P.O.F. primer or powder causing rust in my bore?

[Bruce_in_Oz]

When I were y'ung lad living in cardboard box.....

We cleaned our SMLE barrels by using the approved funnel to guide copious quantities of boiling water down the barrel. (the trick was to avoid any water getting anywhere near the woodwork). As I have not shot corrosive (mil spec, chlorate based) primers in my Lee Enfields since about 1975, it is not so much of an issue any more.

Some time in the 1960s, the RAAF apparently started using dilute phosphoric acid for barrel washing on their rifle ranges. Anyone else heard about this? Probably safer than scald burns and runaway fires on the range, too.

The mercury is in the "fulminate" component of the priming compound. I suspect that the reason it was used almost exclusively in "British" .303 ammo is that, if manufactured and stored correctly, it will ignite at any temperature found on the planet. Lead based primers are (or were) more fussy about extreme cold. Mercury will not adversely affect your rifle, however, from the instant of ignition, your brass is doomed. The mercury metal liberated on ignition instantly bonds with any brass it contacts and starts breaking the bond between copper and zinc, hence embrittlement. Those of us who grew up reloading the stuff very quickly got used to cracked necks during bullet seating. We resigned ourselves to not picking up cases after even the first reload, because of the rapid rate of failure during subsequent reloads/firings.
That is why the cups for mercuric primers are made using heavy COPPER cups, not reactive brass. Note also the size of the cup (.250") compared to what we now consider a "large" primer, (.210").

All of which comes back to the principle that military firearms are usually built around a cartridge, thus the striker specifications on your basic Lee Enfield ( and P-14) were set to work with mercuric / chlorate based primers with copper cups. Note the striker / boltface changes when the P-14 became the M-17.

The fulminate gets things going and the chlorate adds "brissance", the extended flash that ensures ignition of the powder. Very important when you are loading ammo with compressed bundles of long propellant sticks. Granulated, double based propellants are a lot easier to ignite consistently than the old long stick "Cordite". Note that "contract" .303, (US especially) was loaded with granular propellant and lead-based primers. It is still CORROSIVE, however, as it contained chlorate as per above.

The .30 M1 Carbine cartridge was the FIRST military cartridge in the world to be specified and mass produced with non-corrosive primers. Why? the gas system is "non user-maintainable", unlike the M1 Garand.

Why the slow uptake of non-corrosive primers? Reliability at all temperatures and under all storage conditions. Once again, "surplus" ammo is often "surplus" for a reason, usually poor performance in batch testing.