How will barrel length change exterior ballistics of 45-70?

[Naphtali]

I have a Marlin 1895 rifle with 18-inch cut rifled barrel. I use H322 and IMR3031 with CCI Large Rifle Magnum primers to load moderately heavy loads with 405-grain Remington JFP, and 420- and 425-grain LFN-GC cast bullets. My cast bullet loads chronograph at just over 1700 fps.

Were I to return the rifle to Remington to have an 1895 24-inch barrel installed, how much of an improvement in exterior ballistics should I expect? The benefit of which I am positive is a significant improvement in my accuracy, not the rifle's accuracy when using my Marble tang sight with an 8-inch longer sighting radius. Presently, the rifle and I are 120-yard shooters.
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While I have a superb Burris Eurodiamond 1-4x24 sight (30 mm tube) on my other 1895 18-incher that allows us to be comfortable 200-yard shooters, I strongly prefer to use aperture sights. My scoped 1895 I reserve for guests during hunting seasons.

[jh45gun]

YMMV but I can shoot great groups with my guide gun and a scope on it. The gun I had before with a tang sight on it I did not do that great at all with it compared to the two guns and the previous gun had a 32 inch octagon barrel on it with a target front sight and the rear peep sight. It was not bad groups but I can do a heck of a lot better with a scope at longer ranges. Others may do better with a peep than I can or maybe a better sight was in order. Then I think it depends on your age too I am older which might make the difference with the eye sight.

[NSB]

I'm going to give you a little different take on my answer than you'd expect. I own/owned several different 45-70 rifles both scoped and unscoped. I also shoot a LOT of long distance handgun. When I shot silhouette, I used open sights on the handguns I used. I could consistently shoot all the rams at 200m and most of the shoot off targets at the same distance. The barrels I was using were 10" and 14" and the shoot off targets were very small. I don't think you'll gain anything in practical accuracy by going to a longer barrel. You may gain some velocity but that will not translate to accuracy. Your sight radius is already long enough that your eyes will not benefit from making it any longer. You may, and probably would, benefit from getting a better rear sight such as an MVA tang sight on the gun and leave the barrel alone. You might also consider putting a Merit aperture in the Marbles sight you currently have and trying that. I think the longer barrel will turn out to be a dead end for you and you'll still have to spend more money to get where you want to go.

[BABore]

You'll gain 25-35 fps per additional inch of bbl. This is based on going from 18.5 inch to 22 inch bbl. The rate of return goes down slightly as you go longer. Changing powder burn rate will help when going either direction.

[MtGun44]

Expect small differences in velocity, the 18" barrel has already done most of the useful expansion since this is a straight wall
case. Each time you to an additional 1.5 inches or so, you add a bore volume equal to the case volume below the boolit. So,
by 18" you have expanded your gases by about 10 or 12 times. Think about a bottleneck case with a small bore - how long
of a bore section will equal the case volume of a .220 Swift? Maybe 4" or more? So the amount of expansion on a small bore
bottleneck case is much less - so they benefit from a longer barrel a bunch more. A straight case, not so much.

A .22 LR is the classic case - it is reported that beyond about 16" a Std vel .22 LR actually slows down- gases are fully expanded. This
is why they are so quiet. Not much remaining pressure at the muzzle.

Bill

[MostlyLeverGuns]

The benefit of the 24" barrel will be greatly reduced muzzle blast and noise. Reducing these could improve your shooting. I appreciate the handiness of short barrels,in hand, horseback, and truck; but for shooting enjoyment the longer barrels are more pleasurable. Same for scopes, iron sights carry much nicer, but for accuracy on the range or especially for finding a hole through 80 yards of dog-hair timber the scope works better. IMHO