In my experience with belted magnums (264WM, 7RM, 300WM), the base to shoulder on virgin brass is VERY short. My 264WM grew over 25 thousandths on first firing with Winchester brass. My ADG 7 rem mag brass grew 15 thousandths if I remember correctly. These were both in minimum head spaced X-caliber Savage prefits.
With that much stretch happening, it might be good idea to "fire form" with a jammed bullet for the first firing. I did this using some relatively cheap bullets and a mid-level load. One firing got the brass near enough to its full size, with almost imperceptible growth after the second firing with a "normal" load.
If I were you, I'd at the very least check the growth. Measure a virgin piece of brass from base to shoulder, then fire a couple and measure. If the growth is extreme (>0.010") it might be worth using the jammed bullet method, which forces the case head to rest against the bolt face. Otherwise, the brass will stretch backwards and forwards, instead of just forward, potentially thinning the case walls at the web. If the rifles are good shooters, you'll likely get good accuracy out of the fire forming loads, so use them to plink around, get your scope close, break in the barrel, and just have fun.
After the first firing, you'll be able to adjust your die so that you just bump the shoulder back 0.002" or so, and subsequent firing cycles will result in FAR less case stretching, and therefore considerably longer brass life. A small shoulder bump is also probably better for accuracy and precision.
