The BCY Mercury is very small diameter, so it takes 32 strands for a spun string (which makes for a very round cross section).  32 times around the posts is a bit much compared with 14 for B55.  The BCY 652 requires 20 strands, so it's definitely a fatter fiber, but a bit thinner than B55, for a more rounded string.  Once I set the brace height and nock point the Mercury string is as stable as kevlar fiber (well, minus the bow blowup, thank you very much!  ).  I'm liking the Mercury a lot and will spin one up for the Au Sable longbow, too.  Only problem is the color - I've got five 1/4lb spools in flashy Barebow games colors of hot pink, magenta, purple, neon orange, and mountain berry. 

Now I've gotta go buy another spool of Mercury in a more tradbow-centric color.   

Have you tried the Mercury 2 string yet Rob?  I just had a few flemish strings made with 12 strands with .025 serving size with a net of serving diameter of .110.  Great string material .   Kirk

Yes, but Mercury and not the later and thicker Mercury 2.  Spun up 3 strings, 32 strands each for the AuSable and Montana longbows.  While Mercury didn't work well for me with other bows it works quite well for these two Bears.  Due to Mercury's thinner diameter the resulting strings are perfectly round.