Revolverguy; you are welcome.
The saga will continue... I hope you find it useful by the time it's through.
I've got Longshot loads worked up with Remington 7 1/2 and CCI #41 arsenal primers waiting to test, once the Chronograph is fixed up. Both of those are "hotter" primers than the CCI #400 primers so I'm starting the load .5gr lower. I'm planning on doing 4.6 through 5.4gr Longshot powder tests with them, using the 40gr Hornaday V-Max.
I'm also going to proceed with a 5.7 and 5.8gr test load out of the PS90 using CCI#400 to see if I can nail down the "unsafe" level. Want to get velocity data off of those though, so I need to wait for the chrony to work again...
Now that I know there is a noticeable difference with Longshot ignition on "powder forward" or "powder back", I will test this each time.
BTW, a piece of trivia; with a 40gr hornaday bullet seated to match the SS197 round, my cases are "at capacity" at 5.8gr of Longshot. So powder forward or powder back should not make any difference at that point. But that's a pretty hot load out of the rifle. I lost a primer at 2339fps, which is about the mark where 5.8gr would be if the fps gain trend continues past 5.6gr.
Also, I *have* confirmed that the lost primer in the first round of testing at 5.5 grains was due to a misthrow of powder; there were not any serious pressure signs at 5.6 grains, with "powder forward" or "neutral"; however, I had flattened primers, increased recoil, and increased noise with "powder rear".