Update
Well, the 5 shots with BP were beautiful. Stinky. But beautiful. They were shot at ambient temp of about 66 degrees fahrenheit. I'd guess humidity to be maybe 20%. So, cool and dry. They were shot a couple maybe a few minutes apart. No hurry. The report was a deep basso "boom" as opposed to a "crack." A look down the bore I could see loose fouling and barrel steel all the way down to the muzzle, as opposed to a fine sandpapery appearance as I have experienced before. The fouling was nice and loose. One wet patch of moose milk got all the fouling out, with no crusty places in the bore.
The second wet patch of moose milk got just a hint of color on it, and the dry patch was a faint grey.
I cannot say that it would go several rounds, of course, but the gooey texture of the fouling on the patch suggested to me that bullet lube was adequate for these conditions, and a grease cookie was unnecessary.
After some reflection, I have tentatively concluded that it is the Old Eynsford 1.5F powder that is responsible for the good all round performance. Last time I loaded BPCR it was 3F Elefant powder, in a 40-65, and fouling from that stuff was like sandpaper, even one shot. Before I get all excited and start dancing or something, I think I'll try to repeat this with 20 rounds and see what sort of performance I get.
I don't recall black smelling so bad. Folks at the range kept casting disgusted looks my way, as the smoke was retchid. My kitchen smelled like bad flatulence as I was soaking those 5 fired cases. They turned color almost immediately upon immersion in the warm soapy water. The empties look quite dark now. Soaked them for an hour and a half maybe, and swished them around in the soapy water, and rinsed them. They are discolored but clean, and I didn't even brush them as I used to have to do with 3F Elefant.
So far, I think I have a winner.
Mehavey, is there any obvious indication to you that you need to use the blow tube? I mean, any obvious change in appearance between the fouling before and after you blow down the barrel? Or is yours more a performance sort of experience, like if you don't use the blow tube you get bad accuracy or something?
DJ, it seems your admonition to ditch the grease cookie idea was well taken. I may yet try it out, but if the cartridges continue to perform as they did today, I think the need for grease cookies will not be proven.
Again thanks guys.
ETA short video of one shot. Maybe you will get a chuckle out of it. Perhaps I should edit it to remove the question to the spotter.
https://youtu.be/gLaITeO2HdY