Sir Scotty, I started to do that to my standard SAAMI .458 WinMag M70 but since it is a later South Carolina/FN-Winchester, my Connecticut Classic mag box is a different fit. The mail-order parts at Midwest Gun Works out of Missouri are for the M70 Classic, not the SC/FN M70. I would have to rob another SC/FN .375 H&H rifle to make the switch. There are all sorts of possibilities ...
I will illustrate the SC/FN mag boxes and followers for .30-06, .338 WM, .375 H&H, and .458 WM ... to be continued. I am most comfortable with recommending a switcheroo on the Connecticut Classic M70, having done that myself, a .270 WCF 3.4" to .400 Whelen 3.6" which was as slick as your conversions to long box .300 WM and .338 WM.
But I take heart in this:
My SC/FN .458 WinMag has an inside box length of at least 3.410" which easily handles my 3.380" COL with 404-gr Shock Hammer, and 450-gr CEB Safari Solid at same 3.380" COL.
The unaltered rifle will also eject a single-loaded, unfired round with COL of 3.540". Easily ejects 3.480" COL with the 404-gr Shock Hammer. No change in ejector/boltstop required. No milling on the receiver required. No magazine box switcheroo required.
Having a +3.6" mag box is desirable but not necessary. The factory +3.4" box will handle a magazine feed of 3.380" ammo, no problemo.
Loaded with WW/Hornady brass the 450-gr CEB/3.380" gives +2350 fps in a 24" barrel. Loaded with Norma brass, the 404-gr Shock Hammer at 3.380" COL gives + 2500 fps, equivalent to the preferred 3.480" load of 404-gr SH in the WW/Hornady brass.
The 404-gr Shock Hammer may be loaded to 3.480" COL with Norma brass, and that will be better than trying to load it to 3.580" in WW/Hornady brass. Not enough bullet remains in case for 3.580" COL with the 404-gr Shock Hammer.
Makes sense RC. Mines a Classic. I haven’t outgrew the 3.4” power level yet…. I might not ever
