I assume you mean "throat length" (which affects where you seat a bullet) and not "headspace" (which affects how the brass fits the chamber).
For your M4 and your Mini 14, load to a length that fits your magazines. You can either use the industry standard 2.26", or you can try a longer length so long as you reliably fit *your* magazines.
I would suggest at least trying a few magazine-length 69s from your two semiautos. They might shoot well enough to make it worthwhile using 69s for all your .223s.
Also, if your Stevens arrives before your Sierras, try the Nosler 60 BTs in it - I would expect them to shoot very well, out to at least 600 yards.
If you have several different brands of brass, you could choose to use one brand for the boltgun and the other brands for your semis. Commercial Winchester is quite good, it would be a good choice for the bolt gun. (and use disposable brass for your Mini 14, since it does such a good job of "disposing" of brass - better than thirty feet away, always into grass, every time I've fired one! ;-)