When a rifle fires, it actually responds to the powder charge and bullet motion by undergoing a "whipping" motion - even though your eye is not "quick enough" to see this phenomenon. This is actually the barrel's harmonic response to the forces imparted upon it. Shorter barrels and thicker barrels are affected less than longer barrels and thinner barrels, but they all exhibit this behavior to some degree nonetheless. Knowing this and understanding it's effect on accuracy is important to those who want to reload their own ammunition to maximize their rifle's accuracy.
By adjusting the seating depth of the bullet, the chamber pressure is changed, as well as the distance that the bullet must travel before it exits the muzzle. These factors will affect the barrel's harmonic response, as well as at what point in the vibration pattern the bullets leave the muzzle.
Consistency is the name of the game. If every factor was perfectly consistent (every pressure generated, every bullet, every velocity, were all identical), including the shooter, then every bullet would go in the same hole (ignoring, of course, external factors such as the wind). However, we live in the real world, so we can only get close to exactly the same.
Now, if you can tune your load so that the bullet leaves the muzzle at a point in the muzzle's vibration where it changes direction (this is called a 'node' in the vibration pattern), it is moving the slowest (and even actually stopping before changing direction), and is therefore much more forgiving of a lack of perfect consistency. Tuning your loads to take advantage of these points in the barrel harmonics will yield tighter groups.
I have come to the conclusion that the seating depth of the bullet has a greater effect on accuracy than the type and charge weight of the powder (as long as it is reasonably suitable for your load).
Here is a case in point - I performed the following tests while trying to develop a load using 100 grain SPEER HPs in my .270 for varmints, using IMR 4831 and IMR 4350 powder, and a Redding Competition Benchrest Seating Die.
I loaded the bullets to different seating depths, in 0.010" increments. Different charges of different bullets did not have close to the effect in accuracy as the seating depth - even using the same charge of a given powder.
@ 0.00 die setting, 59.0 grains of IMR 4831
4/5 went into 0.825". The fifth hole was a screw-up by yours truly.
@ 0.00 die setting, 58.0 grains of IMR 4350
5/5 went into 0.908"
Now, here's evidence that SEATING DEPTH IS IMPORTANT:
@ -0.10 die setting, 58.0 grains of IMR 4350 - SAME AS ABOVE except for seating depth.
SAME RIFLE, POWDER CHARGE, SHOOTER, DAY, ... successive groups
4/5 went into 1.715"
5/5 went into 3.455"
@ -0.20 die setting, 58.0 grains of IMR 4350
5/5 went into 0.953"
@ -0.30 die setting, 58.0 grains of IMR 4350
3/5 went into 0.712" ( The other two were difficult to chamber, and surely raised the chamber pressure. They landed a couple of INCHES to the left)
@ 0.00 die setting, 58.0 grains of IMR 4350
3/5 went into 0.374"
5/5 went into 1.313 (I guess I was getting tired, but still good results, confirming the above 0.00 setting)
Shoot safe.
12 July 1999