I don't often disagree with 4Mesh, but it does happen. I use to shoot a 6.5/06 Ackley Improved, which is about 4 grains more capacity than your plain 6.5/06. H4831 is a good powder. Rel-22 worked better for me, but there were a few people who got better accuracy with 4831 (Larry Jones, for one, Phil. Both Joel and I shot Rel-22, and for a year, we'd trade off who won group, who won score. And yes, this was with the 142 SMK. Bit stiffer charge than he's using, though.)
One downside of Rel-22 is it's burn rate varies a fair bit from lot to lot. You have to test each new lot. There are years when the whole line of Reloader powders seems fast, years when it seems slow. In the very fast years, you can usually go to Rel-25, in the slow years, Rel-22.
It's rumored that Norma MRP is held more tightly lot to lot. Maybe, but both the Norma and Reloader double-based rifle powders are made by Bofors. That would be a bulk powder, which can vary by 20% in burn rate. Norma and Alliant repackage it and sell it as canister grade; canister-grade powders are suppose to be held to 10% variation. It often seems like Alliant (Reloader) uses all of that. Perhaps Norma controls things better, I do not know.
I go on at length about Rel-22 because so many have found it just about right for the 6.5/06AI.
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About testing at 100 yards: You can, but you have to be able to trust your shooting. Is 3/4 of a bullet hole worth of vertical caused by velocity variations, or less than "quite good" shooting on your part?
I do most of my load development at 100 yards. If there is a bullet hole or so of vertical, I'll still use the load, but make a mental note. If I'm getting vertical at 1,000, I know where to look.
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Statistically, it takes about 10 five-shot groups to be significant. I've seen it argued that to get reasonable Standard Deviation numbers, you need around 70 shots.
Well, that's too many rounds on a barrel.
The usual load development procedure for an experienced BR shooter is to shoot 3-shot groups, where the purpose is ruling things out. If three shots are bad, the fourth and fifth aren't going to make the group any better. Again, you have to trust your shooting. Once you get a powder/bullet combination that isn't bad, it is time to start the 5- or 10-shot groups.
Another thing to remember is Benchrest competitors -- particularly 1,0000 yard BR -- are going to test several powder and bullet combinations with each barrel. Short range benchrest is primarily the 6 PPC for group, or .30 BR for score. Most everybody shoots a rifle of the same weight, with a barrel of the same length, etc. Most of us stick with one powder, and maybe several types of bullets.
This is less true with the 1K stuff. In other words, along with the 142 SMK, I'd at least include trying a Berger-grain 140 VLD, and maybe a GTB 140, or *something* else. That would give you a tangent ogive (Sierra), VLD-secant ogive (Berger), and less radical secant (GTB).
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From your two targets, I'd say your load is promising, but not there yet. That's on the assumption your shooting is "quite good." If so, I'd go up in powder, if you're not seeing any signs of pressure now. Half-grain at a time. Of course, if you're seeing pressure, you have to go the other way.
Slight improvements in group size, and big improvements in consistency come from getting the seating depth dialed in for that bullet/barrel combination.
EDIT you reposted while I was writing:
For contemporary loading data, the 6.5/06 (plain Jane) is very, very close to the 6.5/284. Just start with that data.
Just ignore that 3,200 fps stuff with a 140 grain in a 6.5/06. I've done that with an Ackley Improved (3,300 actually), but that's with a very stiff charge of Rel-22 in a 30-inch Shilen ratchet-rifled barrel. BAT action. With your setup, I'd guess the 2,900 region is about right.
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If you're near Burney, say hello for me. Its been 50 years. Was real nice country then . . .
