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Mule Deer has written, as have several other authors, very interesting stuff about actual pressure and the first indications showing on your brass. We have learned much since the days of PO Ackley and "load until you see ejector marks, then back off a grain."

And we have learned what pressures Ackley was actually producing. It is very fortunate that our rifles are over engineered to the extreme.

Interesting. Anyone have a link to these tutorials?

Charlie Sisk performed an interesting experiment a while back. It was in print, not online, so I don't have a link. It had to do with case shape, case capacity and velocity. The twist was he was able to compare two cartridges in the very same barrel.

The cartridges were the 300 H&H and 300 WSM, which despite their radically different shapes have the same capacity. The barrel was first chambered to the H&H. After velocity/pressure testing it was chambered to the WSM and tested. Done in this order each chamber got to use the same length of the same barrel from the case mouth to the muzzle crown.

The results were that the same quantity of the same powder in same size chambers produced the same velocity.

AND the 300 H&H has far more reliable feeding and extraction under adverse conditions than any WSM.

Brits, back when, were way smarter than our current "experts".

See Rolls Royce Merlin and angled flight deck if you have any questions.

Agreed. No way you can get around P = F/A.

Was this Charlie Sisk any relation to the bullet maker? I have a bunch of his bullets I've been wanting to try in my 22-250.

As well, there is no problem with feeding, as it is a single shot.

Send a PM to Mule Deer, John Barsness, ask him which book you should buy. You will want the one which explains the 4 to 1 rule.

In short, a 4% increase in powder yields 1% increase in velocity.

In your case, a 15% decrease in powder yields about 4% decrease in velocity at the same pressure.

That is gratifying to know. I missed that .300 H&H to .300 WSM comparison by Charlie Sisk somehow.
Thanks.
My own verification of the principle has come from comparing the .458 B&M with 2.240" case length (shortened and necked up RUM case)
to the .458 WinMag with 2.500" case length.
Both made with R-P brass, and gross water capacity is identical.
No magic came from the shorter powder column with same weight of powder.

Throating might be an issue in the Charlie Sisk experiment, if the .300 H&H throat was changed with the .300 WSM throat.
The SAAMI .300 H&H throat is a "leade-only" throat with a 2-degree angle starting at 0.3080" diameter and tapering into 0.300" bore diameter.
Basically NO THROAT.
The .300 WSM has a free-bore diameter of 0.3099" that runs a length of 0.1884" before running into a leade of 1*30' (1.5 degrees).
Maybe Charlie Sisk did them both with the .300 H&H NO THROAT using a special .300 WSM reamer ?

My .458 B&M had a short throat (proprietary), but it was re-throated with a .458 WinMag (SAAMI) reamer.
I used a fat, short cart and a longer cart with same throat and gross case capacities.
I agree with Charlie Sisk's findings.

Everybody here is neglecting throat effects.
They are not insignificant.
Free bore reduces pressure and may be seen as doing so by increasing the effective case capacity
that is containing the burning powder.
Universal gas law: PV=nRT.

Accuracy issues and COL allowed by a longer throat: A different can of worms.

Just like moly, increasing distance to the lands reduces pressure as every experienced rifle ammunition loader knows.

Reduced pressure always results in reduced velocity. One adds powder weight to compensate.

Aw shucks, moly and boron nitride:
Don't they work by increasing lubricity of the bullet, reducing start pressure mainly ?
I have tried both and prefer naked bullets such as Hammer bullets
that have reduced drag in the barrel as well as in the air after leaving the barrel.
Use those in any rifle without coating.
Very little fouling too.

Nothing wrong with having a special rifle with bore coated with boron nitride,
using only boron nitride coated bullets,
and adding the little bit of extra powder to get pressure and velocity back up.
I know of one crazy Arab Shiek who has a pet wildcat and he shoots only moly-coated bullets.
Nothing wrong with that as long as you stick to the moly with all the bullets.
In general, moly is too much trouble and mess for use in my full armory.

I do like powder-coat-paint on all my cast bullets for smokeless loads.
That is all the lube needed there.

Throat effects can be much greater than bullet coating effects.
Also throating long can allow long COL and actual increase in net powder capacity, especially in a single shot like OP has here.
Every increase in COL of 0.100" in the .458 Winchester Magnum adds 4.166 grains of water capacity net.
Increase the COL of the .458 WinMag to 3.600" with a 500-grain bullet, and you have a .458 WM+ load.
That is not a SAAMI load, it is a wildcat load.
.458 Lott maximum loads become .458 WM+ starting loads, and very accurate loads with yet more free-travel to go than in the .458 Lott at 3.6" COL.

Still, in any case you do not gain velocity by decreasing powder capacity, and powder charge. Except with increased pressure.

In the case of the OP, he is beating the velocity of RUM, and Weatherby cartridges with less powder capacity Those are not factory cartridges deficient in pressure.

That ought to make for an interesting magazine geometry problem for stacking and feeding!

Retraction - After reading some of the varied posts and watching the associated videos, the velocities obtained by my range games are NOT statistically relevant. My apologies to all for erroneously stating what I assumed to be correct data. Clearly, more testing (shooting....YAY!) is in order. When a more statistically correct sample has been achieved the information will be forwarded on. Until then, I will still comment on what each session reveals, but it will be more of a "one-off" than an absolute. Good shooting to all. Off to the range....

"That ought to make for an interesting magazine geometry problem for stacking and feeding!"

As Freddy said: "There can be only one!"

It's 2023 not 1923 !

staggered round detacheable mags with center feed eliminate the much squawked about "feeding issues" with the WSM cartridges

one must research current improvements instead of parroting ancient old wives tales

Y'know what else the Brits were good at ?

not bitching about recoil ..... just shot the fkn things and enjoyed it

unlike the current sissy trend in the USA where every mothers sob whines about recoil