BJlooper,
Well, in general terms, a loose barrel will allow higher velocities because the bullet is not swaged down in diameter as much as it is with a tighter bored barrel.
Those wanting to shoot hard bullets will especially see this although a loose barrel used with a bullet such as a Barnes X bullet will aften see poor accuracy results because the hard, solid bullet will not bump up under pressure to fill the bore diameter if slightly larger then the bullet diameter.
A tight bore is generally more accurate because the bullet does not have to bump up under pressure much if any for a quality fit to the bore.
Velocity may be limited because it takes more force to drive a bullet through a smaller diameter hole then a larger one but accuracy generally is better.
As far as barrel life, I am not sure there would be much difference at all as long as the loose bore is not so large as to allow gas to pass around the bullet body and cause flame cutting.
In all honesty, the difference between loose and tight can be measured usually by less then 1/2 thou. Most aftermarket barrels are pretty tight on average. I would say Lilja barrels are some of the tightest and this may result in lower velocity potential.
May seem kind of strange that these barrels are the ones I use exclusively in my Allen Magnums as they are based on high performance but I will take extreme accuracy over an extra 50 fps any day of the week.
Again, I would say the difference between say a Lilja, Rock, Broughton, Hart or Kreiger in bore diameter would be very small if any. There is a reason all of these are very good shooting barrels.
It is not as big a deal as some believe it is but there are some differences, mainly velocity potential, although this is pretty small.
If someone is saying they are getting +100 fps more velocity with one barrel brand over another, all else being equal, I would find that hard to believe without a change in pressure.
Good Shooting!!
Kirby Allen(50)