28" seems to be pretty standard barrel length for inlines. What, if any performance would be lost with a 24 or 26" barrel ? Would a max charge of powder be completely burned in a 24" tube ?

It is possible you may loose some fps but then there is the theory of barrel harmonics. Not all barrels react the same way and in some cases a 24" will produce better accuracy.

My 54cal DISC carbine will out shoot my 50cal DISC with the same bullet and amount of BH209 (max). So it not all about length. Knights original DISC had 24" barrels and shot as well or better than the longer barrels.

I will take better accuracy over a 50-100fps gain any day.

Handling and balance are another issue. On a bench it isnt as pronounced but carrying a lighter and better balanced gun all day is a big plus to me. The shorter barrel on the new Omega Dream season vs a standard Omega was quite noticeable in this regard.

Form should follow function/application so there is no perfect answer for everyone's preference or needs.

I have an older T/C Firehawk .54 inline. This has a 24" barrel. I shoot 100 grains of FFG Goex with a Hornady white sabot, and a Hornady 225 grain FTX, .430". I have never tried more than 100 grains, as it kills deer with great accuracy with this load.

I prefer a longer Barrel 27-28". A longer Barrel will Stabalize the Bullet Better and more Powder will have a chance to Burn. Plus Im Tall and like a longer Mler for that pourpose also, a better fit.
(BP)

I have a White g-series whitetail and it has a 22 inch barrel. I have never had any problems with this short barrel. Its a extremely accurate rifle. I have shot deer at over 150 yards with it. Have to admit its really nice if you have to swing and shoot quick.

Barrel length has little to do with bullets stabilizing. It may make you more stable while aiming. Matching the bullet weight/length to the correct rate of twist is the main factor. My Savage uses a 24" with a 1-24 twist yet the fps is very close to longer barrels using BH209. Their OAL though is pretty long due to the action length and they are heavy.

I shoot 385-400gr bullets in sabots out of my GMB54 without any issues...besides a lot of recoil

Barrel length does NOT affect accuracy or stabilization.
The difference between a 24 inch and a 28 inch in velocity can depend on a number of things, for instance a light load of say 70 or 80 gr and a fairly heavy bullet in a sabot say 290 gr will burn all the powder efficiently in less than 24 inches so differences would be negligible.
While a heavy load of slow powder behind a light bullet say 150 gr of rs behind a 50/40 bullet of 200 gr would lose a couple hundred FPS.
and would gain from a 32 inch barrel.
There is no black and white here knowing the load and bullet would help though.

I've been shooting 50/45 cal. 250 gr. Shockwaves, 110 gr. FFG 777., Win. 777 primer. I'm just thinking a shorter barrel would be handy.

A shorter barrel is a good thing, at times. That is why I keep my T/C Thunderhawks. (21" Barrels) One in .50 caliber, that shoots Maxi-Balls, accurately, and one in .54 caliber that shoots saboted bullets accurately. IMHO the reason 28" barrels are now popular, is that they will accomodate 3, 50 grain pellets, or 150 grains loose powder, and get a more complete burn, & velocity with these loads. I neither shoot, or need these heavy loads, so a shorter barrel is fine for me, even though I do own a T/C Triumph with a 28" barrel. Funny thing though, the 28" barrel of the Triumph, and the 24" barrel of my T/C Firehawk (Plus the 4" length for the striker, built into the breach area) The overall lengths of the 2 ML's is about the same.

I have an original Knight Disc (they had 22 inch barrels)...With 80grs of Goex FFF she stacks the .430 Hornady XTPs on top of each other...I've killed 4 deer with it this year...Previous to this I had also killed deer with the 250 SSTs and the 300gr Barnes MZ...Accuracy is better than most can hold as long as a fellow knows how to work up a load...

I don't see a disatvantage to a shorted barrel, to really stoke up a muzzleloader you need to go smokeless and that doesn't appeal to me...