WAY WAY underloaded or something?

The surplus stuff I have seems to kick a lot harder than the new hunting ammo.

The answer to your question is yes.  There are two versions of the ammo.  When 8mm originally came out, it had a .318 bore loaded for a weaker Mauser action.  Around 1904-1905, this caliber was beefed up to a higher pressure and bore diameter increased to .323.  Commercial ammo is loaded weak because a few weak .318 "J" bore 1888 Mausers are still floating around.  If you had one of these and fired the larger and higher pressure "JS" cartridge in it, things could get really exciting.  Basically it's about liability.  The pressure difference is 37,000 cup vs. 50,000 cup.  

P.S.  I remember Sportsman's Guide advertised 1888 Contract 8mm Mausers.  Those would be the ones to look out for.  Don't fire high pressure European 8mm ammo in those rifles. Your Kar 98K, Egyptian FN-49, and Turk 98's are fine though.

Sellier & Bellot prints a warning on their 8x57 JS ammo stating the ammo is for use only in "S" caliber bores (7.89 mm). I don't know if that means it's loaded full power or not.

American 8MM ammo is underpowered for the reasons described by Wadman.  Some European 8MM ammo is not.  Supposedly Norma (Swedish made) 8MM approximates a 30-06 in performance.  However, I suspect it is both hard to find and pricey.  

The ammo I got for my 8mm mauser is real cheap, uses corrosive primers, but it kicks like a fucking horse!

The S&B 8x57 JS (green box -- 196 gr. FMJ #2910) appears to produce 2588 fps and 2870 ft/lbs. This is after doing the conversion from mps and joules. It looks about equal to 30-06.

Georgia Arms has just started producing a load with a Ballistic Tip in it that I am anxious to try in my M-48A.

JBR, is that the Turk, Yugo, or the Ecudorian FNM? I heard the Ecudorian is good so I bought a case from ammoman for $115 delivered.

The most recent stuff I've tested is 50's vintage Turk 8mm.  Seems like everyone's selling this stuff.  

About the only non-corrosive surplus 8mm Mauser I've ever seen or used was Portugese FNM.  Very good ammo and expensive (approx. $60/400 rnds).  

Chronograph results for the Turk ammo averaged 2860 fps from a 24.5" Yugo Mauser.  This result combined with less recoil than I remember lead me to dismantle a round.  Instead of the usual 198gr spitzer bullet, The Turks loaded it with a 150gr bullet.  Either way, 2860 fps with a 150gr FMJ is cooking.  Wonder what this stuff does in a 29" Gewehr 98.

I would consider all European made 8mm to be the full pressure .323.

I've been loading my own 8mm for a couple of years,I like the 8mm, it's a really good cartridge, It's a shame that there's not a good selection of bullet weights for this caliber,No doubt in my mind that the 8mm will do anything that a 30-06 will do, Yeah, you won't get the full effect of the 8mm round until you get some good surplus or some european commerical stuff, the commerical stuff here is a little on the lame side,but at least you can reload the brass....