Cdoc42,
Glad to help. If you go to the JBM ballistics site, they have a stability estimator at the bottom of
their calculators page. It's input is bullet weight and length and velocity and atmospheric conditions, and it output is the estimated gyroscopic stability factor, a number such that marginal stability is at 1.0, and any number below it is unstable. A Sierra tech I spoke to puts good stability as a gyroscopic stability factor of between 1.3 and 3.0 for "hunting accuracy" and 1.4 to 1.7 for match accuracy. For bench rest accuracy, Don Miller says about 1.5 is best, but Harold Vaughn estimates 1.4 is best. I've found a couple of other authorities that like 1.5, 1.6 or even 1.7., but 1.5 is where the majority come in for rifle shooting. So you can measure your bullet lengths and your barrel twist and see what happens.
The estimator is one Don Miller wrote, so I use his number, 1.5, most of the time. What makes one of these numbers better than another is the compromise it sets between robustness of stability and spinning the bullet fast enough to cause eccentric wobble in flight due to any tiny mass asymmetry that may be present. There are other factors, like increasing spin drift, lowering BC by increasing the yaw of repose, not giving the bullet enough angular acceleration to cause core stripping or disintegration on the way to the target, but adequate stabilization after recovery from post-muzzle yaw and not causing wobble are the two main effects traded off against one another in picking the best number.
I shot moly in my accurized Garand with original military barrel for years. That original barrel was a shooter, and would put ten rounds into well under an moa, but it was so rough it would actually accumulate metal fouling fast enough that my scores would start dropping during the slow fire phase of the National Match Course. The deterioration began at about 35-40 rounds. Then it would take hours of repeated re-wetting with Sweet's to get it clean again. This was late 80's/early 90's, and the fast modern copper cleaners like KG-12 or Cu++ weren't available then. When I switched to moly, that problem stopped and I could shoot 100 rounds without cleaning and still not see the accuracy deteriorate. So I know it didn't merely cover up fouling. Also, cleaning times were noticeably shorter.
Walt Berger opined that the reason moly-plated bullets sometimes show slightly lower ballistic coefficients that their unplated parent bullets is that the lubricant easing the bullet into the rifling helps it center better, so there is lower yaw to correct. Friction with unplated bullets also smears small tails off the back of each rifling groove that interfere slightly with the air flow over a boattail, and that may be a factor.
The trick with using moly bullets is cleaning properly. A lot of folks thought that because the metal fouling didn't build up fast meant you could go a whole season without cleaning your gun, and then a few complained about rust and pitting showing up. Well, that took two mistakes. One was trying to substitute cheap moly that had minute amounts of free iron, more free sulfur, and larger, less consistent particle sizes than the lab grade stuff NECO first created the kits with. Many just vibrated the powder on instead of using NECO's steel shot impact plating method that helps break the particle size down further so it mixes better with carbon for easier removal. And the second was just not cleaning bores in a way that removed the moly/carbon matrix that accumulates in the imperfections and sometimes in a couple of other places.
But today you can handle all that stuff easily. I've always used the NECO kit or bought Sierra or Normal moly-plated bullets made under patent license from NECO. Second, I discovered Gunzilla will break down the carbon and moly matrix overnight. Just wet the bore with it and leave it 24 hours and all the moly patches out with the carbon the next morning. You can now also buy special moly bore cleaner made by Boretech, and while I haven't tried it, by reputation it works very well.
I'm curious how many rounds you had through the STW? The hot guns often don't last well. One of the complaints about the 6.5-284, for example, has been some barrels not making it even to 1000 rounds before best accuracy starts to drop off, with nobody getting to 2,000 rounds with best accuracy. Another is muzzle funneling by large gas volume cartridges, causing the need to cut the muzzle back to get a good crown again. IIRC, Bruce Baer said he started his .300 Baer Magnum rifles with 32" barrels, then cut an inch off every 600 rounds or so to keep the funneling from affecting accuracy.
I would have liked to have taken a borescoping and slugging tour of your barrel after it started going south, to ascertain the cause of the problem.