halfbreed: if you want to reload for that 8mm FN49, find yourself some new factory ammo, like the S&B with the 196 gn bullet, see how she shoots with that ammo, and then reload to those specs.
The military semi-autos were designed to handle mil-spec loads and I believe the pressures on the 8mm were/are similar to those on the mil-spec 06 and 308 ammo, somewhere around 46-50k psi. These are not hot loads - velocities with the nominal 147-150 gn 308/30-06 loads were somewhere around 2700'/sec. Match/sniper loads using a 174/177 gn slug in the 308 were 2550'/sec and in the 30-06 with the same weight bullet at 2650'/sec. That would probably put the 8mm with a 196 gn load somewhere around 2450-2500'/sec. Some of the best mil-surp 8mm I have encountered is the Portugese 8mm with a 196 gn bullet.
I would look to european loading data for that cartridge. American ammo makers have traditionally loaded down the european calibers in favor of the latest American craze and have not treated european calibers well at all. Literally the best ammo for the euro calibers that has come out in recent years is the Sellier and Bellot - all made to original mil-spec for those calibers which sets them up right. The only way I would opt for a 308 over an 8mm is if I couldn't get good 8mm ammo and even then I wouldn't bother with the 308, I would find something else.
Using Speer data, you can load the 8mm 200 gn spitzer soft-point to 2469 with 52 gn of H380, which is a max load. But, 46 gn of IMR 4064 gets you 2434'/sec and 53 gns - compressed- of IMR 4350 gets you 2432'/sec. All these loads should make that FN function fine. Speer also lists a reduced load in 4198 with 28 gns getting to 1726'/sec but I don't know if that would function your piece. Most folk I know who relaod for semi-autos, including myself, prefer the faster burning powders for the semi-autos. You can load the slower powders but you run the risk of not having enough pressure to function the action (eg, some of the Hornaday Light Magnums rounds are not recommended for semi-autos because they use large charges of slower buring powders that may not reach a pressure peak in enough time (barrel length) to function the action). You may wish to see if Steve's pages list loads for the 8mm that would work for you - that man has an incredible inventory of reloading data.
I would also avoid cast bullets, even gas-checked, to keep from fouling your gas port.
I see one VihtaVuori Oy load for the 200 gn jacketed bullet using their N140 powder that gives 2530'/sec at 49.3K psi; and, two 175 gn loads using the same N140 powder with 46.6 gns giving 2580'/sec at 49.3k psi and 49.7 gns giving 2670'/sec also at 49.3k psi.
HTH and good luck. Let us know whatcha come up with. Mikey.
