Not necessarily!

If you are talking about a muzzleloader, the answer depends NOT on the length of the barrel, but the STRENGTH of the barrel. If the muzzleloader is rated to take the so-called "magnum" charges of powder. Generally with muzzleloaders, the term "magnum" means any muzzleloader rated to use more than about 120 grains of blackpowder or equivalent, and up to 150gr or equivalent. Barrel length has nothing to do with the ability to handle heavy charges of black powder (or subs.). If once took a "magnum" rated ML and cut the barrel down to 16" it would still be safe with "magnum" charges of powder. The only way barrel length effects the max powder charge is in terms of efficiency. BP and subs burn very slowly and inefficiently compared to smokeless powder, and as a result a large charge may not have time to burn while the projectile is in the barrel if the barrel is too short. The result is particles of still burning powder ejected from the barrel after the bullet is gone. This leftover powder does NOTHING but increase recoil (by increasing the mass of ejecta). With a ML, the shorter the barrel, the lower the efficient charge weight and vise versa. That's why most "magnum" muzzleloaders have 26-28" barrels. The longer barrel is useful only in that it gives the huge loads of powder a little more time to efficiently work. However, I have yet to see a ML shoot a 150 grain charge of powder without spewing a ton of unburned and still burning powder embers out of the bore. Makes for a cool fireworks show, but doesn't do squat for performance. From my experience, all muzzleloaders with barrels shorter than 28" reach their point of diminished returns somewhere between 120-130 grains of loose powder. Shorter barrels will reach this point with less powder, and longer barrels quickly become too heavy and impractical to carry and shoot well. Pellets, by virtue of being already highly compressed, tend to be more efficient and burn more completely with higher charge weights, but have always been prohibitively expensive so I haven't worked with them much.

Mike