I have not seen any great results with the slug barrel; nobody writes about it, which tells me nobody is impressed with it. What videos there are don't show any groups for a reason. My own tests are not complete, I just got 5 more slugs to try but I have not found anything, yet I would shoot out of it at 100 yards after over $200 in slugs. The difference in the slug barrel and the non-slug barrel is the non-slug barrel shoots pellets well if you get the top hat kit to slow them down, I do not have the top hat kit. I like Airforce guns for the power, light weight and simplicity but I would never recommend any Airforce Utility gun for competition shooting; they are hunting guns ment for pellets. If you are shooting shorter ranges like 30 yards they will shoot slugs fine. The .257 or .308 Texan could compete some but not the smaller calibers without a custom barrel like my TJ.
You would be better off looking at something like a FX DRS or a BRK for long range or competition shooting. FX and BRK are known for being able to shoot a wide range of both pellets and slugs from the same barrel really well and at long range. If you want more power, you are better off moving up from a .22 to a .25 caliber. These barrels have twist rates designed to stabilize certain ammo, usually pellets. Yes, you can usually find one or two slug lengths that will work with enough testing, but you will normally get better results with better accuracy for power increases by moving up in caliber rather than by moving up in bullet weight. The best accuracy out to 75 yards will always be with a certain pellet unless the gun has been made specifically to shoot slugs. The Condor slug gun was not made to shoot slugs, all they did was cut off the choke that made it shoot pellets okay if you slow them down and glued on an adapter. That's not good enough. I have a hard head; it has taken time to convince me of that but now I believe it, many have come before me and they know. That is why I shoot different caliber guns for different proposes. It depends on how much power I want to get the job done and I really don't shoot far enough to have any need for slugs I just think they are cool. Big bores usually have a good twist rate for slugs but not always with .300 calibers.
I should add that I am still a learning beginner myself and making lots of mistakes. You can learn from the Ham testing and from
Mountain Sport Airguns