Bronc,
here's what I wrote a couple of months back on this subject in an article on handloading the old Russian 7.62X54R military number in the December 2012 issue of the free to download magazine TargetShooter online (www.targetshooter.co.uk). While a rimmed cartridge, the same issues and solutions apply:
Headspace
There is another issue related to the rimmed set-up – case to chamber fit and adjusting the die in the press. What do I mean by that? Rimless cartridges ‘headspace on the shoulder’, or to put it into plain English, they must be a close fit lengthwise between the bolt-face and shoulder section of the chamber – just enough clearance for the cartridge to be chambered easily. In precision ammunition, we’re looking for a mere 0.001†to 0.002†clearance. Rimmed (and belted) ammunition is different. Headspace is determined by the fit of the rim (or belt) between the bolt-face and the front end of the rim / belt recess machined into the rear of the barrel / chamber. As the cartridge is securely located at its back end, this set-up allows the manufacturer to chamber the barrel with deliberately oversize dimensions up front where the top of the case-body and shoulder section lie giving a loose cartridge fit in this area. This offers advantages to military users and dangerous game hunters, principally 100% reliable chambering and ignition even when the barrel and its chamber are filthy, glowing red-hot from continuous firing, and/or the ammunition is dented, dirty, or out of specification wartime manufactured stuff, and so on. Where clearances exist, the case simply fireforms up to the chamber dimensions. This might be great for soldiers and lion hunters, but doesn’t do much for the target shooting handloader – resize that blown out case back down to something close to its original dimensions, and a massive amount of brass working takes place on each firing / resizing cycle doing nothing good to case life, or probably accuracy. One answer is neck sizing, but that’s not necessary if you set your FL die up properly.
As with rimless designs, it comes back to measuring and comparing fireformed against sized / unfired cases straight out of the die and adjusting the sizer die in the press to avoid pushing the shoulder back more than a few thou’. Unlike most of the .303 Enfields I’ve owned over the years, visual inspection of the cases and the amount of effort needed on my Redding T7 press-handle to full-length size them fooled me into thinking my M1891/30 was fairly tightly chambered. Nevertheless, in the interests of doing things properly in researching this feature, I thought I’d measure the ‘before’ and ‘after’ case-head to shoulder dimensions using a Hornady L-N-L comparator and headspace gauge insert. The results surprised me – fired cases measured 1.659†– 1.660â€, while after full-length sizing with the die set to the ‘default position’ (base in hard contact with the shellholder at full press operation) the reading changed to 1.622â€, 38 thou’ of shoulder set-back, far too much. The answer is to back the die out and reset it using the ‘headspace gauge’ (not an accurate term, but we don’t seem to have a better one) so that sizing only moves the shoulder a little, 0.003-0.005â€. It’s also a good idea to try a few sized cases in the rifle chamber to ensure they still chamber freely before undertaking the final loading steps. Another tip here is to decap the fired case before measuring it as any primer protrusion will give a false reading.
In the days when .300 Win Mag was a widely used long-range target cartridge, people sized them to give minimal or no shoulder to chamber clearance. This usually (invariably?) not only changed the headspacing method to that of a rimless type, but moved the belt fractionally off contact with the front of its recess. Reducing shoulder to chamber clearances to nil / a couple of thou' may not be a good idea in a hunting rifle however unless each resized case is tried out in the rifle chamber before charging and bullets seating. Even in hunting rifles, reducing clearances 'up front' to maybe 3 or 4 thou' is a good idea as it will give better accuracy and case-life.
The reason for the belt is widely misunderstood. Its original (and the correct purpose) as devised by Holland & Holland back in 1912 when they introduced it was to give the positive rear of the chamber headspacing that a rimmed case provides, but without the rim's disadvantage of potential jams in feeding from a magazine fed bolt-action rifle (very bad news if the quarry is dangerous and attempting to get at you / kill you after you've missed, or worse, wounded it with the first shot). The belted case feeds almost as well as a rimless example, but gives the rear-end lock-up and headspace. As noted, there are pluses too for people using belted ammo with lots of built-in clearances in dirty, humid, very, very hot etc etc tropical climates and other off the beaten track environments when the chances of a jam, stuck case etc were reduced.
The belt has nothing much to do with case strength as is usually thought - a well designed rimless case will be as strong as a belted one. (Although as noted, turning the belt off may end up making it weaker than an equivalent rimless design.)
Roy Weatherby created the belted magnum sporting cartridge fad that has only recently started to peter out. Having used belted H&H brass as the basis of most of his hot magnums, and all his early introductions, the market came to expect a belt on a really powerful cartridge. So, for decades, Remington and Winchester made all their top-performance magnums in this form even though there are no benefits and various downsides in using this form on European and North American non-dangerous game. That's not to knock the 300 Win Mag, 300 H&H Mag, 7mm Rem Mag and others. They're good cartridges, they work. Just be aware of the issue of the slack case to chamber top of the body / shoulders fit.
If you set up your sizing die correctly, you'll be head spacing off the shoulder.
you do your own chambering, you have a great opportunity to set your own head spacing. DON'T use the standard belted magnum go-gauge. Make or purchase one that measures .217 at the top of the belt, rather than the standard .220. This will put an end to the bulge above the belt. Start with virgin brass, fire it one time, then set up your sizing die (properly) as you would for a non-belted case. DON'T screw the die down to touch the shell holder. PROPERLY adjust the die to bump the shoulder back .001-.002. One of the good features of the belt is this John: even if you go too far with the chambering and end up with excessive head spacing, you have the safety factor of the case head spacing off the belt. One thing for certain is that like the .375 H&H, the 7MM Remington mag will be with us forever. Good choice on the cartridge 
