aaaa"Correct for you" barrel length depends upon several factors, among them the action type and the requirements of your shooting technique. A balance and feel that is best for you provides the harmony that allows you to achieve your top-level, most consistent performance.
aaaaTo find out exactly what you need demands a lot of experience, shooting and investigation. These must be diverse, so they take time and certainly a few dollars. From a general standpoint, the faster the shooter, the more the nod is toward a longer barrel - not because of any velocity advantage but because there is more muzzle weight and an increase in sight-radius length. Ideal balance, excellent timing and the resultant enhanced swing/follow-through all work hand-in-hand.
aaaaUsually shorter or longer replacement barrels can be purchased from the manufacturer or various supply houses for the repeating-type shotguns. If you value your favorite shotgun highly, a new custom-made barrel can be obtained, or your existing assembly can be altered by a gunsmith.
aaaaBead sizes vary between different gun manufacturers and gun types, as do the distances between the front bead (largest of the two) and the extremely important center, or mid-rib, bead. These beads need not remain as supplied on the gun, nor do they have to remain at their established distance apart. They can even be color-coded if it helps improve your performance. As with iron-sighted long guns and sidearms, an increased sight radius (distance between the front and rear sights) can translate into more accurate bullet placement, or more precise shot pattern placement with bead-sighted shotguns.
aaaaA few years back I decided to do some experimenting with 12-gauge barrel lengths to determine resultant changes in muzzle velocity. Decades-long prevailing opinions, both verbal and published, indicated length has very little, if any, practical influence on pellet velocity. My results, obtained with a variety of shell types (see chart), back up this conclusion.

Here is Jimmy Kwan firing a Rottweil 72 AAT (Adjustable American Trap) at the Ben Avery Shooting Range in Phoenix, Arizona. Trap shotguns, superposed and single-barrel, commonly run 32" and 34" in barrel length, which puts the sighting-plane length into a comparable overall-length realm with pump and semiautomatic shotguns having 30" barrels. Of course, the additional length of the receiver combined with a 30" barrel makes the difference. Skeet guns, over & unders and repeaters usually have 26" to 28" barrel lengths due to their specialized use. Shotguns designed for sporting clays competition usually run to the longer lengths due to the demands of that sport. The important point to remember is the loss of velocity from any length barrel compared to another is considerably less than you might imagine.

aaaaFor the accompanying chart, I shot three Winchester Super-X Model 1s and one Marlin Model 55 Goose Gun. All were 12 gauges. I fired a minimum of five rounds with each load to obtain the averages listed on the chart. My shooting was done over a Model 33 Oehler chronograph at an elevation of 3,800 feet on a day when the mean temperature was about 80 degrees Fahrenheit. The 36" Marlin barrel was shot using a full choke constriction; shorter barrel lengths on the Marlin were with cylinder bore. The Winchester Super-X Model 1 had a 30" full factory trap choke. I used two custom Model 1s, one with a 24" full-choked barrel with the Briley Long Choke System and the other, a specialized slug gun, with a 22" cylinder-bore barrel.

Average Velocities With Various Barrel Lengths
Shell/Brand	Drams Eq./ ShotWt./ Shot Size	Barrel Length & Avg. Velocity (f.p.s.)
>>>		36"	33"	30"	30"*	27"	24"	24"**	22"***	21"	18"
AA Trap/Win.	3,1-1/8,7-1/2	1,234	1,265	1,293	1,245	1,141	1,326	1,210	1,235	1,265	1,207
Dove & Quail/Win.	3-1/4,1,8	1,341	1,310	1,361	1,351	1,288	1,200	1,319	1,336	1,201	1,192
Duck & Pheasant/Win.	3-3/4,1-1/4,6	1,423	1,383	1,415	////	1,361	1,239	-	-	1,216	1,231
2-3/4" Mag./ Win.	Max,1-1/2,2	1,342	1,264	1,296	////	1,233	1,196	-	1,172	1,247	1,234
3" Mag./Fed.	4,1-7/8,4	1,259	1,287	1,234	////	1,220	1,207	////	////	1,224	1,231
Buckshot #4/Fed.	Max,27 pellets,#4B+	////	////	1,333	////	1,269	1,203	-	1,250	1,201	1,242
Buckshot #00/Fed.	Max,9 pellets,#00B+	1,234	1,276	1,278	////	1,224	1,210	-	1,108	1,207	1,202
Slug (Rifled)/ Fed.	Max,1,slug	////	1,558	1,554	////	1,531	1,457	-	1,358	1,309	1,428
*Winchester Super-X Model 1, 30" full trap choke (factory gun) **Custom Win. Super-X M-1, 24" full-choked bbl. w/Briley Long Choke System ***Custom Win. Super-X M-1, 22" cylinder-bore bbl. (specialized slug gun) //// = Not applicable to barrel or not applicable to choke + = Buckshot is loaded on a per-pellet count, not by weight

aaaaThe photos with this article illustrate two different guns and barrel lengths and their uses. I used a variety of factory loads in the various barrel lengths so I would cover a mix of shooting scenarios. The widest velocity spread was 249 f.p.s. for the Federal rifled slug load. The Federal 3" Mag. load had a spread of only 80 f.p.s. As the chart indicates, barrel length does influence velocity slightly, but usually not enough to influence performance.

Maureen, a trapshooting newcomer, experiencing recoil (note the spent shell in front of her support hand). She is shooting with a tuned Winchester Super-X Model 1 Trap gun on which I installed a special barrel shortened from 30" (standard) to 24". It has a Briley Long Choke System (17-4, Series 25). Id had this barrel made up for me so I could ascertain trapshooting performance differences between it and the standard (customized) 30" barrel I normally used with this shotgun. Switching to the shorter barrel cost me an average target reduction of over 30% at the longer handicap distances. At 16 yards, I lost over 20% on average. Other accomplished shooters I had try the gun had similar results.

aaaaWith repeating (slide-action and semiautomatic) shotguns, if you have any question as to exactly how long your barrel is, you can easily check it. Close the action and insert a straight rod from the muzzle end until it contacts the bolt face. With a sharp pencil or pen, place a distinct mark on the rod, then remove it and precisely measure the distance from the bottom of the rod to the inside edge of the mark using a metal tape measure. On over & under or side-by-side doubles measure from the barrel breech face to the farthest point at the muzzle. Pattern size and density are not dependent upon barrel length but on the amount of choke (constriction), which is much more important.