What can kind of performance can be experience from a 20" 50-209

There shouldn't be too much of a velocity loss due to the 20" inch barrel.
I don't know how much less, but it would depend on how long of a barrel that it's being compared to. Barrels usually range from 20" - 26"+ in length, so I would guess that it's possible to be 100 - 200 FPS less.
Here's a Traditions velocity chart but it doesn't indicate barrel length or type of powder.

http://www.traditionsfirearms.com/loadchart.pdf

Necrothread time...

RW, have you had an opportunity to work up any loads with that barrel? I have an Encore as well and have been looking at the 20" 50cal barrels for it. I have a standard 26" barrel (haven't chrono'ed any loads yet), but wanted to know how the 20" ones perform before spending the money.

How is the muzzleblast?

Chris

I'm very unsure about this but in smokeless powders you get a more adequate burn and less muzzle blast by using a faster-burning powder than a standard length barrel would call for. Black powder is not smokeless powder and has a different burning rate but You might try using 3FG and maybe even 4FG to see if you get better results than standard 2FG or a substitute black powder like Pyrodex RS. Pyrodex P may work better than RS if you can't find real black powder.

This is conjecture on my part and may be totally wrong.

Well, a 7 1/2 inch barrelled black powder .44 can give muzzle velocities in the 900 fps range, so the additional 12 /12 inches in your new barrel should get you up to pretty normal rifle velocities. Like so many other questions, experiment is your best way to find out. Put a white sheet or some butcher paper on the ground in front of the muzzle. When you find unfired grains of powder on the paper, you have reached your maximum load.

I would not try 4F powder though; it is recommended only for charging pans on flinters. I think 2F will give you all the velocity you can use.

SB

No....no FFFFg.
I use FFg in a 14 inch Blunderbuss. It works fine...two drams. The 20 inch 50 cal should work OK, kinda like the legendary/ mythical canoe gun.

I use FFFG in my .50, it works well enough to smash cinder blocks and smack steel targets. I've never taken whitetail with it though but I'm sure it would be more than adequate.

That's not the Point. FFFg has a faster burn rate which means a quicker rate of pressure build up.
The top pressure will be just about the same, but it will spike earlier.

It's like a car going 0-100mph

Imagine FFg is a Ford F-150 who does it in 18 s
FFFg is like a Ford Mustang GT and does it in 13.5 s
Then think of FFFFg as a Porsche 9FF GT9R that will do it in 4.9 s

The final speed (pressure) will still be the same but consider the vastly enhanced stress that the acceleration (burn rate) puts on the Porsche and how it must be built to match it.

Now imagine trying to push the Ford pickup to 100 mph in less than 5 s

I'm not worrying about the deer getting killed here Levi