I have an FN comercial action....what is the biggest caliber I can use...urdubob

A .458 Lott will fit with some massaging of the feed ramp. That would be the maximum overall length of any cartridge you could fit in that action I believe.

As far as I know, for a heavily modified, standard M98 type Mauser action, I would think the .500 Jeffery for a factory round, .585 Nyati for a wildcat.

how about a 404 jeffery?

If you have a FN commercial magnum action, you should not use it for a cartridge larger than the .375. The action lower locking lugs may have had too much metal removed in the magnum version and there is a documented history of some of them failing with high pressure rounds. Roy Weatherby tried to use some of these actions early in his career and there were many problems. With that said, I see no reason you could not build a .404 jeff or 9.3X64 on one. I'd certainly keep away from a .458 Lott/Watt/Ackley and don't even consider a 585 Nyati for even a moment.-Rob

Actually, where you get in trouble with the FN actions is with the high intensity rounds like the 7 STW and its ilk.....thats a real no no..I have discussed this problem with Pete Grissel, Jack Belk, and most of todays top gunnysmiths and this seems to be the general consenses.

The 404 and even the 458 Lott work fine as does the 375, but these are all milder rounds...They also require that they be opened as much as possible in the rear and just a tad in the front and the addition of a proper drop box for the caliber from Ted Blackburn is very desirable,if not manditory for function....Same applies to the Model 98 Mauser.