A military rifle has to work, reliably, every time. That includes in hot, wet, cold, dirty, nasty, dusty, or whatever, conditions. The designers spec the chambers to be oversized enough to feed and fire in all these conditions, along with ammo that is produced by a number of different manufacturers, from several places on the globe. They don't reload, so brass life isn't a concern. It just has to go bang, every time. As far as wildcatting goes, your concerns should be twofold. First, is it possible? Select a factory chambered barrel with the bore dia. that you want to use, having a chamber that will clean up completely when reamed to your specifications. Or be ready to spend the bucks for a rebore and rifling job. Then decide if you want to do a recreation of an existing caliber, either recent or obsolete, or if you want to go with a true wildcat, with it's own set of specs. Then calculate the breech thrust on the single shot frame you will be using. If it's a true wildcat, you can do whatever you want, but if it's a duplication of anything that factory ammo is available for, and that ammo is even available, make sure that ammo doesn't exceed allowable breech thrust in factory loads. If the barrel or gun ever passes to someone else's hand, you don't want to be responsible for blowing up a firearm. And most of all, be sure that the barrel is clearly marked, and that the factory chambering nomenclature is removed.
