How Do I Choose A Barrel Length?
(In my opinion)
Barrel lengths and caliber selection are determined by the following factors: weapon weight, maximum required range, terminal ballistic requirements, ammunition supply, and vehicular or mission-based length maximums.
In my opinion, a shooter should try to select the shortest barrel possible to accomplish the given task.
Short barrels are very accurate because of their relative rigidity compared to a longer barrel of equal diameter.
I don't subscribe to the school that thinks short barrels burn powder at differing rates; if that were true, the velocities would differ on the longer barrel as well.
Also, in most cases, the fastest load out of a short barrel will be the fastest load out of a long barrel.
Adding barrel length only increases velocity, which translates to increased range.
If you have a barrel length maximum, but require greater terminal ballistics, choosing a larger round may be a good answer.
There are too many anecdotes to list here. I like the saying, "Overkill is an oxymoron, there are many levels of wounded, but only one level of dead."
Choose what you realistically think you will need, and err on the lighter/smaller side.