Len, I shot the 178 A-Max in my 30/338 Lapua IMP one time so far, 9 shots. The Oehler 43 picked up two shots as light was fading which were 3365 and 3404 fps. That was with 95gr RL25. I imagine 95.5gr would get me a solid 3400, or maybe a tad bit more. I could get 3450 - 3500 without a problem, pressure is still pretty low at 95gr. You run H870 or something and you might get 3600 with the 7mm if the barrel was long enough. I'd guess 3400-3500 in a long barrel for accuracy, and hope for better.
A side note:
Dad used 7 WSM data for the 140 load development on his 6.5 WSM, and even starting loads were way to high, obviously from the step down in barrel I.D. making more pressure, and the 6.5 bullets longer bearing surface.
I know that Mr. Bifurcated's barrel length is 32 inches, what is yours? I am planning a 34 inch barrel this time. STL's is 32 and if we are ever seen together I want to have a "bigger one" than he does. Size does matter, you know. By the way, you should see the shape of the brake on the end of his thing. Its diameter is larger than the end of the barrel and its shape is like a.....no, I can't say in public!
The Hogdon online reloading data indicates that the 300 Rum gains about 200 fps when going from a 220 caliber bullet down to a 190 grain (same pressure). That's in a short barrel. I don't know if a longer 32 inch one would show a larger or smaller difference. The Hogdon data shows about a 200 fps difference between 175 grain bullet velocities 7mm vs 30 cal.
So that would suggest adding 200 FPS to STL's 3,300 with the 210 grain bullet when working up a 180 grain load. And then drop 200 fps when switching to the 7 RUM pipe size. Resulting in 3,300 fps for a 7 Lapua with 32 inch barrel and the 176 grain Cauterucio bullet. Add about 40 to 50 fps for my 34 inch barrel for a total of 3,350 fps. So, 3,400 may be pushing it a little.
Incidentally, Hogdon doesn't sell H870 anymore and they warn that any H870 being sold today by others may not match Hogdon's data for H870.
That's interesting about chronograph brand velocity reading differences. I know Oehler is supposed to be the most consistent but I never would have expected that much difference.
I measured the water weight capacity of one of my 300 WAMBOMAG cases and it was 116.1 grains. This was with a primer installed upsidedown and filled to the top of the neck, level not rounded. I added a drop of dishsoap to about a cup of water, don't know why, just heard it helps.
168 SMK shoot fine, 168 Bergers keyholed but I didn't try running them real fast, they may have stablized.
I was not really interested in shooting 180 grain bullets when I bought the barrel. I thought 168 SMK and 162 SST/AMAX would do fine for my intended use.
I was concerned with barrel/throat errosion and wanted the 160+ grainers to go real fast. So I went for the 11 twist.
If one were to go with a barrel that was 34 inches long and get some serious velocity, I think a 10 twist would be fast enough, even for big bullets.
Would be interesting to know what twist the other 7MM RUM/Weatherby users have.
I know that a lot of people will be thinking 9 twist is right for the real big bullets, but I think the real fast twist contributes to the burned out barrel/throat thing. If you can get them to higher velocities they will stablize with a slower twist.
The bigger the bullet and the faster the twist the harder it is to get them spun up and pushed down the barrel. Something to do with overcoming the rotational moment of inertia of the bullet and the added friction due to a larger bearing surface. I think this holds the fire ball closer to the throat for a longer period of time. This is purely a thought exercise and I have no experience or proof to back it up though.