So, one thing I'd been curious about was the effect of, not only barrel length, but the presence or absence of a barrel/cylinder gap on various rimfire loadings. So I scooped up four different firearms and four different rimfire loadings I've used a lot in the past and headed to the range. There were Smith wheelguns in both 2" and 4" flavors, my trusty and much-abused bargain-basement fixed-sight 4.5" Ruger Mk III 22/45, and my beloved little Papoose, now sexied up with an inexpensive Tasco red dot.
For ammunition, I brought 20gr Aguila Super Colibris, Remington's 40gr Target blue box standard velocity, my ammo can of Federal Champion 36gr High Velocity bulk pack Wally World fodder, and a box of CCI's 40gr flat-nosed SGBs.
From the 2" revolver, they averaged 522 fps; they were a little faster from the medium-sized handguns (543 from the 4" revolver and 532 from the 22/45) but the surprise was that they gained velocity in the Papoose. With only the primer to burn, the point of diminishing returns was probably halfway along the barrel someplace, but nevertheless they still managed a 623 fps average. They were probably moving at similar rates of speed from the Browning BL-22 I used with good effect on garden pests some years ago.
It was ever so slightly more consistent from the 16¼" barrel of the Papoose, but in the carbine-length tube it belied its Standard Velocity descriptor by turning in an average velocity of 1124 fps, which is probably flirting with being supersonic at 68° and ~700' above sea level and I don't know what the humidity was but it wasn't too bad yesterday.
The Federal plated 36gr hollowpoints averaged 917 fps from the 2" revolver and 1021 from the 4" gun and had the rather dubious distinction of a velocity spread of 162.7 fps in a ten shot string from the 4" Smith, which is the largest I've yet seen in my chronoing experiments.
From the Ruger, they averaged 999 fps, and 1203 fps from the Papoose. It's hard to say too much based on a ten-shot string from ammo where the outliers can be so outlying, but in general the longer barrel and lack of a B/C gap again seemed to have a positive influence on consistency.
Even from the 2" snubby, the round averaged 911 fps with a spread of less than 50 fps between the fastest and slowest, and 978 fps with a similar spread from the 4" revolver. From the Ruger pistol, the average was 971 fps, and it was 1190 from the Marlin carbine. In no case was the Standard Deviation for a string over 20 fps, and it was actually 14.86 in the Kit Gun. I'd be nodding my head approvingly if that was centerfire duty ammo from a service pistol; for rimfire loads from a 2" revolver, that's freaky good. Good enough that I want to get some from other lots and see how much of a fluke it was.
I've got a fair amount of 22 match ammo, Wolf and Eley and Gold Medal Match and suchlike, squirreled away, and now I want to sacrifice some of it for science, too.
Anyhow, the takeaway from this, broadly, is you get what you pay for in consistent performance from ammunition. If you're plinking cans and it doesn't matter if the speed of your bullets varies by almost 200 fps from shot-to-shot, it's probably not that big a deal. If you're shooting a bullseye for score or a squirrel for the pot, you want the next bullet to go to the same spot as the one you used to sight in the gun. And that's why some kinds of .22LR cost more than others.