My head is spinning from the advice on this thread. No offence, but the reason original ML rifles did not usually go below 32 cal. is simple: ramrods and fouling. In the past, what could you use as a ramrod for a sub 30 caliber bore that wouldn't break in ordinary use or carry, and especially if the bore was fouled from BP? A metal rod would have worked, if you happened to have one from a military musket and it was small enough to go down the bore (unlikely, and its weight would have been unfortunate plus it would have continually wanted to fall out of the ramrod pipes.) And what would have allowed for putting a patch onto a miniature jag to clean out the bore?
Now the other major problem: BP fouling. That's why BP rifles and pistols have deeper and wider grooves, to take care of some of that fouling and still allow the patch (or bullet expanded at firing by bump-up (obturation) ) to get the ball or bullet to spin and thus stabilize. Then a sub-problem exists too: twist rate. A modern .22 barrel has a rapid twist rate and a ball pushed too fast is going to simply "strip" and fail to take the rifling. Hence, you are shooting a smoothbore. In this case, the microgroove rifling is going to fill up very quickly with fouling and here again you are then shooting essentially a smoothbore.
In former times, the smoothbore rifles and pistols were larger bore, as in .54 to .80 or so. That's because a larger ball has more momentum and thus it is likely to fly farther in the direction of aim, rather than curving away to the side. A smaller shotgun pellet loses velocity faster than a larger one and thus has more liklihood of veering off course, plus its trajectory curve is worse because of the lowering velocity.
My bottom line: find out what the twist rate was in a former .22 pellet rifle, and hopefully in the early .22 pistols and rifles of the later 1800s. A round ball does not need as much stabilization as a bullet, due to its form, but as you say, you are swaging your roundballs to an elongate form (i.e., a bullet form). So all the more you need intact, functioning rifling. And the more BP you use, the more fouling you create. Even ten grains is going to be too much, in all liklihood, especially with your fast twist, very shallow rifling. The faster you try to push your lead "bullet" the more likely it is to strip the rifling and accuracy goes out the window. I you can do it, use the known velocity of the early .22s as in this thread, and calculate that RPS (revolutions per second) of that bullet at, say, 800 feet per second. That RPS is what you want to try in your .22 ML rifle and hopefully your "bullet" doesn't strip the rifling.
I'd try this: get yourself a musket nipple that has the same threads as your current one (probably 1/4-28, try TRESO as supplier) and shoot your patched bullet using solely the musket cap (hard to find, tho). I has the same brissance (firing power) as a modern 209 shotgun primer. It may well give you adequate velocity in your very small bore.
Also,a musket cap will probably light off a very small charge of Blackhorn 209 BP substiture powder. Since you want to shoot the powder with the least fouling, that would be BH 209. It is loaded at 70% weight vs volume of BP. ( An aside: 4F BP gives you more pressure and velocity, but you don't want that. But it might possibly burn cleaner and leave less fouling,which you do want -- if using BP). So if you were using, say 4 grains of BP, your load of BH 209 would be 4 X 0.7 = 2.8 gr. by weight. Your modern steel barrel will handle a lot more than that, but keep in mind two things: the smaller the bore, the easier you build pressure, and in addition, the faster you try to push your lead "bullet" the more likely it is to strip the rifling and accuracy goes out the window. If you can chronograph your shooting trials, see what you have for accuracy at 7-800 fps. Do not try for "heavy" loads of BP like 15 or 20 gr, or the equivalent of BH 209 using a musket nipple. It's way too much and fouling will very quickly remove any accuracy, plus your "bullet" will strip the rifling.
You may also want to try a "Mag-Spark" nipple from Warren Outdoors, in whatever thread your current nipple has (likely 1/4-28, as said). It fires using a small rifle primer, and that alone should propel your small "bullet".
I'm always saying maybe I could be wrong, so I look forward to your shooting trials. Good luck. (Addendum: apologies, read thru this thread and was off thinking about caplocks and later 1800s .22s. Forgot the original message said flintlock!)
Aloha, Ka'imiloa
