I'd like to be able to "plink" with rounds that I can load myself. Why? I don't know, random thought.

So what would be the closest round to a simple 22lr, but obviously a re-loadable centerfire cartridge.

It may be worth splitting the conversation into both a pistol round and a rifle round.

While it might not be the smallest, the 5.7 x 28mm will be the smallest/most practical cartridge to load sub-sonic.

25 acp I suppose with a cast bullet and find a barrel blank for a single shot or contender or something to make up a rifle for it.

5.7x28, .22 hornet and its family, 218 bee, and the zillions of other similar rounds come to mind.

They also make reloading kits for 22lr, but it seems very impractical.

I think 32 ACP already borders on subsonic.

25acp and 22 ccm would likely be the smallest commercial cartridges. there is a guy who made a legit reloadable 22lr. he uses 25acp cases and swages them down in a few steps then turns the excess at the web and the rim. however his uses a standard .224 bullet rather than a heeled .223 bullet. he chambered the barrel with a standard chucking reamer and made his own dies. alot of work.

a 22 squirrel would be a easier to come by as it uses hornet brass and it shortened and improved, then theres the 22 flea which is a 32 acp necked to 22 cal. on the larger end of the spectrum there is the 22 tcm.

I did a cartridge called the .223 short which is based off a .223 case shortenend to 1.15" full power loads go 3200+ with a 40gr bullet (18" barrel and 13.5gr of powder) and i've done loads with titegroup and it does 2000-2200 ft/sec with a 40gr bullet and 3.5-4gr and 1.5 moa vs just under 1 with full power loads.

you can even do a 17 or 20 squirrel if you wanted.

Supersonic, but "The Load" scaled down to .223 Rem is fun.

~50gr bullet.
4.0-5.0gr Red Dot - visually inspect for any doubles.

About 1500fps.

25 Auto is too damned tiny to load.

32 Auto is too close to 9mm, with much less selection in firearms.

9mm can be loaded with 124gr subs that function pistols.

And even lighter subs that will function in blowback guns, I have had 124s down to 850fps.

Rifle or pistol, 9mm Luger is it for ecconomical subs.

5.7x28 with a 55gr bullet is fun, but kind of a pain as it's a single stage or 550B only operation and the coating on the cases has to be protected.

7.62x25 Tok is more fun and runs like a mini-300BO. I use 147gr pulled bullets for giggles, but have gone as heavy as 180gr Sierra SPs.

I'm not sure I truly understand your goal.  If you're looking for the smallest subsonic reloadable round, then without going exotic, it has to be .25 ACP.

Having said that, though, you could likely shoot subsonic 9mm or 38 Special for less money.  And you'd most likely be using a firearm that was more fun to plink with.

Any .45 ACP to 850 fps?

22 TCM. Comes in a rifle and a pistol.

I've done it recently with 37g PCC bullets.

1100 fps out of my tcm rifle and 850 to 900 fps out of the 1911 pistol.

Cost is almost the same as 22lr as well.

I'm still developing the load, but 2 to 2.6g of unique is where I'm still testing.

32-20!

It used to be popular in NRA hunter-pistol silhouette competitions back in the 80's.

Accurate out of a 10" TC and low enough recoil to use a rifle scope.

Just trying to be funny?  This is the smallest caliber that you can reload and subsonic: 2MM Pinfire guns. Takes one #8 shot pellet and a cap.https://www.youtube.com/watch?v=R1-8bCuL3CE