COAL as listed in most reloading manuals is there due to the SAAMI (or CIP in Euro) spec.
It is NOT however, an accurate measurement for adjusting the amount of freebore that the bullet has to jump through before engaging the lands. One needs to measure THE bullet one intends to use at that bullet's ogive, for a given rifle. Setting the ogive at a given (optimum) distance off the rifle's lands generally improves that rifle's consistency, or precision, resulting in smaller groups.
Since all brands and bullet designs have a different ogive length, this may change with bullet weight within a brand, and when changing brands, or when the load is used in a different rifle.
It is just one more step in making consistent loads, to provide more consistent groups for a given rifle. Even different rifles within a brand (say Remington for instance) may have different freebore, even though they are of the same caliber.
This is also the reason for a custom rifle's chamber to be spec'd with a known freebore (sometimes known as leade). This allows the bullet of choice to be seated far enough out that it's base doesn't protrude into the case, taking up valuable room for powder, and providing the optimum bullet "jump" (through the freebore) for precision (group size) and pressure control.
A bullet comparator setup and a little experimentation with your rifle and handloads will tell you when you have found your combination's "sweet spot" for the distance off the lands.
Ya all good points 6.5.... dont forget that in most factory rifles the jump is so much that by the time one gets a good group the COAL has far exceded the magazine lenth ... A bad situation for someone hunting dangerous game...LOL ... In a custom rifle this is not to much of an isue. But in my factory size actions I try to find an accurate load that still fits in my magazine.
Even the bullet bto length varies, to some degree for a given cal and type. Measure a box of bullets of the same design and you may sort bullets into 3 or 4 groups by the ‘base-to-ogive’ dimension varying by as much as .003”. Some don’t do this, others do, since it affects bearing surface.
I wouldn't say most rifles Dusty.
But the mag length has been known to be the limiting factor with some rifle/cartridge combos.
I can't get my Rem .270, '06AI or my 6.5 Mauser pills out too far to fit the mag before they hit the lands.
Heck, the 135 SMK .270 pill won't reach the lands at all (before it's no longer in the case) and still fits in my 700ADL magwell.
The Mauser was custom built (throated) so the 140gr 6.5 pills are seated out as far as needed (no case protrusion) and they still fit the mag, and that's a long bullet!
It is just something that one should consider before choosing the action for a new build.
"How does cartridge length influence the accuracy or for that matter grouping?"
Opinions differ but I don't think anyone KNOWS, at ignition, lots of things happen very rapidly in a dark place. I think it has to do with barrel viberations and bore time.
"Are there guidelines for various caliber sizes?"
No. Each rifle is a law unto its self so we must experiment. SOME rifles like bullets seated close to the lands, especially tight chambered bench rest rifles. But, in my experience, that is seldom true with factory rifles and our common ammo. Every factory rifle I've ever found a good shooting load for was from .025" off the lands to as much as three or four times that much.
Our rifles aren't BR types so their loading methods are often of no use to us.
One thing I have found is that when measuring COAL (from tip to csae) the differences from one to another in the same batch of bullets changes alot too. I dont have a micrometer type bullet seater either so that may play some of the role. But If I measure from the ogive its much closer... telling me the bullets from the same box are not uniform. So when one starts playing with bullet seating to get the right device(bullet comparitor) like 6.5 said to measure with... When I started out I did not have the right stuff and it was frustrating...
Ya 6.5 I should not have said most. It just seems that way for me... all of my rifles are limmited by the mag... but I use heavy bullets too... Im sure a 40 grain bullet in my .22-250 would fit fine and have some jump too.
rem 7mm 700
243 mod 70
300 rum 700
all my ar-15
kimber 300 wsm