Why use graphite? PTFE (Teflon) powder has a much better friction coefficient, and at airgun temperature PTFE is stable (<200C).
Also Graphite gets its lubricity from lattice crystalline structure (needs thicker layer) compared to PTFE which is not latticed hence more efficient.
My recommendation would be to strip and clean the hammer assembly, use Dupont teflon-resin spray to bind Teflon to metal surfaces, and brush in some extra PTFE powder when reassembling. That should give highest possible performance if you are looking for the that last ounce of juice.