DiracSubstitute67[exp] inserts the explicit definitions of the chirality projectors \gamma^6 and \gamma^7. DiracSubstitute67 is also an option of various FeynCalc functions that handle Dirac algebra.
Overview, DiracSubstitute5, DiracGamma, ToDiracGamma67.
DiracGamma[6]
DiracSubstitute67[%]\bar{\gamma }^6
\frac{\bar{\gamma }^5}{2}+\frac{1}{2}
DiracGamma[7]
DiracSubstitute67[%]\bar{\gamma }^7
\frac{1}{2}-\frac{\bar{\gamma }^5}{2}
SpinorUBar[Subscript[p, 1]] . GA[6] . SpinorU[Subscript[p, 2]]
DiracSubstitute67[%]\bar{u}\left(p_1\right).\bar{\gamma }^6.u\left(p_2\right)
\left(\varphi (\overline{p}_1)\right).\left(\frac{\bar{\gamma }^5}{2}+\frac{1}{2}\right).\left(\varphi (\overline{p}_2)\right)
SpinorUBar[Subscript[p, 1]] . GA[7] . SpinorU[Subscript[p, 2]]
DiracSubstitute67[%]\bar{u}\left(p_1\right).\bar{\gamma }^7.u\left(p_2\right)
\left(\varphi (\overline{p}_1)\right).\left(\frac{1}{2}-\frac{\bar{\gamma }^5}{2}\right).\left(\varphi (\overline{p}_2)\right)