Last time this barrel averaged 1.400" with the 100 gr. GC and WC844. That's about as good as I expect a 14" twist to shoot at 2700+fps, so if I were smart I would declare a victory and leave it alone.
But naturally I want to keep searching for the perfect load, so I'll be trying a few different powders, and a hybrid PC coating.
Also, the Remington's bolt was pretty clunky and the rear of the bolt would drop noticeably when the trigger was pulled. I dunno if that matters but to get rid of the slop I knurled the rear of the bolt, knurling progressively deeper until the metal was raised up enough to make a snug fit in the receiver.
As it came from the factory the bolt measured about 0.6955".
After knurling just enough to feel some light drag as the bolt is closed:
With the knurled bolt, the crosshairs still move just as much as before when the rifle is dry fired on the bags, so apparently the sloppy bolt was not the cause of the moving crosshairs. It may simply be due to the inertia of the firing pin?
At any rate today I used bullets cast of J.R. brand reclaimed shot, tumble-coated with one thin coat of hybrid powder, cured for 30 minutes at 400F, then quenched in water. I did not measure the BHN of this batch but past experience puts them at 25 - 30 BHN.
I made a homemade COL gage for this barrel, similar to the one I made for the 30BR, and used it to adjust the sizing and seating process, though I ended up seating deeper than last time -- more on that later. As always the COL gage is neither 100% accurate nor 100% repeatable, but it's better than nothing. For now I seated right at the measured contact point. As time allows I'll experiment with different seating depths.
I used Quickload to get a feel for which powders would be most likely to succeed in the 7BR. Besides WC844 (H335), 748, 760, 8208, RL10, RL15, N133, and H4895 looked decent in Quickload, in the sense that they hit the desired velocity with a high loading density and moderate pressure.
I started with WW748 ignited by Remington 7 1/2's, but experienced 6 misfires, or at least they seemed to be misfires -- more on that later. The primers had a big indentation so that did not seem to be the problem. The bullets were stuck in the throat and had to be knocked out. Powder spilled into the action when the case was extracted -- much cussing and knashing of teeth ensued.
Repeat this process 6 times over the course of a 10 shot group.
The group was a poor 1.85" but considering all the problems with misfires and spilt powder it's amazing it even shot that well!
After shooting the one group with 748, I then "shot" 5 empty primed cases from the same batch of Remington 7 1/2's. They all went bang. Hmmm ....
Then I switched to WW760 but still with the Remington 7 1/2's. Quickload warned that this would be a compressed load, but by using a drop tube it fit with a little room to spare. Accuracy was fair despite some significant mirage making the target dance around. There were no misfires and velocity variation was decent, too, better than with 748 or 844. Even though it didn't match 844's 1.400" average, it acted like it wanted to shoot.
Mirage is pretty bad during the month of August so load testing is confined to early morning and late evening when the mirage subsides. That's as far as I got this morning, I'll try again in the afternoon. Observations:
-- last time only the GC was seated in the case, my goal today was to seat the GC and half the bottom-most band. But I miscalculated and actually ended up with all of the bottom-most band inside the case. Next time adjust the nose-sizing depth to allow the bullet to seat just a bit further out. WIth the Ardito sizing/seating method, best accuracy is generally with only the GC seated, but I find that for reliable functioning -- getting the bullet seated straight and not falling out of the case when the cartridge is handled -- it's better to arrange for at least half the bottom-most band to be inside the case.
-- in hindsight I suspect the primers did fire with 748, but they simply failed to ignite the powder. The primer alone was enough to jam the bullet in the throat, though, so that's why all the bullets were stuck and powder spilt out. It might be worth trying 748 again with a magnum primer.
-- even though there were no misfires with 760, it may benefit from a magnum primer, too.