I tried a grossly undersized ball in my .69, just for kicks. It was a .662" ball and to tighten it up a bit in my bore, I used a doubled .017" patch- thus .034" per side.
Thus .662" + .034" +.034" = .696" = .730", thus actually having .013" compression in each groove.
Here's the results, a 5-shot group at 50 yards, plus a flier- me flinching. I was getting a bit punch-drunk (no I wasn't drinking punch) by the time I shot this group.
The group wasn't too bad, being about 1 1/4" for the 5 shots. The patches were burned and badly scorched from blow-by, but the accuracy actually wasn't that bad. No wiping needed.
I used a water-based lube, being Blue Thunder Winter Windshield Washer Fluid + a couple ounces of Neetsfoot oil per quart. Shaken, then poured over pre-cut patches, soaked, then gently squeezed out. The patches are VERY wet in the tin.
html upload imageThus, if you use a thick enough patch, it might work. Trouble with the smaller bores, is increased pressure. The above load(82gr.)develops about the same amount of pressure as perhaps 60gr. 2F in your .54 as it is only putting these balls out at a guessed-at 1,100fps. So, whatever charge develops 1,100fps in your .54- THAT is the equivalent load to what I just did. Due to this low pressure, I was able to get away without building up fouling. If using a decent charge in a .54, you may not be so lucky. Experimentation is FUN - have at it.
The higher the pressure, the more fouling you will get with loose loads.
The day I shot that target, I also shot this one but with a tighter combination, but still, a well-undersized ball, being .675" and pure lead, in the .690 bore. It is a 10 shot group with the same rifle.
The heavy denim patch, is the 12 ounce I normally use in this rifle. It measures .030" compressed hard in my calipers, .025" compressed hard in my mic.
Thus, .675" + .030 + .030 = .735" in the .704 groove diameter. If you use the .025" mic, measurement, that is .725" in the .704" groove diameter barrel. They are tight, but not difficult to load with the rifle's rod. That hickory stick tapers 3/8" to 7/16" and has been used since the rifle was made in 1986.
I also noted this load shot 2" at 100 meters, but that target is missing. Can't find it.
As well, there is a mistake in the recorded data on the target. The lower group should be 1.11" as the ball size was not the normal .684" that I used to use & still use for 'accuracy' work.
For trail walks, the .675" is easier loading - with the heavy patch. A friend gave me some 14 ounce Denim that was a bit too snug in his .54 with a .535" ball. I am now using that material, that runs .030" mic'd and .035" compressed in the calipers. With the .675" ball, loading is sung, and quite nice. Accuracy matches the a-fore mentioned .684" and .030" denim.