Ludwig,
there are so many 'rules' and 'wrinkles' surrounding this issue, it's often one of the most difficult issues for tyro handloaders to understand. There are 'rules' or guidelines, but there are also in practice many, many exceptions to those rules.
It would help forum members if you state the cartridge, barrel, what sort of throating the manufacturer or rifle-builder has given you (in general terms), and what you're going to use all this for. We're also all or mostly assuming (a dangerous thing!) that given the nature of your question, you know how to measure / ascertain the bullet position in relation to the lands, but are unsure of where you should set it ??
The general rules are:
1) VLD type bullets, or others with aggressive secant ogives (long nose with a sharply defined junction between the parallel shank and nose sections) - seat into the lands, 0.010" and up. (15 thou' is a common starting point for people loading Berger VLDs.)
BUT ...... sometimes VLDs work better with very large jumps of up to 0.080". Berger used to, probably still does, give advice on this issue on its website.
A potential problem with jam-seating is that the bullet may be pulled from the case and left stuck in the leade if a live round has to be unloaded for any reason. As well as leaving the bullet in the barrel and having to be knocked out, powder is usually spilt in the lug recesses and will stop use of the rifle until every last kernel is cleaned out.
As others have pointed out, jam seating increases chamber pressures, all other factors being equal, so a switch from jump to jam may require a reduction in powder charge weight and working up again.
Using QuickLOAD internal ballistics software to model a .308 Win load gives the following results for the Berger 190gn VLD at 2.950 COAL over 44.0gn H. VarGet:
out of the lands (initiation pressure 3,626 psi) 60,071 psi PMax
in the lands (initiation pressure +7,200 psi = 10,826 psi) 69.034 psi PMax, well above SAAMI maximum allowed pressure.
That's an estimated 15% pressure increase from this factor, but in real life it'll depend also on far and how hard the bullet is jam-seated which is partly about COAL the bullet is seated to, but also about case-neck tension on the bullet. If you run with very light tension, the bullet will be pushed back slightly into the case and limit the amount of jam. This may also stop you de-bulleting any round that that has to be unloaded.
2) traditional ~7-calibre nose-radius tangent ogive form bullets typified by most (but not ALL) Sierra MatchKings that are said to be 'jump tolerant'. People may run from 5 to 50 thou' (or more for magazine fed rifles where the COAL is determined by the magazine depth) jump, but 10-20 thou' is a typical starting point in load development.
Most people who've looked at this subject recommend a minimum jump of 10 thou', not because small jumps don't work, but
to ensure every bullet is jumped. Depending on the bullets' dimensional consistency and the accuracy of your initial measurements, trying to run at say 5 thou' may see some bullets jump at up to that amount, but others just kiss the leade. Any inconsistency in this is almost guaranteed to increase group sizes, maybe produce really bad 'fliers' outside the group. Personally, I've always started at 0.015" with this type of bullet and it usually works well in most barrels and chambers.
That's not to say such bullets don't work well seated into the lands. If you look at German Salazar's Rifleman's Journal weblog
http://riflemansjournal.blogspot.co.uk/
and look for his .30-06 loading articles that feature both traditional Sierra tangent ogive number such as the .308" 190gn MK and out and out VLDs, German seats them all 'in'. This is a guy who knows what he's doing and if he does this, it's because he gets more consistent results than jumping his bullets.
3) some bullets that don't seem to play by the 'rules'. The new 155gn Sierra Palma MK (p/n 2156) has a reputation for liking a large jump and many users are running at ~0.040". My experience with some .30 cal Berger Hybrids is that they either have to treated as a VLD and seated 'in', or (and I've not tried this) may be like the new Sierra and need a larger jump than a conventional tangent ogive type.
The final thing about all this is that as everybody else is saying, let your barrel tell you what it likes, and that's even more about bullet model than COAL. Sometimes a bullet that won't perform is magically transformed by a change in jump, but often the barrel simply doesn't like that model for some reason and playing around with COALs won't cure the problem.
Thanks Guys, you helped me a lot.
