I've owned a Winchester Trapper in 44 Magnum ( 16 in Barrel ) for some time now

I was thinking about a Marlin in the same caliber and the one I was looking at has a 24 in Octagonal Barrel

I know the 44 magnum benefits from the 16 barrel over a Pistol length Barrel but does anyone know how the longer barrel will affect ballistics ?

I've done the Google search and couldn't come up with anything

Pure speculation as I don’t even have one, nor can I lay my hands on any data.

Jumping from 16” to 24” will probably gain you some small increase in Muzzle Velocity.

I suspect that since we are dealing with a pistol cartridge there is a point using factory loads where there is little gain.

Factory loads are for handguns and they can be used in rifles too. However they are optimized for short pistol barrels.

Handloading is different story. Listed loads often contain warnings that they should not be used in pistols. Likewise you will see ‘slower powders’ listed for Rifle loads that do not appear in the Pistol load data.

1700fps to 1900fps 240gr loads are listed using the Ruger 18” in my books.

I was under the impression that Going from a Handgun 4" 6' or 8" to a 16" the 44mag gains alot

I just don't know at what point it's too much barrel to where it would be detrimental
Although it's a different beast 45acp actually loses with longer barrels

You do get a good gain for 44 mag. It's a big cartridge for a pistol caliber. I can't recall the numbers when I was checking on them. I opted for 444 marlin with a 20" barrel. The mag just can't touch it as far as ballistics but ammo is cheaper and the magizine holds a few more rnds if you go 44 mag.