I used to have a.300WeathMKV 26"bl. The loads I used were 180gnBT 81-83gn IMR4831. What sort of pressures and velocity do you think I was achieving?

I'll bite...but I think you have a loaded question for some odd reason. Have you at least Chrono'd the load?

Take into effect that lots of powder can easily vary 5% so there are only guesses here unless you have more concrete proof....

But my guesses...81...3180 fps...60,000k psi
82 3215 fps...63,000 psi
83 3249 fps 68,000psi.

Don't ask me how I got my numbers....just a mix/match of Quick load and personal experience with the wby. BTW--rem brass or anything besides norma or wby brass blows these numbers upward quickly.

Impossible to say with any sort of accuracy. Primers matter and so does powder lot and so does the cut of your barrel and chamber. As kraky indicated, an fps reading would have narrowed the range of uncertainty.

That said, the A-Square manual has pressure tested .300 Wea data for the 180 BT in a 26 barrel, with IMR-4831. Your loads are way beyond their max of 78 gn, so extrapolation is required. In their pressure barrel with their lot of IMR-4831 over their lot of CCI-200 primers in their lot of A-Square brass, your 83 gn load would have run about 65,000 CUP, well beyond a SAAMI max load and about 2/3 the way to a proof load. True pressure would have been over 80,000 psi. The fps would have been around 3370 if those pressures were attained.

I used Weatherby brass,Federal Magnum primers.
And if I remember correctly,Primer pockets were beginning to come loose after about 3 loads(83gn charge) from new brass. I didnt buy the Weatherby for it to be a flameless dragon. And flame it did. When spotlighting hog on the wheatfields,the flash was impressive, So was its effect on game.

Add about upwards of 10% to the pressure for those primers. It should qualify as a proof load.