▁defect ▁in ▁deep ▁hole ▁dr illing ▁which ▁can ▁often ▁cause ▁the ▁dr ill ▁to ▁break . ▁A ▁special ▁cool ant ▁is ▁usually ▁used ▁to ▁aid ▁in ▁this ▁type ▁of ▁dr illing . ▁ ▁Gun ▁dr illing ▁▁ ▁Gun ▁dr illing ▁was ▁originally ▁developed ▁to ▁dr ill ▁out ▁gun ▁bar rel s ▁and ▁is ▁used ▁commonly ▁for ▁dr illing ▁smaller ▁diameter ▁deep ▁holes . ▁ ▁The ▁depth - to - d iam eter ▁ratio ▁can ▁be ▁even ▁greater ▁than ▁ 3 0 0 : 1 . ▁ ▁The ▁key ▁feature ▁of ▁gun ▁dr illing ▁is ▁that ▁the ▁bits ▁are ▁self - centering ; ▁this ▁is ▁what ▁allows ▁for ▁such ▁deep ▁accurate ▁holes . ▁ ▁The ▁bits ▁use ▁a ▁rot ary ▁motion ▁similar ▁to ▁a ▁tw ist ▁dr ill ; ▁however , ▁the ▁bits ▁are ▁designed ▁with ▁bearing ▁p ads ▁that ▁slide ▁along ▁the ▁surface ▁of ▁the ▁hole ▁keeping ▁the ▁dr ill ▁bit ▁on ▁center . ▁ ▁Gun ▁dr illing ▁is ▁usually ▁done ▁at ▁high ▁spe eds ▁and ▁low ▁feed ▁rates . ▁ ▁T rep anning ▁ ▁T rep anning ▁is ▁commonly ▁used ▁for ▁creating ▁larger ▁diameter ▁holes ▁( up ▁to ▁) ▁where ▁a ▁standard ▁dr ill ▁bit ▁is ▁not ▁feas ible ▁or ▁econom ical . ▁ ▁T rep anning ▁removes ▁the ▁desired ▁diameter ▁by ▁cutting ▁out ▁a ▁solid ▁disk ▁similar ▁to ▁the ▁work ings ▁of ▁a ▁draft ing ▁comp ass . ▁ ▁T rep anning ▁is ▁performed ▁on ▁flat ▁products ▁such ▁as ▁sheet ▁metal , ▁gran ite ▁( cur ling ▁stone ), ▁pl ates