Hey bwinters …I think we have changed considerably over the last 35 years with regards to what we need in a hunting rifle.I dont recall hearing much about lighter rifles or long range hunting back in the 1980's where the 243/303/3006/270/308/7x57 went about its business for most of the hunters back home in Africa.

The new pioneers came along ... Lee/Brown/Jarret and Forbes with a host of others following in their wake and a whole new candy store opened in town. This enabled us to buy and build rifles for every conceivable hunting scenario. … and we did. What it also did was offer the carrot of a multitude of option to be added to one rifle, including calibers new or revived… in my case, the more I can add into one rifle the fewer I need. Being a minimalist by nature and a hunter is a very painful and frustrating experience for me going on many years now!

If we talk about lightweight/moderate recoil/flat shooter in the same rifle, there is a few considerations:

1)Going lightweight in a long action cartridge means a lot more recoil, you want “lighter rifles that recoil less” ..why? your getting older .. and you will keep getting older which means within a few years from now that rifle will be less tolerable for bwinters and your back asking the same questions. Only option is to be a step ahead of yourself and go with short or even better ..an intermediate cartridge on an intermediate action size… no difference in recoil but usually a tad flatter/faster than a short action brother or sister, and you can build as light as you want with it.
2) Flat shooting cartridges, yes .. if you want 6” drop at mesmerizing distances .. you will be up for shoulder surgery every year. Within the recoil boundary you specify, it seems to me your wanting to hunt up to 350 yards with this rifle
3) Calibers options: The golden thread is 6.5/7mm. Since your initial thoughts pointed to long actions, I’m assuming you want this rifle for medium to larger game. 6mm is limiting and getting 30 cal’s to shoot flat’far results in either compromise on BC or increased recoil.

Intermediate calibers to consider:

6.5x55 (modern loads impress)
7x57, better even 275 Rigby for the cool factor (reloads improve on the 708)
284 Win. (Melvin Forbes knows a thing or 2)
6.5x284 (another star on the 284 case)