"I'm given to the fact that factories have safety factors that MUST come into play so no biggie there...please clarify for me where the "public safety/manufacturing" elements end and where the real world veteran reloader REALLY begins."
All of the above applies. But...real world reloading really begins when the loader buys a press and dies.
When I started reloading few manuals bothered with any OAL suggestions at all and those that did simply gave the SAAMI length, which is for a minimum throat without regard to what bullet it may contain. And gun makers can enlarge the leade as much as they wish because there is NO maximum length for that leade specification, nor need there be.
Only after a few loading books started listing the OAL they used to develop the data they list, along with exactly the bullet they used to do it with (which means virtually nothing to anyone else) did the agonised questions start coming; "What OAL should I use for a Nosler .308 148 gr. ballistic tip-hollow-point boat tail in my Remchester .300 SOL with 21" barrel and Tasco 3-9x65 scope in Simmon's C-through mounts? All I can find is info for a 150 gr. spirepoint and I don't want to blow my gun up!" Forget it! Bottom line; book lengths and the powder, bullets, case and primer data they list are starting point guides, no more than that. And as more and more books include OAL for specific bullets the more the noob's understandable confusion rises. (Including a LOT of web gurus who consider themselves "knowledgeable.")
Thing is, NO firearm cares a bit about where a bullet's meplat hangs in space. If the round feeds and chambers smoothly then develop your load around that OAL. AFTER you find the best shooting charge (optimal), vary the seating in maybe 5 thou steps, in and out, until you find the best (optimal) OAL and stick with it for that load. Few sporters shoot best at the lands so finding MAX OAL with high precision is interesting but virtually meaningless, all we really need as a working reference point that's reasonably close just to keep us from jamming the bullets. Most sporter rifles will shoot best with a jump varing from 20 thou off to as much as 4 times that much so precisely where you start and what a book says matters not a lot.
There are perhaps a half dozen ways to find the max OAL to lands contact, and maybe a half dozen variations of each method. They all work, no one method is automatically better than the others.
A note of caution with the hornady gauge. The brass they supply is non fired and usually 5-6 thousands short, if working with max loads or seated to jam, you may be jambing and not realize it because of the brass being short. If you have a comparator and shoulder gauge you can base the brass gauge off your fired brass and note difference for reference
"..case is indeed shorter...about 4 thou...mmm...on the .308's that I was about to load up I was giving myself a .020 jump...should I "jump" back just a bit?"
Not necessarily. The math says you are still some 16 thou off and that's plenty. Develop your best shooting charge at that point and the experiment by going deeper in 5 or 10 thou steps, as you see fit, until you identify the full width of the good shooting OAL window and then load in the middle of it.
Boomtube, can't get my to completely light up so I confess that I'm havin' a hell of a time get a visual in my head of what's happening when the modified case is shorter than the brass being loaded. I've tinkered with it in my head, on paper and in the shop for 1/2 hour...just can't quite put my head around it. The math is simple...just subtract the brass differential to see how much you're over shooting - I'd just like to SEE it too.
BTW, what's the silver "brass" made of and do folks do anything special when reloading it?
As boomtube mentioned, for most sporters it is non issue. But I shoot with some bench guys that it would be a big issue. Just thought it is nice to be aware if you have a rifle and you think your -.005, but in reality your touching. I t is not hard to visualize or measure case with a shoulder type gauge against a brass fired in YOUR rifle. I have a hand full of the hornady brass gauges, all of mine are short at least .005. If you send in a fired brass you dont have this issue. My 257 WM has enough free bore that a 87 berger is out of the brass to touch lands. Side note I tried to make a gauge for a 338 norma, with my brass and the said tap. I used a drill press and jig to hold brass, but my tap tracked at a slight angle, tried twice. This slight angle was about .002 error on reading coal, because it was crocked. I use a Sinclair gauge and my fired brass. My accuracy load is -.005. The issue is the brass gauge is made to fit any chamber, not yours specific
"Silver" brass is brass. With a very thin layer of nickle plating. Nickle looks cool but it adds nothing and can be a PITA for reloaders to work with. If you get some for free, use it, but don't pay for it.
BR shooters do a lot of things that are not only meaningless for the rest of us but is often actually detrimental to our accuracy. Seating in or on the lands is one.
IF it were true, or even possible, to mathmatically pick the 'best OAL' then doing a lot of measuring and calulating might have some meaning. But it's NOT possible, so all any of us really needs is a STARTING length that safely prevents bullet jamming and use it as a fixed reference to work back from. A seating/OAL range difference of 4 thou is trivial for factory rifles and even for many competitive BR shooters.