I’ve beaten GR 105 with UE4M4, and recently a GR110 with CC3A3M4 in like 13 min. (keep in mind, that last build is about 14 GRs weaker than M6 Mulitshot and N6M4 Multishot), but it proves how incredible S2CC3M4 could be if they ever restored it.
For M6 Multishot, i play it with Bombardiers in offhand and DML in the cube:
5 Sentry M6 Multishot
50% Uptime on Vengeance = 1.2x
DML = 1.66x
Total: 2X
On Bombardier, reroll Sentry% to Multishot%
3 Sentry M6 Multishot
100% uptime on Vengeance = 1.4x
Total: 1.4X
You get faster damage ramp up with 3-sentry but you get an extra affix with 5-Sentry Bombardier.
For N6M4 Multishot, I use DML in the offhand and Yangs in the cube.