The whole idea of achieveing even lug contact is to eliminate any lateral deflection of the bolt under load. For the same reason, some designers favored the three lug system; they felt the tripod effect of a three lug layout added lateral stability. By the way, the same claim has been made for the angled lug seats found in the Enfield rifles.
Like so many variables, differences in performance may be hard to measure but, when stiving for ultimate precision in building or shooting, small differences can be huge. There is little doubt that many of the precision steps which might pay dividends at BR levels would be largely unnoticable in the hunting field.
I think it is safe to say, however, there is a definite difference in performance, at benchrest levels of accuracy, between precision actions and those which are not so well built.
The concept would be relatively easy to test by using a Savage target action. The bolt heads are cheap and it would be a simple matter to face one lug back by five thou or so. No, the bolt heads don't float enough to compensate for this much error. I once laid out a protocol to test this as well as misaligned threads and other errors one might find in an action but have never had the time, money, or inclination to follow through. Perhaps I should do this and include it in the book.
I have never deliberately built a rifle with one lug contacting. I have worked on rifles which had one lug contacting and which shot poorly. After the remedial work, both lugs were contacting and the rifle shot better. The thing is, the barrels had also been recrowned, the actions bedded,and, in some cases, the barrels had been set back and rechambered so the effect of even lug bearing was lost in the shuffle.
The only evidence I have for the efficacy of lapping lugs for full contact is anecdotal evidence but from sources which I respected. GD
