According to BE EN 1090, the clearance hole for an M12 bolt is 13 mm. If this diameter hole is used, then the shear resistance may be calculated in the normal way, without any additional factors.
This value of shear resistance appears in the online Steel for Life version of the Blue book.
Clause 3.6.1(5) of BS EN 1993-1-8 allows M12 bolts to be used in 14 mm holes (i.e. slightly oversize), but applying a factor of 0.85 to the quoted resistance. This factor was applied in the paper versions of the Blue Book (P363) and the ArcelorMittal Orange Book resource. It is clearly conservative to apply the 0.85 factor, though the reduction is unnecessary if M12 bolts are used in 13 mm holes.
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