I am talking about the interior profile. Typically a shotgun barrel has a Chamber, a Forcing Cone, a Bore, and a Choke. All of which may vary between different guns.
Starting with the most common Gauge/Caliber 12 ga. Chambers: can be 2 1/2", 2 3/4", 3", 3 1/2" and probably some I don't know about!
Forcing Cones: can be anything from 1/2" long to 2-3" long. Typically less expensive barrels have short forcing cones because they are cheaper to do. Forcing Cones act as a funnel between the interior of the shot shell and the bore of the gun. This accommodates variations in hull wall thickness and provides a smooth transition between the chamber and the bore of the gun. How smooth that transition is depends on the length of the Forcing Cone.
Trap Guns were the first to use long forcing cones in an attempt to Tighten Patterns and Mitigate Recoil. The Pattern Tightening came from less shot being deformed while going thru the Forcing Cone, and
I can attest to the change in Recoil, because the guns that I have that were modified have a noticeably Smoother Recoil Impulse as opposed to the "abrupt hit" caused by the payload hitting the short forcing cone, and being forced tighter in a shorter period of time. This was noticeable to me on the very first shot fired after the barrel came back. That barrel was also "Back Bored" which also helped mitigate Recoil, another mod spawned by the Trap Shooters..
Internal Barrel diameters: Typically a 12 ga. barrel is.729-.730 Also known as "Cylinder Bore" A Back Bored Barrel may be between .740-.745. Typically Shotguns made for Field use will have standard bore dimensions. Shotguns now made for Target Shooting may have standard bores or more recently Back Bored barrels. My Browning Citori XT from 2007 had .730 barrels. My new Browning Citori CXS bought a few months ago has .740 barrels. Both are made for Target Shooting so times have changed.
Chokes: Chokes are the "constriction" at the end of the barrel that modifies the pattern of the shot and are used to vary that pattern for different distances. A 30" pattern is the standard size that is sought for optimum results on a target at a given range. At 20 yards on the Skeet Field a Skeet Choke will yield a 30" pattern. To get a 30" pattern at 30 yards you will need a Modified Choke, and at 40 yards a full choke would be used. These are not chiseled in stone, they are approximate and vary with things like ammo type and the components used in that ammo..
Chokes and their names are related to the amount of "Constriction" below Cylinder Bore. IE Cylinder Bore is .730, Skeet is .725 or -.005, Improved Cylinder is .720 or -.010, Modified is .710 or -.020, Full is .700 or -.030. There are a few others as well.
The "Constriction" amount is the operative point here. IE: how much below the bore size the choke tube is? I found this out with my new Citori CXS 32" which I had planned to use my set of Carlson's Choke Tubes left over from my Citori XT. Rude awaking, My Existing Skeet Chokes shot like Modified Chokes in the new gun. Reason? The constriction was based on Cylinder Bore being .730 instead of .740 because the barrels were Back Bored from the factory on the new gun and .730 on the older gun..
This gun patterns tight to begin with, but Proper Choke Tubes from Browning that were designed around the.740 bore resulted in 30" patterns at 20 and 25 yards with a Cylinder Choke tube and a Skeet Choke tube which measure .740 and .735 respectively. IE-.000 and -.005 constriction.
There is one more thing about older barrels that needs to be discussed. The "Jug Choke." It was not uncommon for Black Powder Shotguns with Damascus Barrels to be Cylinder Bore from the maker. In order to tighten the patterns and increase the range of the gun a technique known as "Jug Choking" was used.
A 'Scraper" that was about 2" long and could be adjusted for depth of cut was inserted into the barrel about 1/2- 1 1/2" below the muzzle. This would cut a "Relief" or "Jug" in that area of the barrel which would serve to open the payload up and then choke it back down just before it exited the muzzle. The effects were similar to a conventional choke but done on a barrel that was strait from end to end.
Back Boring the barrel and then tapering the cut back down to .730 in the last 2" before the muzzle would accomplish the same thing as Jug Choking. This was also the process developed by Hans Vang, noted Shotgun Smith from Santa Barbara CA, and "Undisputed Guru of Remington 870 Police Shotguns" over the last 30+ years. The Vang Comp Process of "Lengthening the Forcing Cone to 2", Back Boring the barrel to .745 and then choking it back down to .730 at the muzzle as well as Porting the barrel with 62, 1/16" holes about 2" behind the muzzle, has proven to yield 7" patterns of 00 Buckshot at 25 yards while Mitigating the Recoil so that Follow Up Shots can be taken much faster.... I have 3 barrels that do just that!
I am sure that there are some fine points that I have missed in this post,,, so feel free to chime in! I know that there were many Barrel Mods that were generated from the Trap and Skeet Shooting Games which come under the heading of this topic. Also the Paradox Barrels would fit here as well. We need to hear about all of them!
Randy
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