I saw on BookFace that Bloke on the Range just won an auction for a BMS CAM rifle. A bolt action rifle in production since 1980 that uses an AR15 bolt, barrel and magazine. I have seen UK legal "AR15" rifles without gas systems that have a side charging handle.
This is my question for the collective. If you wanted to build a straight pull rifle using primarily AR parts for 5.56 or 7.62 based calibers, is there any need for a recoil spring or even the rear half of the bolt carrier? You need part of it to cock the hammer when cycling the rifle, but is anything else actually necessary? Since there is no gas system and running the BCG forward cams the bolt to a locked position, why include them? It seems to me that some interesting precision rifles could be made relatively cheaply.