Candidates for the longest 8 man checkmates? Are there proven 8 men checkmates that have a good chance to be the longest checkmates? How were they proven and why are they good candidates?
One might try adding an extra piece to the well-known mate in 549, thereby adding at least a move or two. But although these may be plausible to Stockfish, these are not proven. In other words, they are not 100% exhaustively verified by a tablebase engine.
In that sense, the longest proven sequence I know of is a mere 64 moves long. This was given by Guy Haworth in his 112 page online PDF about the longest tablebase checkmates.
[Title &quot;KBBBB-KRR, 1-0, 2014/2/27, 8m, 5-3m, KBBBBKRR= KB4KR2, GBR 0690.00_40/22: SV+C,-/SV+C, +, Annotations By Guy Haworth&quot;]
[FEN &quot;6r1/8/B4rB1/8/1B6/5k2/7B/K7 w - - 0 1&quot;]

1. Bh5+ Ke3 2. Bbd6 Rd8 3. Bc5+ Kd2 4. Bae2 Re6 5. Beg4 Rc6 6. Bf4+ Kc3 7. Be5+ Kd2 8. Bcd4 Rd5 9. Bb2 Rb6 10. Bbc3+ Ke3 11. Bf7 Rd8 12. Ba2 Rb5 13. Bf6 Rd6 14. Be7 Ra6 15. Bcb4 Rb8 16. Bbc5+ Kd3 17. Bf3 Kc2 18. Ba3 Kc3 19. Bad6 Rbb6 20. Bc5 Rb5 21. Bh1 Rb8 22. Bed6 Rb5 23. Be4 Raa5 24. Ba3 Kd4 25. Bf3 Ra7 26. Bab4 Ra4 27. Be1 Kd3 28. Bd1 Ra7 29. Beb4 Rab7 30. Ba3 Kc3 31. Be2 Ra5 32. Bf3 Rbb5 33.  Be7 Ra7 34. Bfd5 Raa5 35. Bb2+ Kc2 36. Be4+ Kd2 37. Bc6 Rb8 38. Bd5 Kc2 39. Bba3 Kc3 40. Bed6 Rbb5 41. Be5+ Kd2 42. Bf4+ Kc3 43. Bb2+ Kc2 44. Be4+ Kd1 45. Kb1 Rb4 46. Bc2+ Ke2 47. Bd6 Rh4 48. Bba3 Rb5+ 49. Bab3 Rh3 50. Bd1+ Kf1 51. Bdb4  Rh4 52. Be7 Rc4 53. Ka1 Rc8 54. Bg4 Rc7 55. Bge6 Kf2 56. Bbc4 Rh5 57. Bd8 Rh1+  58. Kb2 Rg7 59. Bc5+ Kg3 60. Bd6+ Kf2 61. Bb6+ Ke1 62. Kc1 Rg2 63. Bb4+ Rd2 64. Bxd2#