Six of the participants in the Bach to Back DNA project have results for the comprehensive Big Y 700 test. This test is for SNPs, not STRs, and is more accurate for predicting the time to the ancestor in common. The results suggest an even closer relationship than the paper trail supports. Descendants of Harman Jr., Joseph, and Henry are confirmed to share a paternal ancestor born perhaps as recently as 1837. This means there are even fewer mutations for these descendants than the average for the Big Y database. The Big Y results plus the paper trails to Harman Back Sr. of Culpeper Co VA confirm the ancestor in common.

Two of the six descendants are close relatives so their haplogroup has been further refined to reflect that relationship but still within the same broader haplogroup as the other four descendants. All six are listed in subgroup A1.

https://discover.familytreedna.com/y-dna/E-FT198173/story

In addition, FTDNA now assigns a haplogroup to men who tested with Family Finder. That process added two more men to the family group, both with Letcher Co KY roots. They are listed in subgroup A2 along with a man who only tested Y67, not Big Y. These three are also identified by DNA results to be of the same broader haplogroup as the Big Y testers.  It is likely that if these three were to test Big Y they would match those in Group A1.

https://www.familytreedna.com/public/Bach_to_Back?iframe=ydna-results-overview

None of the nine men in subgroups A1 and A2 have matches to anyone except the other members of those two Back/Back subgroups. They are distinctly different from the descendants of John & Jacob Back of Wayne Co KY who are subgroup J. It is not possible for the two groups to have a common paternal ancestor within the time frame of surnames.

The Big Y test changed the paradigm of Y-SNP testing. Before the Big Y, SNP testing was primarily useful before the advent of surnames; but today, SNPs are useful in determining genetic lines of both closely and distantly related men. SNP markers mutate at a much slower rate than STRs, which makes them more stable for determining relationships. But you need to test millions of locations to find the relevant markers, which is exactly what the Big Y test does.-Family Tree DNA

https://help.familytreedna.com/hc/en-us/articles/4414479800463-Introduction-to-the-Big-Y-700-Test#h_01HXCWFG5046BJKD627JT3YC0