I've got a question for you ballistics guys.
In both of my bolt guns (M70 7mm) (Ruger 77 30-06)
both with standard weight bbls, lighter bullets seem to be more accurate. Heavier the bullet, bigger the group. I've yet to have a bolt action rifle that did well with heavy bullets.
Come to think of it, I've never had a semi auto that did well with heavy bullets. Sure, they all will shoot the heavy stuff, just not with any kind of accuracy. So do light bullets stabilize easier? Why the better accuracy with light bullets?

Slacker, you might consider matching your barrel twist to your bullet weight.  (Now someone is going to come in and say bullet length not weight matters but I'm going to assume you are not going to shoot nuclear tipped sabot rounds.)

For the .223 (5.56mm)
1:12 - 55 grns
1:9  - 62 grns
1:7  - 69 grns.

For a .308
1:12 - 168 grns.

etc., etc.

Find out the barrel twist on your specific  rifle and then find the sweet spot (Bullet weight) for that caliber/barrel twist to insure max accuracy.  

Thanks 5subslr5.
Funny how my Bushy is a 1:9 and its more accurate with XM193 than it is with SS109.

Quoted: Thanks 5subslr5. Funny how my Bushy is a 1:9 and its more accurate with XM193 than it is with SS109.

The military uses 1/7 to stabilize 62 grain SS109 rounds.  1/9 is a civilian compromise.  BTW, both my colt uppers are 1/7 and shoot 55 grain ammo fine.  Lighter bullets will be stabilized by a fast twist, but they tend to spin so fast that when fired at close range targets (less than 100m?) they disentegrate.

1 in 9 is fine for a 62 gr bullet.  When the M856(?) tracer came out, it was horribly long, requiring a 1 in 7 twist.  

Back on topic, with all construction materials being the same, a heavier bullet is going to be longer.  The material has to go somewhere.

Velocity is another possible factor in the equation.  An ex-coworker used to build up loads for his 44 magnum, and as he would increase the load, the groups would either get tighter or spread out.