DELETE
This is not 100% what you want, but close. Certainly not cheap…
http://www.davidtubb.com/tcom_images/t2k_images/t2k_tactical.html
do side-charging AR uppers count?
I have to ask - WHY?
this is something I have thought about and have a rough design for it. Several reasons for it. some people just love the AR15 and the ergonomics of the platform. they are familar with it and comfortable with it. trying to sqeeze every bit of accuracy out of it. while some would like it in ban states, that does not matter to me. Bolt action with folding stock and high capacity.
it would use off the shelf parts. the upper receiver may or may not be modified depends on if you go with a recipricating bolt or with a American Spirit arms receiver. the carrier would need modifcitions to allow for a folding stock.
I asked if there was a bolt action upper, for an AR-15 lower, on another forum. They thought I was crazy too.
If you already have an AR lower with a good trigger and an adjustable stock, it would make sense, at least to me it would. It seems like it should be cheaper to put together a .308 bolt-action upper than it would be to put together an ergonomic Savage rifle.
take a look at a company called bohica. i know they make bolt action uppers for AR-15’s. not sure if they are all single shot or if some have mags tho.
OP asked for a .308 upper. Bohica only does .50BMG and .416Barrett.