tvc15,
The target being slightly out of plumb could be the reason the holes appear to be slightly out of round.
Another reason could be that the bullet is not stabilised at the range you're shooting. What is the range, by the way? What size groups are you getting with the load? Some times a bullet will need a bit more than 100 yds to "go to sleep"; that is, settle down and fly straight, nose-on, to the target.
The best way I can think of to describe what I mean, without getting way technical, is a comparison to an everyday, easily observable event.
Have you ever taken a coin, a nickle, say, and spun it between two fingers so that it spins on it's edge, on a table? try it a few times--what usually happens, if you get it just right, is that the coin spins on edge for a few, or several seconds, pretty much with the axis of spin vertical. As it slows down, the axis of spin will start to wobble until it reaches the threshold where it no longer spins on edge and falls onto it's side.
A few times out of ten, it will spin with the axis of spin rotating for a few instants, then settle down to spin with it's aixs vertical, and then wobble. then fall onto it's side.
A bullet fired from a rifle barrel can wobble on it's spin axis for a few instants too, for whatever reason, then settle down and fly point-first, until it hits something. If it goes far enough without hitting anything, it's rotation will slow to the point where it crosses the threshhold of stability. At this point it's axis of spin will start to rotate around the center of mass, and eventually it will start to tumble.
All modern rifle bullets have this tendancy, to a greater or lesser extent, due to their configuration. Their rear ends are heavier than their front ends, so they naturally want to tumble when shot. The only thing that keeps them from tumbling is the gyroscopic force generated by spinning it up in the rifled barrel. The longer the bullet, the more spin is needed to keep it flying nose-first.
If you're getting good groups, you probably don't have a stability issue since poorly stabilised bullets as a rule will group poorly. You could try moving your target back another 50-100 yards; if it's still shooting good groups, you're OK. I once had a 6mm Rem that shot 1-1/2" groups at 100; I thought I had a 1.5 MOA gun, until I moved the target out to 300 yds, and got the same sized groups--1-1/2". I called Sierra's tech line, they told me about this fact of some bullets needing more range in order to "go to sleep". The actual flight path of such a bullet is a spiral around it's trajectory.
HTH
