From PS mag. Feb. '05 issue titled "Heat Checking In Rifle Bores" by Fred Barker;some items of interest:
1/ CM has the advantage of heat conductivity about 70% > than 416, but is partly offset by 416 having thermal expansion coefficients ~ 25% less
2/ Quote " to my knowledge we don't have any quantitative results on the bbl. life (and hence resistance to heat checking)of 416 vs. CM. When 416 was first used.....it was hoped bbl life would be longer, but PO Ackley and others saw little difference
3/ ....."powder burns ~5,000-5,500'F for about a millisecond, and the metal at the bore surface in the throat region is heated about that hot only to a shallow depth ~0.0001"....the surface of the bores throat is thinly nitrided by the N in the powder gas, giving a brittle coating that behaves differently than the underlying steel."
4/ " The most important factor in heat checking...remains the temp difference between the very thin skin of hot steel and iron nitride at the surface and the underlying steel substrate"......this gradient causes the surface to expand (on firing) or contract (on cooling) relative to the substate and give the thermal stresses that lead to cracking "
Of note, the author advised against running coolant down the bore, and to keep the bbl warm when doing extended firing, and cool the barrel once it becomes too hot to touch....but only to the point where it can be handled....all this in attempt to reduce drastic temp. cycles which induce cracking.
1/ CM has the advantage of heat conductivity about 70% > than 416, but is partly offset by 416 having thermal expansion coefficients ~ 25% less
2/ Quote " to my knowledge we don't have any quantitative results on the bbl. life (and hence resistance to heat checking)of 416 vs. CM. When 416 was first used.....it was hoped bbl life would be longer, but PO Ackley and others saw little difference
3/ ....."powder burns ~5,000-5,500'F for about a millisecond, and the metal at the bore surface in the throat region is heated about that hot only to a shallow depth ~0.0001"....the surface of the bores throat is thinly nitrided by the N in the powder gas, giving a brittle coating that behaves differently than the underlying steel."
4/ " The most important factor in heat checking...remains the temp difference between the very thin skin of hot steel and iron nitride at the surface and the underlying steel substrate"......this gradient causes the surface to expand (on firing) or contract (on cooling) relative to the substate and give the thermal stresses that lead to cracking "
Of note, the author advised against running coolant down the bore, and to keep the bbl warm when doing extended firing, and cool the barrel once it becomes too hot to touch....but only to the point where it can be handled....all this in attempt to reduce drastic temp. cycles which induce cracking.
