Thank you, all:
I think you've given me my answers, so I'd like to fill in the back-story for your amusement and history, prompted by Dan1953's questioning if "30-30" and "precision" can be used together.
The firearm is a now discontinued single shot T/C G2 break-open pistol that I first bought in 30-30 Win caliber decades ago. I just wanted something "different" to use for Eastern white-tail deer hunting out to 300 yards. After load development (MOA at 100 yards) it met that requirement. Happy years passed until I tore my rotator cuff and subsequently experienced a good bit of pain each time I shot that pistol. I turned it over to a prominent authority and former protégé of P.O. Ackley for installation of a muzzle brake to reduce the felt recoil.
Instead of adding the muzzle brake to the end of the 14" barrel, he cut into the barrel for the braking effect after assuring me that the loss of muzzle velocity would "not be significant". He was wrong and I was left with a 200 yard deer pistol. Coincidentally, I had just bought hunting property that gave me a potential 400-yard food plot location; and I had the rotator cuff surgically repaired. So, I bought a custom 17" 30-30 A.I. barrel fitted to the pistol frame. (The small, light frame limited my caliber choices.)
I was successful in load development and the pistol was deer hunting adequate at 400 yards. A year or so ago I just happened to drag a wire inside a fired case and discovered incipient case separation that I then confirmed in the remainder of my brass. It was then that I asked for input from this forum's members because I didn't know a reliable way to measure headspace on my break-open action. I was directed to a shoulder comparator that allowed me to fine tune headspace and the problem was seemingly solved. One of the contributors was Bob Wright; he later PM'd with me on new load development and ladder testing. During that time case separation re-appeared. We eventually traced that to the frame-to-barrel flex at firing recoil, and managed that with a combination of headspace adjustments and frame to barrel shims.
Now, months later, the incipient case separation has returned. The explanation: The pistol closes and releases the hammer block when two locking lugs are seated. The lugs intermittently would appear locked, but the hammer block would not release. I solved that by increasing headspace; i.e., seating the brass deeper into the barrel and then forcefully snapping the action shut. BUT, the increased headspace led to the new case separation episode.
In all the years I've been chasing my tail with this pistol, I've never read anything about locking lug problems other than the recommendation to stone them if they were too large to move freely in their recessed position. After driving them out of the frame I found that the frame recess that houses them was full of decades of thick sludge that inhibited their free movement. As simple as cleaning that pocket, the problem is solved.
That brings us up to date and is the reason I'm now fire-forming new brass.
Bob, good to hear from you; you and MagnumMania mentioned crimping and neck tension. I don't crimp after finding that the Lee crimper shrank the extreme muzzle velocity spread but opened my groups. Maybe it exerted uneven neck tension? I set the neck tension to the minimum that resists my hand-pressing the bullet into the brass. That works as long as the bullet doesn't encounter the lands when I snap the action shut.
So, I'm in agreement with Bob, Gorilla@, MagnumMania and RD57 to standardize neck tension, no trimming for now, watch and wait for case lengthening (doubt it will be significant), and after maybe two more firings trim the cases to some uniform length, not necessarily the minimal specification.