Effects of barrel length on shotgun patterning

[Geckgo]

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]

haha, pyro, let me know what happens!!

 

[kozak6]

My reloading manual says that you lose/gain about 5 fps per inch of barrel length, so your velocity figures are somewhat unrealistic.

 

[Geckgo]

^- just more fuel to debunk the sbs scattergun myth

 

[Double Naught Spy]

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]

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]

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]

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]

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]

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.

 

[JohnBT]

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]

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]

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]

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.