I've built quite few rifles on quite a few different actions and can only say some are more appropriate than others for different tasks. A properly built Mauser 98 based rifle (I would include commercial Mauser actions here too) is unbeatable as a serious field rifle. They are simple and well designed. The parts are robust and well thought out. A good Mauser will feed, fire, extract and eject every time. In the unlikely event something breaks, they can be stripped and repaired with a minimum of tools. You can replace the firing pin on your Mauser in the field with no tools. For field use, I even like the military two stage trigger just fine. Mine are set up so the second stage is crisp and breaks at 3lb with minimal overtravel. Just fine for hunting and it's unlikely to ever fail for any reason.
For an accuracy rifle, the Mauser can make a pretty good varminter but it just isn't really the choice for a match rifle. The Pre-64 M70 can work quite well within limitations. I have made some fairly good shooting rifles based on pre-war actions but I don't know that I could make a 1/4 minute rifle. I have not tried to build a pre-war in 6PPC but I have built some 308s. The best was a 1/2 minute rifle on it's best days. In 6.5x55 the same rifle is a 5/8 moa performer at 300 meters. 10 shot groups are usually just under 2". This action is as perfect as I can make it. The threads have been recut. The locking lug seats machined and the receiver faced. The bolt has been faced and the locking lugs trued. I have retro-fitted "bumps" to take the slop out of the rear. By the way, when I say a rifle is a 1/2 minute rifle, it's a REAL 1/2 minute rifle. Not one of those which shoots a 1/2" group anytime the full moon falls on a Tuesday.
A 1965 model (the lowest of the low!) in 6BR is an honest 3/8 moa rifle. 10 shot groups at 300 meters will be right around an inch to an inch and a quarter. This likely has less to do with the action and more to do with the fact that a 6 BR is easier to shoot.
Another M70 push feed is in 308 and will also shoot 3/8. However, a 308 is just plain harder to shoot well; at least for me. Anyone who has shot a full 308 in hunter class will know how easy it is to suddenly shoot an eight for no apparent reason. This rifle is like that. I have shot numerous sub-.3 groups (5 shot)and followed them up with one of .7". Maybe it's just me.
My point is that Winchester M70s of any vintage are capable actions and can be pretty darn good but they are not accuracy actions; not even with considerable modification.
The Remington 700, like it or not, works better. A well built 700 in the proper configuration can be a 1/4 minute rifle. If things go well, it can be even better. This goes doubly for the 40X. It is still possible to build a rifle on a 40X action and get it to shoot under .2. Not in stock condition, perhaps, but with typical truing and tuning it can. This if it is barreled in a suitable caliber, or course. I have a 40X based rifle I built strictly for 300 meter "F" class. This one is a sleeved (stainless sleeve) 40XBR action with a home made bolt. It is barreled with an old Clyde Hart barrel (14 twist) chambered in 308. I have fired 5-5 shot groups at 100 yds and agg'd right at 1/4" with Sierra bullets (very nice conditions, mind you). It is glued into the stock and is really kind of a 1980 style cruiserweight rifle (about 16.5 pounds). I'm quite sure I could get this one to agg around .2 with better bullets and a little effort.
I think the Savage single shot actions have the potential to work nearly as well, maybe just as well, as the 40X actions but in either case, modern custom actions will work much more easily and probably better. This wasn't always the case.
Back in the Seventies custom actions were not always any better than commercial ones. Sometimes they weren't as good. The worst action I ever had in my hands was a custom action. A Shilen DGA, this thing was so crooked, it was impossible to barrel. With the barrel screwed in to contact the receiver, I could stick a .006" feeler guage between the barrel and receiver on one side. To make matters worse, the recoil lug was .0007" thicker on one side. No 40X was this bad! Today's customs are another matter. Nobody dares to make a crooked action and there are more good actions available than ever before. Some, like the Stolles, Stillers, Bats, Farleys and so on, are well known to BR shooters. Some others like the Barnard (New Zealand) and RPA (Great Britain), are not so well known but are of equal quality. The only reason to use a Remington or other commercial action is to say you could and that's not a real good reason!
I still use commercial actions and even reworked military actions but this is more an indication of my obstinance than it is the suitability of these actions. Although I have high hopes for my rebuilt CIL950T, I would have undoubtedly been better off with the Barnard I'm using for a customer's rifle. Regards, Bill.