1. 500 A2. If you start with a CZ550 in 416 Rigby, it is quite easy for a good gunsmith to get it to feed .500 A2 with a very little rail and ramp work. Also, he will have to tighten the extractor for you and install a recoil lug on the underside of the barrel. the CZ550 also needs a new safety (model 70 style) or else the safety can inavertently be pushed to the ON position while working the bolt. And frankly, it could use a more attractive bolt handle and some new sights too.

2. 577 TRex. This has a gigantic rim (0.580"?? from memory) and therefore needs either at least a 0.750" bolt body diameter, or a P14 bolt which has an extended left lug to hold the rim. You could use an Enfield P14 action as A-Square does, but the gunsmithing is so expensive that it is not worth it. Or you could use a Granite Mountain Arms magnum Mauser action, which will work. The cheapest route would be to buy an A-Square rifle for about $3500, install a new stock, new sights, model 70 safety. As a prior .585 owner, I recommend that you go with the 500A2 project at is is more affordable, brass is readily available and good quality, the gun goes not need to be as heavy, and the recoil will not be too overwhelming to hunt with. But if you really want the biggest and baddest, get a .600 Overkill.

3. .600 Overkill. This round pushes a 900 grain 0.620" bullet at 2400 fps and is based on a CZ550 action. You can get one from www.hunting-rifles.com for only $3500 which is a genuine bargain. But, beware of the recoil.