The product was packaged and shipped in such a way that flat spots on the drill may occur. There are two flat areas in the bit, either due to shipping in horrible packaging or poor design. 155mm also measures 6 1/8", which isn't even 6 1/16". When drilling such a large hole it is very important to have a round drill bit because of the torque that can be exerted by the drill bit if it gets stuck on something. Terrible product even for the price and I won't even test it on drywall because of the quality. Not to mention tubeless lights require a 6 1/8 inch size so I would prefer a 6 inch carbide which would probably give me the same hole size as this one as it won't fit. I will definitely come again