I use sandvik 880 (U) tipped drills with 2020 grade tips in 321 stainless and other 'difficult' to machine materials (including c22 and c276 hastelloy). They work very well. Through coolant is a must but you can achieve tip life of about 4metres to an edge. Of course this will all depend on...
I use titex drills for deep hole drilling. I drill 4mm dia holes to a depth of 120mm-150mm without fuss in duplex and other 'difficult' metals. Now superceded by EDM drilling. (although the size you want is a little bit big for EDM.) Gun drills are a good option at that size TBH. Although i got...