That sounds right. I believe the army phased out corrosive primers between '51 and '53, except one lot of Frankford Arsenal match ammo in, I think, 1958 that was corrosively primed.
The culprit is potassium chloride (KCl), a reaction product from the potassium chlorate (KClO3) oxidizer in the corrosive primers. I believe it promotes corrosion when it attracts enough moisture to partially ionize, allowing the chlorine to bring small amounts of surface iron into an equilibrium solution. The chlorine has more affinity for potassium than iron, but, nonetheless, enough temporary reaction occurs to leave iron atoms adrift and easy prey to oxygen whenever the chlorine finds its way back to the potassium.
IIRC, Hatcher indicated there needs to be at least 68% relative humidity for this reaction to start. I unknowingly shot corrosive ammo through my M1A out west one time, in dry desert, I cleaned it (probably with Shooter's Choice at that time) once and saw no rust during time out west. It was not until the gun had been back in Ohio and allowed to sit a few weeks that some surface rust started to appear in the bore. It did no serious damage and was readily amenable to removal by cleaning solvent followed by a scrub with JB Bore compound to polish it.
Hatcher says the old rule of thumb for corrosive primers was to clean immediately after firing, then again the next day. I'm guessing that the first cleaning was penetrating the carbon residue and softening it overnight for easier removal at the second cleaning. Hatcher said the Frankford Arsenal formula worked fine back then. Ed's Red (ER) is the modern counterpart to that formulation. It has both polar and non-polar solvents, so it will dissolve the tiny quantities of potassium chloride left in the carbon deposits if the carbon softens for removal. ER also softens carbon overnight, which is why I suspect that's key.
I think a good practice, especially if you are unsure of your primers, is to run a patch or even a bore mop wet with a carbon softening product through your bore immediately after shooting and before you even head for home. The carbon hardens further over the next hours after firing, so you nip that process in the bud by getting some Ed's Red or Gunzilla in there right away. Boretech Eliminator seems to soften carbon well, and it is water base, so it should dissolve potassium chloride, too.
