Ceteris Paribus, (All other things being equal), shorter C.O.A.L.s tend to reduce the internal volume of the round, which will tend to elevate chamber pressures. This CAN be either a good thing or a bad thing. Match-grade rounds for .38 Spl., used in the famous PPC and bullseye matches were full wadcutters ("flying soda cans"), seated deep until the forward edge is seated flush with the case. THIS reduces the space available in which the powder will combust (initially), which means less propellant necessary to launch the bullet, which means less recoil.
I've loaded "middlin" handloads for .357, .44 Spl., and .45 Colt which gave perfectly acceptable performance, but sometimes were smoky when fired. With several loads, I've "cleaned up" the burning characteristics by seating the bullet a bit more deeply (0.005" - 0.020"), and/or increasing the crimp. The new rounds loaded thus, probably developed just enough extra chamber pressure that propellant(s) used burned more thoroughly. I didn't exhaustively measure velocities, but I got the impression that there was a definite, though negligible bump in in M.V.
While I am perfectly willing to do this in larger-volume (.38 Super or larger) cases, there is potential for hazard if both these adjustments are tried in small capacity cases. I do not recall which edition of Speer's reloading manual had this illustrative tale in it, but they spoke of a 9x19mm load that developed ~29,000 p.s.i. at a certain overall length. When the overall length of the same load was shortened by seating the projectile 0.020" deeper, chamber pressures went into the 60,000 p.s.i. range. Someone with old Speer Manuals can probably relate this incident with better detail, but I think I got the important parts right.
In pistol rounds, USUALLY, greater overall length enables the use of more propellant, without jacking up the pressures out of the "safe" spectrum. In rifle rounds, this is largely true, until some other factor comes along. In single shots with extra free bore, it can often be difficult to load the round too long. Additional powder, worked up judiciously, can and often does give rise to higher velocities.
Limiting factors on C.O.A.L. also come in the form of magazine length, and barrel throats with less-generous free-bore. Even if the rifle's magazine allows greater cartridge length, a tight chamber can mean that the bullet ogive contacts the throat of the chamber, not leaving a small space in which the bullet may "jump" from the case mouth into barrel throat, and down the barrel. This drives up chamber pressures, often to an unsafe level, and generally reduces accuracy, so if it's not one thing, it's something else.
ONCE in a while, a large case loaded with medium-burning rate propellant on which a projectile is seated deep enough that the powder charge is compressed, will result in a load that develops higher velocity at pressures equal to or lower pressures than loads with "UNcompressed" charges. Putative mechanisms range from powder position with respect to the primer flash hole, to the notion that a charge having no freedom of motion inside the case also has less freedom to combust in a random manner, which would tend to spike pressures.
So the "take away" is that, USUALLY, longer C.O.A.L.s mean lower pressures and/or higher velocities, until other factors intervene. And SOMETHING else ALWAYS intervenes.
