Seating Depth - How It Effects Accuracy

dev110

When you hear people saying they have their rounds 10-30 thou off the lands what do they actually mean and how does a 5 thou difference effect the accuracy of your load?

First of all the word 'lands' means where the rifling starts.

Some bullets prefer different things. Some might prefer a small jump, big jump or to be jammed.
I know that SMK's like to be 15 thou off the lands in my rifle and the same goes for the Berger Hybrids.

Can you see much of a difference between 10 thou and 15 thou?





Believe it or not but that small change makes a big change to group size.

Here are two targets that I shot at 300 yards:

The first is 10 thou off. I pulled the last shot and as a result fecked up the true vertical. If I hadn't pulled the last shot it would have been close to the first two.





The Second target the rounds were 15 thou off the lands. You can see a big change in vertical. The vertical was halved by just increasing the jump by 5 thou.



Why do I just measure the vertical of the group?

When you are trying to develop a load you are looking for an accuracy node. These nodes will happen across a range of powder charges. When you hit the right spot the barrel will act the same way with every shot.

It is all got to do with harmonics. When you fire a shot the rifle barrel will oscillate/bend/vibrate. When you hit the right powder charge the barrel will oscillate in the same places and when the bullet leaves the muzzle the muzzle will be in the same place.

I have a target that I did a ladder test on that I might throw up later in the week.

Glensman

So you don't facto in the horizontal at all?