No expert, but here is my story ... I have a set of Lyman dies that I have had for more than 25 years ... At that time I owned 1 AR ( A Colt). I reloaded for it many, many years back and quit .. the availability of "cheap" .223 (Wolf, Brown Bear, etc..) made it impractical for me to reload .... I have since acquired several Bushmasters and another Colt, and the "cheap" stuff is no longer "cheap", so I decided to get back into reloading for the .223. Pulled out the old Lyman Full-length sizing die and went to work. While all rounds chambered and shot just fine, I did notice one problem. If I wanted to extract a live-chamber round, it was VERY difficult in a couple of the ARs .. in others it was not problem. I did my research on and ask a bunch of questions on other gun boards -- seems that everyone came back recommending that I switch to a small base sizing die. I was told that the small base die actually sets the case shoulder back a "hair" ... I purchased a set of RCBS small base dies and my problem is no longer a problem. My ammo functions fine in all the ARs and in the Mini-14.
I have since purchased a set of small base in .308 and 30-06. I frequently use the .308 (M1A) with issue ... have not yet loaded any 30-06 (Still shooting cheap ammo!!)
Captain, A small base body die does set the shoulder back at the same time it it decreases the outside case dimensions just a hair more than a FL size die will.
A shoulder bump back die sets the shoulder back to remove the dreaded click you get with bolt actions without reducing body taper.
I would not use a small body die on for bolt action rifle unless I was actually measuring the amount you are bumping shoulders back. Otherwise you can overwork your brass and can eventually experience a case head separation. Even measuring the shoulder you are probably over working your brass if using a small body die in a bolt action.
I know I have to use one to get once fired brass to go into my match chambers, after that I usually neck size only until the brass grows tight. Then I use my bump back to move the shoulder back until I have a slightly snug fit chambering the brass with the striker removed.