It's a pretty well documented phenomena that any accuracy-minded auto rifle smith would know about. I guarantee Holliger & Frank White carefully control for this when they do a barrel. But when factories do them, all they do is screw on an extension and run a reamer in until a bolt closes on a Go.
Bigger rifles are more susceptible to it due to the increased mass of their bolts.
Pulling the bolt back and letting it fly (slingshotting) generates more energy than the automatic functioning of the rifle.
The OP can check this out with a Hornady Headspace gauge.
http://www.hornady.com/store/Lock-N-Load-Headspace-Kit-With-Body-1-Each/Use the gauge to measure the headspace length of a couple unfired cartridges. Next, slingshot them into the chamber and re-measure. Note the new numbers. Ideally, it shouldn't change or be more than a thou, maybe 2, different. You gotta have some clearance in the fit to allow for function and this is one reason why automatics will never equal bolt actions in accuracy.
Then do the same thing with a few more cartridges, only this time feed them out of a magazine by firing a round to chamber them. Don't shoot them, take them out and measure them, comparing the numbers to the new, unchambered and the slingshot fed.
It probably won't do a whole lot of good to single load every cartridge when shooting for groups. The loose headspace produced by single loading doesn't help accuracy.
