...is there a potential load where the barrel length would no longer gain velocity for the caliber its pushing?
Not sure what "the caliber it's pushing" means, but in a practical sense I think the answer is; No. With black powder, more barrel length typically always means more velocity, all else being equal, and significantly more. If you were shooting extremely small powder charges though, I could see the pressure dropping during the time the bullet is in the barrel, to a point where the pressure is only enough to maintain velocity and no longer increase it. In shooting 22 LR for example, because the cartridge uses so little powder, there is a barrel length beyond which you no longer get more velocity, and beyond that you may even get less velocity because the powder has "run out of steam" and no longer overcomes the friction between bullet and bore. At that point the extra barrel length is a liability, slowing the bullet down rather than accelerating it.
With anything resembling a "normal" powder charge in the Remington Carbine, with its fairly short 18" barrel (the originals IIRC were offered in 24", 26" and 28" barrel lengths) you'll always get more velocity than from an 8" barrel, and the difference will be significant. 18th Century American Long Rifles tended to have very long barrels, in the range of 36" to well over 40". One of the loads I tested in the carbine gave me the energy of an extra 15 grains of powder, meaning that 30 grains of O.E. 3F gave me the same kinetic energy as 45 grains in the Walker, with the same bullet.
I've posted several sets of chronograph reading on this very section, so you can find those and see for yourself. Also; I've found that the heavier bullets gain more kinetic energy than the lighter ones, and far more than round ball. They may go a bit slower, but the energy is greater. For example I got up to 600 foot pounds energy from the carbine using a 240 grain bullet, and no 200 grain bullet yielded near that much, even with five more grains of powder. The longer barrel delivers more power. Pure and simple.
It can seem complicated, but it really isn't. The heavier bullet is resulting in higher pressure, which translates to higher temperature, which speeds the combustion of the powder during launch (I call it a hotter burn). Thus you get better energy efficiency out of the powder when using heavier bullets. Lighter round ball then leaves more unused energy spitting out the barrel after the ball is gone. Thus, although a round ball will come screaming out of the carbine at the highest velocities, it will not be carrying as much kinetic energy as the heavier conicals.
The faster round ball would be great if you want to slap the snot out of a coyote at close range, or explode a water jug for fun, but less good if you want full penetration and bone-breaking power on a large Mule deer. For the latter, you definitely are better off with a 225 tor 240 grain conical, going slower, but carrying more energy with more momentum. the longer the bullet, the less powder capacity however, so there's going to be a point of diminishing returns when it comes to bullet weight. Back ion the 1860s the heaviest bullets used in the 8" pistols was around 255 grains. Also; SuzukiBruce done went and ruptured a Walker cylinder trying to use a 285 grain bullet, so more pressure isn't always a good thing.
More barrel length then is also going be more advantageous when using larger powder charges. So it is that I'll predict that a Wakler using 45 or 50 grains of powder and a 225 or 240 grain bullet, and having a longer barrel of 18" up to 30" would gain more velocity per inch of added barrel
than the Remington using 30 or 35 grains of powder and the same bullet. Again; at some point, as you REDUCE the powder charge, there will be no added velocity due to adding barrel length, and BELOW THAT (below some very small charge of powder) adding barrel length will slow down the bullet. It's just that we're no where near using so little powder or so long a barrel.