As Jim in Iowa mentioned, I manufacture chamber tools for bullet seating depth and optimum case trim length. Check:
http://www.cactustactical.com/reloading.htmlI have done extensive testing with all types of rifles and calibers. Some basic info:
Make sure the bullet is seated to at least one bore diameter.
If you shoot a Sako, Tikka, or a newer Savage, they have a forcing cone in front of the free bore. This will spoof your measurements. The forcing cone concept is fantastic for out-of-the-box accuracy so you don't need to seat your bullets out farther.
If your gun has an eroded throat, seating the bullet out farther will help a little but is just a band-aid. The barrel will need to be replaced.
When you change bullet seating depth, it changes the chamber pressure and velocity. The farther out you seat the bullet, the lower the pressure and velocity will be. This means you will have to add a little powder to get the same velocity as a deeper seated bullet. I recommend seating the bullet out to .010 off the lands then working up your powder charge for optimum accuracy. If you adjust your powder charge first, then change seating depth you will chase your tail because you change the pressure at each different seating depth.
The only reason for the .010" is the metal in the gun will expand in all directions when it gets hot. That means if you seat to touch the lands, you may have problems chambering a round on a hot gun. Contrary to what many people think, seating the bullet where it touches the lands does not increase chamber pressure.
One of the main accuracy issues is "bullet strike damage". This is where the bullet is seated deep and builds up a good head of steam before the bullet strikes the lands. This causes the bullet to be damaged slightly because a bullet seldom enters perfectly centered. Once the bullet is launched, it will whiffle and open the groups. If you can minimize bullet strike damage, you will see a vast improvement in accuracy.
Some guns are more problematic than others. Just because SAAMI sets chamber dimensions doesn't mean manufacturers always follow those specs. If your gun shoots well with recommended seating depth, there is no advantage in playing with bullet seating depth. If you have a gun with a deep cut throat, then seating the bullet out farther and maintaining your brass length will help accuracy a lot. Some throats are so deep that seating a bullet out to optimum length would not allow the cartridge to fit in the magazine or would violate the minimum of one bore diameter seating depth. In those cases, you just have to experiment to find an acceptable depth and powder charge.
I see a lot of chatter here on the forum about a particular gun having exceptional accuracy and another owner with the same brand, caliber, and model will respond with poor accuracy results. That's because the factory starts with a new chamber reamer (thus a deep throat) and continues to use it until it dulls or breaks. If you got the last barrel made before the reamer wore out, you would have a tight throat. Recently I was testing a Remington 700 BDL in .223 Rem. It had the throat cut .060 deeper than SAAMI specs. I have a 700 ADL in 223 Rem with the tightest chamber I have ever seen. Factory ammo will not chamber without forcing the bolt. When you remove an unfired cartridge, there will be rifling marks in the bullet. The tight throat 700 ADL is one of the most accurate rifles I have ever fired and the deep throat BDL was one of the worst I have seen. It struggles to get 5" groups at 100 yds with factory ammo. Once the bullets were seated out, accuracy improved to about an inch at 100 yds. Unfortunately, the gun became a single shot because the cartridges wouldn't fit in the internal magazine. Meanwhile, the ADL will poke a sub-MOA group with about any ammo you can feed it.
