I mean to burn up per length of the barrel,what good is it???
What use is 120 grains of powder in a 24 inch barrel? Can or will it all burn?I mean in the magnum inlines.
I was just wondering.

I used 120grs FF to see how my rifle liked it, but this was with a .54 flintlock and a 38 inch barrel...

Settled on 80 for that gun...In my Knight, groups open up past 85 grs of FFF Goex...

mounting man

A chronograph will inform you whether or not, adding powder is accomplishing anything. Myself, i 'know' that 120g of powder gives more energy to bullets shot from my rifle than 100g of powder. My rifle also shoots bullets faster over 140g of powder than, 120g.

This season, my load will be 270g Deep Curl, 90g Blackhorn powder, crush rib sabot, and W209 primer.

Well, the key to getting a good answer to your question is the kind of powder the weight of bullet and what ignition is used.
with Black powder FF you would still be burning powder when the bullet left the barrel with a light bullet but with a heavy conical it could burn it efficiently.
With Blackhorn a good tight sabot and a 250 grain bullet and a hot 209 it would burn in a 22 inch barrel.

This chart might help... It is for a 45 cal. rifle but you can see the various velocities and when they top out

Ive shot max 120gr loads of BH209 in a 20" barrel with very little velocity loss vs a 24" and the fps was still increased vs 110gr. You can get a full or near full burn in a 24" barrel under the correct conditions.

Im am normally using heavy bullets (300gr+) though which help get a good burn. Ive shot even higher loads in special built rifles made for extreme pressures but i normally only recommend those to very well educated ML enthusiasts with very good quality barrels.

BTW don't get caught up in the MAG load craze. I only do it because my rifle shoots that load very well. Many max loads will not shoot as well or require a lot of tinkering to get them to shoot a max load as well as say.....90-110gr loads.

Shoot what gives YOU the accuracy YOU need for YOUR hunting situation. 70gr of real BP has killed more game than you can imagine and it did it very humanely.

I personally never judge how much powder and barrel X length can handle, or burn efficiently. I use what I call diminished returns. Lets say with bullet X I shoot 80, 90, even 100 grains, in barrel X, and after 100 grains the accuracy drops off. Well even if the barrel is four feet long, I know for that bullet, accuracy wise, after 100 grains, I get diminished returns. Meaning, no more accuracy improvement. So even if the rifle COULD burn more powder efficiently, there is no need to. So instead of trying to set a magic number on a length of barrel.. instead I see why the barrel looses accuracy with X bullet after a certain amount of powder.

Now for some range observations.. when I shot my 24 inch Knight barrel over snow, anything over 120 grains of powder gave me unburnt powder laying on top of the snow. And the diminished returns of that rifle was 110 grains of loose powder. So there was no problem.