Barrel length has minimal effects on patterns and velocity. But you are correct on the faster swinging, The problem is, the shorter/lighter the barrel, the more quickly you can swing it, but also the easier it is to stop the swing. Many shooters, even experienced ones, tend to stop the swing as they acquire the proper lead on the target. Of course, if you stop the swing and pull the trigger, you'll shoot behind a moving target. Longer (i.e. heavier) barrels take longer to get swinging, but also tend to stay in motion when you pull the trigger, due to momentum being greater. So it counteracts a person's tendency to not follow through with the swing.
So if you are pheasant hunting over dogs or shooting skeet, for example, where targets are close,crossing and moving fast, a shorer barrel tends to work better. Where targets are a little farther away and moving at an angle away from you, such as trapshooting, then a long heavy barrel works better.
Will a longer barrel shoot farther?
It depends on the length. Using the same load, a 34" will shoot farther than a 18". Powder burn is the key. Modern powders are fully burned by the time the shot load gets to about 22". So, up to that point, the pressure will increase behind the shot and increase the velocity. Beyond 22" the pressure will not increase and therefore, barrels longer than 22" (maybe out to 24") offer no ballistic advantage.
In the normal range of hunting and target barrels, 26" - 34", there really isn't any difference.
Will a shorter barrel swing faster?
Again, not as simple as it sounds. If shorter barrels swing faster than longer barrels, why does my 30" Cynergy swing slower than my 34" Ljutic? It's really about weight distribution. How the weight of a gun is distributed makes all the difference. The Cynergy has a heavier barrel and therefore swings slower. So you see, you can't just say that a shorter barrel will swing faster. Usually a shorter barrel will swing faster if made by the same manufacturer, but not always. Perazzi, for example, makes barrels of different weights.
Following is the best explanation of pressure and velocity versus
length in a shotgun barrel that has ever been posted here.
Almost all shotgun powder burns within the first 2 inches
of the chamber. A typical fast burning powder will burn
in the first 3/4 inch of the chamber. This is where
maximum chamber pressure is attained. A typical fast
burning powder like Red Dot might give a chamber pressure
with a certain load of about 10,000 psi at that point.
This is why a chamber is very thick on a shotgun barrel
back at the chamber and gradually becomes thinner as
it gets further from the chamber.
After that first 3/4 inch, the pressure and tempurature
starts reducing. When the shot has travelled twice as far,
then the already burned powder which is now gas has
twice as much space available and the pressure therefore
drops to 1/2 what it was at 3/4 inch travel.
So when the shot has traveled 1 1/2 inches the pressure
drops to 5,000 psi if it was at 10,000 when the shot
had traveled 3/4 inch and was at max chamber pressure.
Likewise when the shot travels twice as far again, to
3 inches the pressure drops to half again. So when the
shot has traveled 3 inches the pressure is about 2500 psi.
This can be carried out on down the barrel and the
pressure keep reducing by half as the shot travel
doubles onward down the barrel. Here is a table giving
shot travel vs pressure.
So as you can see as the shot travels further down the
barrel the pressure reduces. Since the pressure is
very high early on, the shot accelerates rapidly early
on. With 10,000 psi back initially it is accelerating
very fast. Once it gets out to 24 inches though the
pressure is only 338 psi, so the shot is accelerating
very little after that point.
Adding barrel length does give extra velocity, but its
not a lot once you have a barrel length approaching 30
inches. If you had a 2 inch barrel and increased it
to 4 inches though your velocity increase would be
very large. Increasing from 28 inches to 30 though
might only give you about 10 feet per second.
If you make the barrel very long, say something like
8 feet the pressure has dropped so much that the
gun hardly makes any noise. This barrel length thing
is why an 18 inch riot barrel is so much louder than
your 30 inch trap gun. The pressure wave let loose
upon the air at 18 inches would be substantially
higher than the pressure wave let loose from a
30 inch barrel.
The above explanation is not entirely accurate,
because barrel pressure also drops with temperature
as well as with getting more space available, and
the powder is also cooling as pressure drops, but
the above explanation is accurate enough to think
of it that way as a close approximation.
Barrel length and velocity go hand in hand.
The longer the barrel, the higher the velocity. The difference may only change 5 or 10 FPS per inch after about 20" or so.
Matunas quoted in his Ammunition Book an expected velocity change between 22" to 32" to be 5-15 FPS every 2" change.
The pressure inside a barrel is substantially different in my memory than what was simply calculated and previously quoted, but the gist of the action is within the realm of reasoning that should satisfactorily improve a novice's understanding of ballistics.
That velocity increase will add distance to the pellets and improve the retained energy, but since pellets lose the supersonic velocity most quickly, a fast pellet loses more of that high velocity faster than a slower pellet, but it never does slow to a velocity less than an initially slower pellet.
A pellet going 200 FPS faster than another a the muzzle may be down to 60 FPS difference at maximum hunting ranges, so you need a lot of extra up front to have just a little left on the far end.
So- to answer 2 of the questions:
a longer barrel will have more velocity and shoot farther, but not to a meaningful degree if only a few inch change is compared, and even a foot change from 18" to 30" is not a great boost in effective range with the same shells compared.
Pattern change from a longer barrel will be more substantial than the change in velocity, especially if the setup is geared toward making a high performance pattern. With all things being equal, there is no substitute for barrel length in that situation, but as in all things shotgun, the longer the barrel, the less the difference in improvement (per inch or whatever) as length increases.
Here's my take on it, since I'm actually in this situation right now.
I have a 20 ga. single.
It has a 25" mod. choke barrel.
There is no "weight forward" feel to this gun.
My brother has a 16 ga. single
It has a 28" full choke barrel.
There is a definate "weight forward" feel, but not too "weight forward".
The 16 swings very nicely and if I were tracking a bird, that is the gun I would use.
My gun, being lighter and more agile than the 16, is great for "snap shooting" and since I have a tendency to hunt that way, it works great for me.
...SO, My take is that...
if you put a gun up to your shoulder and the balance feels neutral, or even a little to the rear, the gun is better suited for snap shooting.
If you put a gun up to your shoulder and the balance feels as if there is some weight to the front of the gun, its best suited for tracking the bird.
Get the gun which best suits your style of shooting.
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