Many shooters are not aware of the dramatic effects that bullet seating depth can have on the pressure and velocity generated by a rifle cartridge. COAL is also a variable that can be used to fine tune accuracy. It’s also an important consideration for rifles that need to feed rounds through a...
www.bergerbullets.com
Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 2 by Bryan Litz for Berger Bullets. <a href=http://www.bergerbullets.
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The above links are one article in two parts from Bryan Litz, if you're not familiar with Bryan Litz, he is the end all be all of knowledge when it comes to ballistics.
So when it comes to seating to CBTO I will explain what I do, others may do something different but this is what has worked for me.
I use a
Hornady OAL Gauge (which will need a modifief case) and
Hornady Bullet Comparator. I usually start by finding the lands with the OAL gauge, modified case, and whatever bullet I intend to use. I will lock that in and measure with the comparator attached to my calipers. I typically repeat this about 5 times to make sure I get a consistent measurement.
One thing that you should know when you are doing this is also the base to ogive measurement of the bullet you are using. I can tell you from first hand experience that Berger VLD's vary. Usually when I buy a new box of bullets I try to sort them by ogive measurement to make sure everything is consistent. I just put them in bins or cheap IKEA Tupperware with a note taped on it for the measurement. This is not necessary in hunting ammo, I do it because I'm chasing tight groups, low SD's and like to shoot long range, too.
Once I know the measurement to the lands I back the bullet off some for my seating depth. A good starting point is usually .020" off the lands. Some bullets will like it closer and some further, if you are loading Barnes then start at .050" off the lands.
This is where a chrono comes in handy because you first want to find an accuracy node in your powder charge and having the speed helps you find if you are on track or not. You don't need to invest in a lab radar, a pro chrono or Magneto Speed sporter would do great.
When you have found the powder charge your rifle likes, you can start to tune the seating depth. You can start at .005" of the lands and back it off in .005" increments, so .005", .010", .015", etc.
If you keep notes and track all of the data you are going to find a load that is extremely accurate in your rifle.
Just a thought, in terms of neck tension, I'd recommend you take the expander ball out of your sizing dies and invest in a Sinclair expander mandrel with the appropriate sized mandrel for your caliber. I've got a 300 WSM that I'm F/L sizing with a standard RCBS die and expanding the neck with a mandrel. It is a consistent .25" gun with SD's in the 4s.
