Shooting

Question
Is a longer barrel more "accurate" to shoot. Why or why not?  

Answer
In modern firearms, all things being equal and within reason, the answer is no.

A Glock 34 is no more accurate than Glock 26 at pistol distances, and 7.62 NATO rifle with a 20" barrel is no less accurate than one with a 24" barrel.

But there will be other factors that will affect the performance of guns with different length barrels that will cause pronouncements of more or less accuracy.

Subjectively, the longer the sight radius, the less movement there is in a two-point sighting system, so a longer barrel on a gun with iron sights will be more accurate, subjectively speaking. If you use a sighting system that has a single aim point (scope with retical or red-dot reflex) the subjective accuracy difference will disappear.

When extreme distances come into play, a longer barrel will, because of burn rates and internal physics of the propellant, have a greater muzzle velocity, which will enable a projectile to cover more distance before destabilization takes place. As an example, if you take a particular cartridge in .357 Magnum, fire it out of a 2" revolver and chronograph it, and then fire the same cartridge make out of a revolver with the 6" barrel, the difference in velocity will be considerable.

So, for example, while you will not generally see 7.62 with a 20" barrel used for ranges greater than 800 meters, the same gun with a 24" tube may be used for distances out to 1000 meters and beyond.

To sum up then, "accurate" is a relative term. At extreme distances, longer barrels will generally be more accurate (due to velocities), and subjectively if iron sights are used, an average shooter will get more accuracy out of a longer sight radius.