I assume you mean a 300 Win mag necked down to .284. Jarrett Custom Rifles has such a chambering and calls it the .284 Jarrett, as bellow from Jarrett's web site: A very interesting round.
284 Jarrett
140 gr. Nosler Bullet...........3450 to 3500 fps
150 gr. Nosler Bullet...........3350 to 3375 fps
160 gr. Nosler Bullet...........3250 to 3300 fps
We are amazed that the 300 Winchester Magnum had never been reduced to 7mm with enough success to be well known. The wheels really began to turn wondering what the exact volume of the case would be. We designed a different case taper than we’ve ever used, our traditional 35 degree shoulder along with a lead angle in the throat to accommodate 140 and 160 grain bullets. We had a reamer made and proceeded to build the .284 Jarrett! After the case fire forming was completed, we did analysis of comparisons to the other big sevens to see where ours fit in.
We checked all of the cased by water volume and this is what we came up with: (Volume to base of neck)
7 mm Remington Magnum ..........73.5 gr.
7 mm Weatherby Magnum ..........78.0 gr.
.284 Jarrett ...............................85.5 gr.
7 STW ......................................98.3 gr.
It’s interesting to note where we were volume wise, 12 grains greater than the Remington Magnum and 13 grains less than the 7 STW.
The next step was to shoot this thing. Foremost in our interest was how accurate it was. If it’s not accurate, the velocity doesn’t matter. The cartridge responded well to a variety of powders and without effort we were under our half minute accuracy with velocities we thought should be correct for this case capacity.
http://www.jarrettrifles.com/petcalibers.aspx
