The feeler holes , same size as holes for lug pins , finish nails ...and no regular space is necessary ...I usual drill about four and just wing it , just kinda space em.out evenly ... Its great to kerp track of that darn drill bit . make sure it dont go down ... When slabbing off the side , lock side ... The measurement was like .300 and something ...I used the calipers and uped it a tad to .400 and scribed it the length of the lock along the barrel , then used ruler to extend that line the length of the lock , already marked and a bit extra .... Now , down the barrel , lock forward its just haphazard no measurement needed ....I just used marker , about same distance as lock mark , and took it to the Muzzle ....when cutting I just keep saw blade out side of that marker line , when at lock mark I do try to be a bit more straight and accurate , just on outside of the line .... it leaves a tad extra ....take that extra off later ...