It's simply thinking through the process of sizing/trimming bottle neck cases...
The sizing die is *Supposed* to knock back the shoulder to create proper headspace,
Then length is strictly from Datum line to case mouth...
When you are clear on the case lengths
(there are two, head stamp to Datum line, then Datum line to mouth),
Then it becomes chrystal clear you need a Datum line to case mouth trimmer.
All bets are off if you can't produce a case with the Datum line the correct distance from the case head, that means you can't produce a case with the correct headspace for your chamber,
So NOTHING from that point forward makes a difference...
Once you have case body correctly sized, Datum line in the correct place to create correct headspace,
Then it becomes a question of length to the end of the case chamber (Neck Length Clearance).
Since that measurement is from Datum line to case mouth, the cutter should also index (measure from) the Datum line.
A lot of words for a simple concept, simply measure from the head stamp surface to the Datum line for head space clearance for YOUR rifle (or to SAAMI specification),
Then from Datum line to case mouth for overall case length.
Two measurements added together for one total length, easily done...
Guys using just total length measurement on bottle neck cases are screwing it up,
(I'm no exception, I screwed it up for years before figuring it out)
Total length for STRAIGHT WALL CASES is measure directly, but that's a screw up with bottle neck cases (or belted magnums).
I tried W-T-F, IN MY OPINION... useless piece of inaccurate junk that dulled immediately.
W-T-F went from 'Worlds Finest Trimmer' to 'What The Frack?' real quick!
Worst waste of money I made on a Datum line trimmer, and I bought 4 calibers at the same time...
I tried the bench mounted manual & drill powered units, I was looking for a trimmer that gave me Datum line indexing, the flat faced full case length trimmers weren't the answer.
Worked fine on stright walled cases, just didn't do bottle neck cases accurately.
Girard works pretty well, but the price of the bench top unit left me butt hurt,
Trim It II', the champfer & deburr version, fit the bill and at around $130 wasn't the biggest money mistake I was ever going to make...
Turns out, with a little adjustment, it does a VERY acceptable job,
Less slop in the insert bushing made for closer to square cuts than the W-T-F usually less than 1* off square (while the W-T-F was hard to hold angle to 5* off concentric),
And the Trim It II carbide cutter didn't dull out immediately, and had three cutting surfaces on each carbide cutting insert!
Never stuck doing the last 300 cases CHEWING off the brass with a dull cutter face!
Since the length of the neck means almost nothing in accuracy ammo, I trim to minimum (once the case shoulder is back where it belongs) and generally never have to trim again.
Keep in mind I case roll size to take the bloat/swelling out of the lower case, so my cases DO show a little growth, even once fired,
(Guys shooting the same cases through the same guns won't have to case roll, if the case fit once, it will fit again, I shoot my ammo through a dozen different guns, some with really particular chambers)
Where standard top down punch sizing dies leaves the bloat in the extreme lower part of the case making it shorter than factory new... So I run sized cases through the trimmer just to make sure they aren't over length and I'm done, as close to perfect cases as any common reloader will ever get...
Nothing quite like SAAMI specification lower case size in a speed gun, seriously reduces failure to feed and lets the bolt hit full battery lockup every time...
