Mass of simulant material required to achieve 10% compression with bullet seated: 20.2 grains (as thrown from Hornady L-n-L measure). CSJ (Carter S. Jones) High Performance moly-lubricated cast bullets: Brinell 14, 0.430-inch RNFP, seated at 1.59-inch OAL (0.385-inch seating depth). Case: Once fired, Star-Line 44 Magnum; full-length resized, slightly belled, using 0.425-inch expander; flash hole and primer pocket cleaned up using K&M Services tools; mouth deburred and inside polished. Test gun, Ruger Super Blackhawk.
If we divide 0.85-inch by 0.70-inch, we see that adding simulant reduced average bullet movement 21.4% (about 1/5). These results are unequivocal: Adding simulant material reduces primer-induced bullet movement, even in a load with an unusually high degree of charge compression (by rifle-load standards), and, by logical extension, so would adding propellant, see table. This result offers dramatic support for the contention that heat transfer from primer gases onto propellant granules outweighs any effects stemming from decreased air space in any normal loading in any reasonably large case (wishing I had the Meacham 22 Hornet for some related testing!). This result occurred despite the fact that the "charged" test cases showed zero evidence of primer leakage on outside of cases, while primed-only cases were almost completely blackened – report was perhaps ten times louder with primed-only cases. In progressively larger cases, difference in potential bullet movement will bias toward progressively less bullet movement with charge installed.