Playing with 303 bullet seating depth

[toddpotiphar]

I spent some time at the range this morning testing some different seating depths for my No4MK1. The results were very interesting to me and not at all what I expected.

Here is the load data:
Hornady brass, once fired
CCI BR2 primers
41 gr IMR 4064
180 gr Sierra ProHunter bullets

Measuring from the base of the case to the lead meplat (not the most accurate way measuring, I know; but I don't have a comparator) I seated rounds at 3.055", 3.030", 3.005", and 2.980". Book max says 3.075", but that's a really tight fit in the magazine.

I've had pretty good results with 3.055" COAL with the previous batch of brass (consistently 2MOA or better), and it performed fairly well again. The target is a little difficult to read, but all the data is there. (I forgot to bring my posterboard backing, but fortunately there was a brand new piece of OSB on the target board so I was able to track and mark my hits.)

I didn't follow best practices and shot each different COAL as a single group before shooting the next. This allowed me to track my shots better. Unfortunately, there was a slight breeze (c. 10 km/hr at about 45 degrees) when I shot the 3.055" and it had become nearly dead calm by the time I shot the last group (2.980") so I don't think the horizontal dispersion tells the whole story.

All this is to say that I was very surprised that the shortest COAL produced the best results. So much for getting the bullet as close to the lands as possible!

The group measurements are as follows (shot at 300 yards):

3.055" - 10.45"/3.48 MOA (but the vertical dispersion is only 1.13).
3.030" - 9.182"/3.06 MOA
3.005" - 4.788"/1.60 MOA
2.980" - 3.826"/1.28 MOA

 

[Ganderite]

Don't argue with the rifle. It seems to know what it likes.

 

[Leeper]

The standard throat in the 303 is a fairly long taper. This throat configuration often favours more jump.