From all the testing and feedback I've seen, most hollow point pellets expand very little if at all (usually not at all). Pellets are very different from powder burner bullets. Accurate pellet speeds are typically less than 900 fps, speed bleeds off very fast due to the low ballistic coefficient (much less velocity at the target), and on top of that, the hollow point cavity is very small. The designs with the best reputation seem to be the Predator Polymag and JSB Hades. But even the Hades which is a fresh latest-generation design, people seem split 50/50 whether they get good performance from it. The trade-off with expanding pellet designs is that they are usually less accurate than their domed counter-parts. All of these reasons are why many stick to domed pellets, especially if longer ranges are in play. If your requirements are only for short range, you might have luck finding a sufficiently accurate expanding design and running them as fast as possible.
With regard to 'slug barrels', there is a lot of misunderstanding out there. A big part of this is that FX offers their superior 'standard' barrels and also 'slug barrels'. The main difference in the slug barrel is a faster twist rate that is needed for heavy-for-caliber slugs. Since a lot of people shooting slugs want to hot-rod and use the heaviest slugs, this is what you hear about a lot. The 'standard' barrels in most pcp airguns (not just FX) have twist rates suitable for light to midweight slugs. FX superior standard in .22 cal for example, comes with a 1:24 twist rate (which is slow for a .22 barrel) and is good for up to ~25 grain slugs. Something to keep in mind is that slugs are not forgiving like pellets are. The twist rate may be appropriate, but a gun may not just like a given slug, or it may like a slug but then a small manufacturer change throws things off. Also, when you are shooting slugs, even light ones, your tune is going to be hotter to get them out the barrel so shot cycle movement increases as well as hold sensitivity. Where slugs really shine is shooting 60+ yards, especially in the wind. It's pretty awesome to be able to hit a 1" spinner at 100 yds even with some wind. Good slug accuracy after much trial and error is around 1 MOA for 10 shot groups.
To give you an example - I found that my 600mm .22 Impact M3 shot NSA 20.2gr .218 diameter about 1 to 1.2 MOA. Enjoyed that for about a year, then something changed and now its 2 MOA. I think I got a really bad batch of slugs, and then the absolute latest batch is better but still not what it used to be. .217s also seem to do better now. I don't know for sure and I've been going through validating everything on the gun to be sure. In the middle of all this, I put 10 JSB 18.1gr pellets in the magazine, turned the power wheel down so they'd shoot ~890fps, and I proceeded to shoot a 1MOA group at 50yds. If that 60+ yd performance wasn't oh so sweet with slugs, I would have gone back to pellets.
