I have my 458 Whitworth worked on a while back and one of the changes was a deeper, 4 round mag.

Well, I never measured the new after market bottom metal or shot it yet (I'm getting the stock checkered), but inside length is 3.550" on the bottom metal and the bottom of my action is 3.6". Net, longer than the factory specs. So my gunsmith opened up the action to accommodate the aftermarket bottom metal. And I can seat out to 3.5+ inches with the Northfork bullets I have.

Question - anyone load the 458 WM out to 3.5" (or close) with 450-550 gr bullets? Any results?

My OAL right now is 3.340" w/Accurate 2230.

I'm getting 2,225 fps for 450 gr NF SS and FPS bullets. Very happy with this, but wondering what else I could do - either faster 450grs with a bit more OAL (need to maintain caliber diameter depth in case) or getting longer 480-550 gr bullets and seating to 3.450 or so.

I could also rechamber to a 458 Lott and open up the bottom metal inside length to 3.6" or so.

Early on in THE MISSION thread ("458 winchester magnum"), in this forum:

450-grain North Fork FP solid
Hornady brass
Federal GM215M primer (same as F-215)
COL 3.485"
24-7/8" CZ barrel (0.459" groove, 1:14" twist)
58*F

AA-2230 81.0 grains >>> 2411 fps MV

H4895 81.0 grains >>> 2382 fps MV

Accuracy Powder charges:

H4895 79.0 grains >>> 2335 fps <<< 0.52 MOA 3-shot

AA-2230 77.0 grains >>> 2348 fps <<< 0.69 MOA 3-shot

Search the above thread and you will find a bunch more.
I converted one .458 WIN Whitworth to .458 Lott out of ignorance of youth.
I converted another one to .458/.416 Ruger with Winchester Throat due to factory ringed chamber of the .458 WIN.
I still cry myself to sleep at night over both of those.

I converted a factory .458 Lott Ruger No. 1 to .450 NE 3.25" by rechambering and extractor work only.

I converted a factory CZ .458 WIN to .458 Lott and then had it re-barreled to .458 WIN with a Shilen barrel.

I traded in a factory CZ .458 Lott, after my enlightenment, because there are so many more rifles I druther have than a .458 Lott.

You cannot improve on perfection.


.458 Winchester Magnum Perfection
In maxima potentia parvum spatium.
Id venit et vicit omnis.

Thanks!!