TireurDelite:
This is something that I have been measuring and tracking for quite some time. If you could make every round absolutely the same length, measured to the ogive, then each bullet would be the same distance away or into the lands as the case may be.
If you take this very precisely controlled batch of ammo and vary the length by .005" to .030" (I've seen up to .030" variation) in small groups of rounds loaded to each different length, you might see groups varying up to an inch or more apart vertically because of the reaction of the gun to the seating length. This vertical variance will vary from gun to gun but normally you will see bullets seated to different lengths off of or into the lands printing different vertically. If your particular gun happens to group the .005" at a certain point and the .030" at a point 1" or more away then if you happened to load all of them and never check the OAL you would have a built in variance to your loads and groups of 1" before you even start shooting, because of the variation in OAL. I'm not saying the variation will be 1", I'm just using that as an example. I have had a couple of guns that would group ¾" apart by varying the oal by .030"
What I do is set my seater to seat the bullets about .005" long and then seat all the rounds I'm loading at the time. I then measure them with my Stoney Point gage and separate them in groups of .001". Out of 50 to 100 there will probably be 5 or 6 or even more different measured group lengths. Almost all of the time they will be too long without fail. I then adjust the seater to make up for the difference in the groups seated depth and re-seat them. Depending on how much time I have and how anal I'm feeling I may take them all down to within .001"-.002".
What causes this I'm not absolutely sure of but some reasons could be the shape of the bullet and any variations in the shape, neck tension, neck friction, case shoulder flex etc. I don't know that you can prove what causes it, because in any batch you are loading I think there will be more than one reason for the variation, but I know how to control it. Another thing to make sure is that the seater in your die is not interfering in any way with the tip of your bullet.
Good luck with your measuring and testing and if you try what I've mentioned and test your gun with different loaded lengths then you will know the built in variation for your particular gun and I believe you can eliminate a lot of the variation which should result in correspondingly reduced group sizes.
Maybe this will help you and maybe it won't. Might just be psychological but I believe it works and if there's something I can to produce a more consistent and precise round then I do it.
