Found a 1903 Springfield with a massive barrel

[MarshallDodge]

I'm sure that years ago, before the 300 Winchester Magnum, etc became a thing, they were trying to squeeze all the velocity possible from the 30-06.

 

[Mister Ridge]

It's also worth noting that a longer barrel gives a longer sight radius which is usually better when using irons.

I was also under the impression that velocity relative to barrel length depends greatly on the cartidge (and probably powder) being used, with some cartridges experiencing diminishing returns the longer the barrel gets.