(tho technically you gain a bit of muzzle energy from that same pistol ammo, tho the end result is mostly academic) and the stopping power of a rifle or a shotgun)This is actually only partially true. For most of the your semiautomatic pistol cartridges, it is correct. Perhaps a 100 or 150 fps can be added with the addition of a 16 inch barrel. This is due to the fact most all semi-auto carts are loaded with fast burning powders. They very quickly burn up the powder and if the barrel cannot go much past 16 inches before velocity begins to degrade. However, if you look at revolver cartridges, something magical happens. Most revolver cartridges are loaded with slow burning powder. Where fast burning powders have already reached their full kinetic force, slow powders are still building up.
Buffalo Bore provides an excellent example of this with their heavy(not +P) 125grain .357 magnum load. From a 4inch revolver this spicy load dishes out muzzle velocity of 1603fps. That's about 150 over the more classic hot anti-personnel 125grain loading. However, in 18.5inch barreled Marlin lever action, this load has an incredible 2298fps. It is firmly in the realm of the 30-30 and the classic 7.62x39 soviet rifle loads in speed and kinetic energy delivery.
It is a similar tale with other calibers such as the .45 colt and .44 magnum. Another example is Buffalo Bore's .44 magnum 180grain Anti-Personnel which exits a 18.5 inch barrel at 1987fps which is 570fps over the same round from a 4inch revolver.
I've known this for some time, but Weerd's comment made me think about this fact a bit more closely. I think decided to hit up Google and see if anyone has any slow burning loads for automatic pistol cartridges. While, I found no commercial loadings, I did find the results of an experiment one fine gentleman had loaded for himself to try out with a hi-point 9mm carbine. The results, if correct, are very impressive. He used Wolf 9mm and bulk Winchester white box as control groups, both using 115grain projectiles. They averaged at about 1250fps and 1300fps respectively. Then he used 3 loadings of his slow powder each with 115 grain projectiles. One load was near the maximum pressure for 9x19 using Blue Dot powder. It gave out an amazing average of 1700fps! WWB has an average speed of about 1150fps from a handgun. That is fairly normal for a non-+P 9x19 load. 1700fps out of the same cartridge in a 16inch barrel is not just academic. It's a game changer. Concealable class II bullet resistant vests could not be trusted to stop that sort of velocity. It adds a lot to the platform.
Despite these impressive findings, I am still a bit concerned with a few questions. Would it cause problems to the firearm being that its slow powder? Will it overheat faster? Can the barrel take the velocity? Will it cycle properly?
I shall try to remain optimistic about this. After all, the slow powder experiment was carried out successfully with a Hi-Point 995, which means I think ANY 9mm carbine in good condition could to it too. 9mm AR-15s are becoming more and more popular as time goes on and ammo prices mount. Some of these are going to be dedicated platforms too. They may be earmarked for younger or weaker family members in case of disaster. Uzi carbines, kel-tec sub-2000s, Beretta Storms, and a smattering of others are also available and would also be solid choices for a 2nd line defensive carbine or a primary depending on the user. A few magazines of higher velocity carbine loads would certainly go a very long way for the normally low power of the caliber. If viable, 9x19mm carbine specific loads might be something we see ammunition makers, big and small, work on next.