14 strands of B55 is larger then B50.B55 is .020 diameter per strand and B50 is .018.Not a big difference but doesn't make a larger diameter string even with the .017 diameter serving.I assume you have swaged nock ends.A 9/32" nock fits the best but the throat size of my 9/32" nocks are not as large as I would like.American/BJ nocks have a larger throat then most but you still may have to sand or file them for a good fit.The best and largest throat 9/32" nocks I have found was some NOS Bjorn nocks I found on the big auction site.