You can figure on a reduction of somewhere between 180 to 200 fps with the 7mm Rem. Mag. fired from a 500mm barrel.
Button rifling causes the metal in the bore to displace around the die and is usually much shallower than conventional cut rifling.
Over time I've used slow burning powders in my .240 Weatherby, but always with a Magnum primer.
All .257 Magnum rifles built before 1973 had a rifling twist of 1:12" which is too slow to stabilise spitzer bullets much longer than those weighing 100 grains.
Your old load of 62.5gn of Re-19 would have driven the 90gn Sierra H.P at about 3500fps
It not only balances nicely and swings smoothly, but has plenty of accuracy and knockdown power.
The internet shows some astounding velocities for the .303 Epps, so I've decided to have a rifle built in this calibre.
62.5gn of Re-19 would have driven the 90gn Sierra H.P at about 3500fps.
If I were you, I'd try within two grains of your regular powder charge with the Woodleigh 225gn bullet.
. I've used this .25-20 load on goats out to 150 metres with excellent results.
When fired in this rear-locking rifle, cases will not withstand as many full power reloads as .308 cases fired in a bolt-action rifle.
Its sectional density of .250 compares closely with those of the following calibres and weights: (see article)
Neither neck reaming or turning will be necessary. Cases will not need fireforming,