As you can see from the answers above, it's a simple question with a complicated answer.

Assuming your powder charge hasn't completely burned, then adding barrel length will increase muzzle velocity, and thus, terminal velocity.  There is a point of diminishing returns, though, beyond which you can LOSE velocity due to friction.  But no modern rifles will have barrels long enough for this to be a problem.

Additional muzzle velocity will get the bullet to its target faster, so it spends less time getting pushed around by the wind.  The trajectory of the bullet will be flatter, so aiming corrections may be easier.  Both factors can increase accuracy, especially beyond 300m or so, where wind starts to become a bigger factor.

The trade-off is that a longer barrel will have more flex than a shorter barrel of the same diameter, and will be heavier.  If you are taking shots at ranges of less than 300-400m, you might be better off with a shorter, heavier-profile barrel, which will be stiffer.

None of this addresses some of the other, very important accuracy factors, such as the straightness of the bore, whether the chamber is properly centered and concentric, the size of the chamber, the overall machining quality of the bore, and of course, the ammunition used.  The reason a *real* match rifle (and not just a rifle with a heavier-profiled barrel and "match" in the name) is so expensive is because lots of time and effort is expended to get each of these critical areas right.

-Troy