's ammo means little in the velocity department, but it has everything to do with swing dynamics, balance, handling, MOI, etc.

scythefwd
July 6, 2011, 10:46 AM
seekher, I would suspect that swing is more related to balance than length. If the short barrel was designed with a front bias in balance, it could swing very much like it's longer counterpart. It'll be lighter and ick more but it would feel the same until you pulled the trigger. Using a lighter buttstock, either by virtue of material choice or by skeletonizing or hollowing out the butt) would allow you to keep the balance point the same between two different lengthed guns. The balance is why my uld mans double 20 ga with a 20 inch barrel swings better than my 28 inch single (though mine shoots slugs better:) )

Bud Tugly
July 6, 2011, 01:17 PM
I've mostly hunted upland game with break-open guns. They've had barrel lengths from 20" to 26", giving an overall length of 36" to 42". I've had a lot of success with these guns over 45 years experience but have never shot clays except for a few hand thrown.

When I pick up and shoulder a pump or semi-auto with an overall length of 48+" it feels like swinging a log by comparison. I'm sure folks used to the longer guns would find the shorter ones I prefer to be whippy and nearly toy-like.

IMO barrel and overall gun length and fit are so subjective that no generalizations are really possible. If what you're using works for you then don't let anyone talk you out of it just because something different works for them.

Double Naught Spy
July 6, 2011, 03:11 PM
That's because it is the choke that determines pattern and spread, not barrel length.
Right, and this I noted above with no choke and testing patterning via barrel length.
Basically, once the shot left the barrel, expansion was at approximately the same rate for a given shot and load.

roadchoad
July 7, 2011, 01:09 PM
Geckgo

Still think I miscommunicated the point of my question, so let me clarify. How do you know that the same shot load in two different barrels at different velocities expands the at the same rate from each barrel if choked identically? I understand that the difference in velocity will cause the patterns to be different due to the time of flight, so that is unrelated to the question.

Basically, since you are on a theoretical level, how can one state that the constriction of the choke on the shot pattern is irrelevant to shotload velocity in mathematical terms?

I'm interested in the physics of it. I don't really give a hoot whether a 12" barrel shoots a larger or smaller pattern than a 36" barrel. I miss most of the time anyway. :D

JohnBT
July 7, 2011, 01:14 PM
Because 2 friends and I tried the same ammo in 3 870s with the same choke - 21", 24" and 28".

And 2 1100s, a 26" and a 28".

And there wasn't any difference we could see. Oh, and after the lead shot we tried Hevi-Shot. Same deal, similar patterns.

Geckgo
July 7, 2011, 07:58 PM
roadchild, so you are wanting to know about the physics of the choking itself, that is a little bit tougher and I haven't completely made up my mind on that one, but I'll take a stab at it and let you know what I got so far.

basically, I ignored choke because I wanted to focus only on barrel length.

So, as far as I can tell, spread pattern is more or less linear with time, as expected for any kind of projectile motion. From looking at the highspeed pictures on wiki and exploring the problem, we can start with a cylinder choke. The shotcup separates from the group almost instantly on leaving the barrel, and doesn't seem to affect the shot too much. What does affect it, afaik, is that now you have this column of shot moving faster than the speed of sound in open air. Just like racecars drafting eachother, the shot at the front of the column is going to catch most of the wind resistance.

So the BBs up front will slow down faster then the ones "drafting" off of them, and recede into the group, flattening it. This "squishing causes the group to expand horizontally. Once they are no longer squishing or banging into eachother, they will continue on whatever path they started, except they are only really slowing down in the direction of fire, which causes a slight curve in their actual flight path. This is regular cylinder choke.

Now, the point where all of this starts to get a little beyond me, I think when the choke constricts the shot, as it squeezes the shot and makes the "tube" longer, it also opens up room between the BBs? Perhaps the deformation of them plays a part too. Either way, a skinnier column is going to have more BBs exposed to open air (the ones on the outside of the column) so I would expect that the group is a bit more uniform as far as wind resistance goes, which causes slightly less of this "squishing" effect, and because of that, they spread a little slower, making tighter patterns. This is all speculation, but if I were to try and figure out how chokes work exactly this is where I would start.

hope that was at least somewhat helpful. Sorry I haven't investigated chokes more thoroughly let, and they've always seemed odd to me, so I may look into some theory tonight.

SeekHer
July 7, 2011, 09:19 PM
seekher, I would suspect that swing is more related to balance than length. If the short barrel was designed with a front bias in balance, it could swing very much like it's longer counterpart. It'll be lighter and ick more but it would feel the same until you pulled the trigger. Using a lighter buttstock, either by virtue of material choice or by skeletonizing or hollowing out the butt) would allow you to keep the balance point the same between two different lengthed guns. The balance is why my uld mans double 20 ga with a 20 inch barrel swings better than my 28 inch single (though mine shoots slugs better:) )
Yes but it's barrel length that gives you the proper balance point...I have a SxS, and an O/U and a semi auto all with extra sets of barrels..Main set is 30" and choked Full/Full, Full/Imp Mod and Full for ducks and geese and for upland birds a 27" SxS Imp Cyl/Mod, 26" O/U Imp Mod/Mod and 28" Semi Mod...They were all balanced to the longer barrels and become slightly butt heavy with the shorter barrels...They aren't too, too bad since they're magnum gun to start with.

My brother shoots a duplicate of my O/U but with 32" barrels and it handles far better then mine--swing is smoother or for a better word steadier and the balance point is just a bit further from the hinge joint then on mine.

My oldest daughter has the same SxS but she ordered it with Imp Cyl/Imp Mod 61cm barrels (24") direct from the maker and they gave her a slightly lighter gun--not as dense wood and a thick Schnabel instead of a Beavertail forearm to compensate for the lack of weight in front...Her gun is a dream to shoot in dense cover or in open fields and balances just inside the hinge...If you stick my 30" barrels on it, it becomes a pig and would require quite a lot of weight to the stock to balance it.

The thing is, most guns makers make their shotguns balanced to longer barrels (usually 30")...But putting a longer barrel on isn't as bad for balance then too short or worse yet, shorter and thinner barrels produce...Slightly muzzle heavy is a slight shift of the hands, butt heavy is a different position on the shoulder.

dfariswheel
July 7, 2011, 11:49 PM
The NRA Dope Bag Technical staff did a real-world investigation of this years ago.

They bought a Marlin Goose Gun with a 36 inch barrel.
They attached a choke to the muzzle and shot it for velocity and pattern.
Then they cut one inch off the barrel, reattached the choke and tested it again.
They continued cutting one inch increments off and retesting until the barrel was down to 12 inches.

Their test results indicated:
1. Barrel length had no effect on patterns. Shorter barrels patterned just as well as the longer barrel.
2. Anything that was going to happen ballistically in a shotgun barrel happened within 18 inches.
3. Anything over 28 inches and the gun actually started to lose velocity from friction.
4. Things didn't "get out of hand" until the barrel was down to 12 inches.
5. The idea that a longer barrel will shoot "harder" or farther is a left over from the days of black powder when longer barrels burned the powder more efficiently.

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