Okay, so I got my hands on a 1# jug of IMR 4166 to experiment with in my 308 as a Varget substitute.

I've had excellent results using Gordon's Reloading Tool and its OBT calculator to ballpark a pressure/accuracy node, then dialing in 0.1gr at a time from there. To get the most reliable output, I went the extra mile: case capacity in H2O, seating depth as accurate as I can get, the whole 9 yards.

Here's what I used:

Alpha Munitions lrp brass, 1x (and only 1x) fired, sized at a 0.002 bump with a 0.331 bushing
CCI 200 primer
IMR 4166 powder, charges from 40.0-40.4gr, 5 shots each
Sierra 175SMK

Here's where things went awry: on the 40gr cases everything was fine. Every "warmer" charge backed the primer out by just enough to catch on my fingernail. Given there are folks out there reporting 28+ loadings on their Alpha brass, it's highly unlikely that the pockets are that worn out after 1 firing.

A piece of primed brass actually got stuck in the die because I'm a moron and forgot to lube it, so I ended up buying a second die. In the process of setting it up I did some measurements and math (my nemesis) and found that I was way oversizing the neck, because the bullet swelled it out to 0.335", meaning there's 0.002" of tension on either side. (I will be recovering the first die, but that's another issue entirely.)

That bushing was picked for the Hornady and Federal brass I had left over from my old barrel, which has a much higher case capacity due to the thinner walls. With a 0.333 bushing in place the dummy round I put together passed the table test, so I think I'll be good to go with the larger bushing regardless.

I'm wondering just how much my neck tension plays into my pressure situation. My guess is not much, but you don't know until you either ask or experiment.

I've read Handloader Magazine for a long time and never read an article on this subject.

However, I will say that I strongly doubt that it makes much difference at all.

That's honestly where I'm sitting myself. Little if any effect on pressure, much effect on accuracy.

Agreed. Basically the neck tension and crimp are strictly to keep the bullet from moving in the neck of the case until it's fired. Nothing more.

I could experiment with this idea, or I can just bring the bullet out 0.005" and call it a day. Between the depth I'm currently at and the lands I've got about 0.030" to play with.

Or the bullet was set too far back in the case. Brought it out 0.005" and the problem went away.

As stated in my OP, I use software to give me a reference point where it thinks a pressure node will be, then I load in 0.1gr increments around that. I've done this very successfully with 6.5 Creedmoor, 140HT, and IMR 4451, and I did it again with this rifle after I brought the seating depth out. All told, I actually use fewer components this way, as I only need to shoot the 4-5 groups of 5 shots compared to shooting in 0.2gr increments across the entire load range.

I started over; nary a single primer out of the 20 I loaded this way had a backed out primer or any other pressure signs, with the same charge weights of the same powder.

Not that it matters all that much now because I have a brand new Bartlein barrel waiting to be installed on this rifle...

I don't turn necks. Yet.

An interesting question so lets do the math. Pressure is Force/Area. Lets assume a Force of 50 lbs required to pull a bullet with "normal" neck tension. Area is simple, that is 1/2 Bore Diameter time Pi. To keep it simple lets use 308 for the bore diameter.

So, 50.00/(.154^2)*Pi = 671.09. If your neck tension requires doubling this value you only have a change of 671 psi for a load that is likely running over 50,000 psi. That is a 1.3% change in the pressure.

Conclusion, Neck tension produces such low pressure changes that it is NOT a significant factor. Odds are that if you are throwing chages using a volume based powder measure you see much larger swings in pressure due to variations in the thrown charge weights. BTW, I have taken the time to run Capability Studies on every powder I load with and with Varget every single charge gets hand weighed and trickled to the exact value within the precision limits of my Beam Scale. Because Varget meters like Dog Poop.

Well, thanks for walking me through the math. Appreciate the insight.

I hand trickled every charge to the tolerances of the cheap portable scales I was using up until I got both a Chargemaster Lite and 1500. Truly life changing in both ease of use and time spent per charge. If I had the money to throw at it I'd have bought a A&D FX120I and a used ATv2, but I'll keep slumming it with the CM's for now.

And yes, Varget, IMR 4451, 4166, 4895, 4320, and RL15 in my experience all meter like crap. The powders I use for pistol are another matter, but we're not talking about those. I legit keep a pair of tweezers next to my tricklers so that I can reduce the charge one grain at a time until I get the weight I want when it goes over.

I actually brought the bushing out again to a .334 because I was having other tension related issues.