RE Shoulder indexed trimmers vs overall length trimmers.
To each his own.
Assuming none of the brass I am describing here end up being longer than safe spec in the rifle they are intended for, IE good reloading practices throughout......................here is my spin.
Overall length trimmers are feel good trimmers imo ......... because for the most part (unless a piece of brass is so short it wont hit the timmer), all the brass will come out the same length, and that feels good. These type of trimmers don't care if your brass headspaces aren't the same. If brass headspace measurements are all dead nuts on the same, all is well, we end up at the same place. However, IF YOUR BRASS HEADSPACES AREN"T ALL THE SAME, for example on the longer headspaced brass, the trimmer will remove neck brass until they reach the uniformity or the measurement you have the trimmer set for.......and you can't get that neck brass back easily. Or said another way, with this type of trimmer the neck lengths will vary in length by the amount your brass headspace varies, and your overall brass lengths will be the same (assuming the brass hit the trimmer). They may all be the same length, and while this may feel good, this clearly doesn't mean they are "all the same sized brass" if the headspace measurements weren't equal..... in fact they aren't, and now the neck lengths vary too!.
All is well (not really in my book, but continuing on) ....... until you size them again. Now, the same (shotty in my example) sizing process used the first time, is still producing brass of variable headspace. Go back to the trimmer, and on the brass that you shaved the neck down shorter, some will be now be "shorties"/not touch the trimmer.........while others will seem to really need some trimming again.... weird huh? Not so much. The cycle continues to the next time they are sized (shotty again), and for some magical reason there are a few long ones and a lot of shorties, causing the reloader to sort or dial his trimmer down to make them all uniform length, and the cycle starts all over again.
Conversely..... on a shoulder indexed trimmer, IF YOUR BRASS HEADSPACES AREN"T ALL THE SAME, you set your trimmer at a consistent neck measurement and leave it, you don't chase overall length to the knats hair. You realize the variance (if any) is caused by poor sizing techniques causing variable brass headspace, and you set out to correct that (or at least you should). You haven't taken too much brass off the necks on your slightly longer headspaced brass that you can't get back, because the shoulder index saved you. You still have the opportunity to make all the brass exactly alike, by simply correcting your headspace sizing technique now or in the future. With this type of trimmer, the overall lengths of the brass will vary by the amount your headspace varies, and the neck lengths will be uniform (assuming the brass hit the trimmer and you set it and left it).
Lastly, and IMO most importantly, where the rubber meets the road.......if best accuracy is your goal, you are incented to get consistent brass headspace AND consistent neck lengths. IF you are producing brass with consistently uniform headspace measurements, either style trimmer will work. It's just that the shoulder indexed trimmer will give you a second opportunity if your headspaces aren't dead nuts consistent at some point in the brass's life, while the overall length trimmer will remove material on your longer headspaced brass that you cant get back when you size him right the next time.
IT's a personal preference, but for me, I prefer shoulder indexed trimmers. In 223 for example, I have my Tri-Way set and it doesn't change, ever (within reason). I know that while the headspaces I size brass to varies by rifle, at the setting I have it on, none will exceed safe overall brass length in those rifles (YMMV in your rifles, burden of safety is on you). Or said another way, the neck length on my all 223 brass, is exactly the same...........which means I produce brass with consistent brass headspace for a given rifle coming off the sizer (different for each rifle), and consistent (actually exactly the same across all my 223 rifles) neck length coming off the trimmer. Both are important to me.
Hope this helps someone understand the different styles and what you are actually cutting, if they didn't already.
To the guys that choose to lathe your brass OAL's down to a reported LESS THAN a 1/2 a 1,000th consistency.......if your brass headspace measurements aren't equally as good I think you are kidding yourselves (if they are, never mind, you are now the best sizer operators I have heard of lol, all good)