Questions concerning barrel length as it affects shotshell ballistics generally revolve around how much barrel length affects velocity, with occasional questions about how barrel length affects pressure. I can put pressure concerns to rest with one ballistic fact: While it was an issue with different-burn-rate black powders, it simply is not an issue these days with smokeless powders. The reason is that smokeless powders used in today’s shotshells, regardless of their burn rate, are essentially all burned up within five inches of the chamber.
As to how much velocities of smokeless-powder loads are affected by barrel length, that is a bit more complicated. But to simplify, it works essentially like this: Depending upon the shotshell load involved, with smokeless powders, for every inch of barrel beyond 20″ out to 40″, about 4 fps is gained at the muzzle. Therefore, a modern load developing a muzzle velocity of, say, 1,200 fps at the muzzle in a 20″ barrel probably would develop something like 1,280 fps at the muzzle in a 40″ barrel.
Periodically, a reader will contact me concerning achieving maximum velocities who is worried that his 26″-barreled double (which many ruffed grouse and woodcock hunters love) is not long enough. Having heard that shorter barrels do not develop muzzle velocities with a given load as high as longer barrels do, these individuals worry that they ought to have 30″ or 32″ barrels or 28″ at the shortest. Well, given the above ballistic fact, the difference in muzzle velocities between a 26″ and 32″ barrel would be at most about 25 fps at the muzzle. But note: This difference will diminish rapidly downrange.
All things being equal and, because of the physics of balls, pellets emerging from a longer barrel producing a higher muzzle velocity will slow faster than the same pellets would coming slower out of a shorter barrel. Because of this and by the time the pellets reach 25 yards or so, the 25-fps difference that existed between the two at the muzzle would become virtually negligible.
For those who go in for this sort of thing, you might wish to familiarize yourself with the latest technical details concerning velocity that can be found in current SAAMI Standards. When you buy factory loads or assemble handloads, be aware that the velocities listed for those loads were taken three feet from the muzzle—if the shells are American-made—and that the velocities were measured in a SAAMI-spec test barrel. Those specs vary by gauge.[inpost_leaderboard_middle_2]
For example, the velocities listed for 10-gauge lead ammunition, if SAAMI procedures and standards were followed, were measured in a 32″ Full-choke barrel. For non-lead, a 32″ Improved Cylinder barrel was used. In 12 gauge velocities listed for 2¾”, 3″ and 3½” lead loads were tested in 30″ Full-choke barrels, and non-lead was tested in 30″ IC barrels. Test barrels for 16 gauge were 28″ Full choke for lead and IC for non-lead. In 20 gauge 2¾” velocities were obtained in 26″ Full-choke barrels for lead and IC barrels for non-lead, while 3″ loads were tested in 30″ Full-choke barrels for lead and IC for non-lead. As to other sub-gauges, velocities for 2¾” 28-gauge lead loads were measured in 26″ Full-choke barrels and in IC barrels for non-lead. And for .410 bore both 2½” and 3″ lead loads were tested in 26″ Full-choke barrels for lead loads and IC for non-lead.
So from this one can see that if barrels are longer or shorter than the test barrels that generated the velocities listed, then the velocities achieved are going to be slightly different than those listed.
Last, for any given load and barrel length, the tighter the choke, the higher the velocity will be that is produced at the muzzle. Full chokes can produce muzzle velocities that are 30 fps or higher than those produced by IC chokes. So if your target or hunting barrel does not match the choke constrictions listed for the shot types for the test barrels above, you are getting slightly higher or lower velocities than those listed on the factory boxes or in the reloading data. But again, all of these muzzle-velocity differences are minor, so no worries.
In the final analysis, the principal importance of barrel length is to provide the length of sighting plane and gun weight/balance point that works best for each shooter. Longer barrels are more desirable for longer-range shooting—not, as we now know, for any ballistic reason but rather because such barrels provide weight-forward dynamics that greatly assist in keeping the guns swinging on crossing and overhead shots. Such is not necessary in upland hunting, where the predominant targets are typically close range and simple variations of going-away shots.
And there you have the long and short of it.
To consult with Tom Roster or to order his manuals on reloading lead and bismuth shot, reloading HEVI-Shot and HW 13, or having shotgun-barrel-modification work performed or his instructional shooting DVD, contact: Tom Roster, 1190 Lynnewood Blvd., Klamath Falls, OR 97601; 541-884-2974, [email protected].