I'm still in the slug testing stage of a gun and haven't been as successful as I'd like, getting about 3" 10-shot groups at 100y. Because I'm waiting for an opportunity to get new slugs down to Peru — slugs that
will give me tighter groups....
However: I've read everything on airgun slugs I can get my hands on, and I've asked some of the slug pro's.
And sure, the
barrel twist rates get kicked around a lot. And the
minimum muzzle velocities. However, people with
"regular" barrel twist rates, and
800fps MV are getting good results (very achievable with some of the lighter slugs now on sale, e.g., 17.5gr and 20.2gr in 22cal). One guy here at AGN is even running a scientific test of higher MV really do get better groups — and as far as I can tell 950fps MV alone is
not necessary for good groups.
But it seems like there is
one factor that determines more than other factors if a slug works or not — and it comes up again and again in the discussions. And it is this:
The slug diameter has to fit the size of the barrel, and in case of a choked barrel, it has to fit the choke.
There is a big difference between 0.217 and .218 — too big of a difference for
slugs anyway, because they have much more bearing surface than pellets.

So, the main issue seems to
find a slug that has more or less the "right" diameter (and for that you can "slug your barrel" and measure what that is, if you have the necessary equipment). (After finding "that" slug, you can tweak precision by varying your MV.)
The problem is that most .22cal slugs are
ONLY sold in diameters of .217 and .218, some manufacturers only offer you
one size...

So, not many options to try and find what your barrel likes....

And for people like me who don't have the equipment and who don't want to get into swaging my own slugs — our Nick (of www.NielsenSpecialtyAmmo.com) comes to our rescue.
Because he offers us not just .217 and .218.
Instead, he offers us the following diameters: .2155 | .216 | .2165 | .217 | .2175 | .218
(and in 7 different weights from 17.5 to 31.2gr in .22cal)

And just last week he started offering us
economical sample packs of 40 slugs
of any diameter—weight combination, for just 5 bucks. (Buying a full pack of 300-400 results in
a shot costing 5-6 cents, only slightly more than a quality pellet.)
So, it seems very likely that
with that many options I WILL FIND THE RIGHT SLUG!!

Am I super excited about all this? You bet I am!!
Am I thankful that there are companies out there that make a living by selling stuff to us airgunners —
without jacking up their prices? You bet I am.
Matthias
See for yourself what slugs are out there: Attachment: Slug Table .22: Prices, diameters, weights, lenghts, BC, contacts and links
View attachment SLUG Table. 051. 2020-10.1603211387.pdf