Since getting my new rifle I decided I had to measure my chamber so that I could accurately control the bullet jump. For slow fire with 80gr bullets I have been advised to seat the bullet 20-25 thou off the lands for best results.
I have an OAL gauge like the one in this picture:
This gadget works with a specially modified case that screws on the front and holds the bullet. I also own a bullet comparator which attaches to my caliper to enable me to measure a round from base to ogive instead of base to tip. Bullet tips are not consistent, but the ogive is. I decided to buy a new digital caliper that could be zeroed and give me true measurements. The comparator adds about an inch otherwise and the measurement becomes a bit abstract.
With a little experimenting and a few fails I managed to develop a technique with the gauge that gave me consistent measurements. I had read in one forum that best results could be achieved by tapping the back of the rod lightly with a piece of wood to ensure the bullet is engaged with the rifling. When I tried this my results varied more than I desired (a couple thou each way) so I settled on tapping lightly with the rod itself. This way I managed to achieve variances of less than 1 thou.
I came up with an average of 1.978 inches base to ogive using a Nosler 80gr bullet. The next step should be to subtract the required jump from this number and seat bullets to that depth, however the first time I did this I had a “brain fart” and added 20 thou instead of subtracting. I didn’t realise my mistake until I was on the way to the range. Luckily it is easier to seat bullets deeper than to pull them out and my error had no serious consequences.
In my recent efforts to load more ammunition with the 80gr bullet I have been finding it hard to get a really consistent seating depth. I have the press set up with a quality die but the bullets seating depths are varying from 1.956 – 1.961″ (my base to ogive measurement to achieve a .020″ jump is 1.958″). I don’t know if this is an error in my process, my press and die, or my calipers. I would be interested to hear any opinion on this. I have managed to reduce variation by trying to always use the same pressure and speed on the press arm, but this implies to me there is flex in the press and I didn’t think that would be likely.
A thou here or there is probably not significant in the end, but I am still curious. I know reloading can become an endless (and expensive) pursuit of consistency, the ends some shooters go to in search of accuracy blows my mind.