Who exactly advises this?
This was an official directive from the Royal Army Ordnance Corps.
Cordite is brutal stuff and this was well known fact from the outset (Cartridge, Cordite, Mk.I) in 1891. The Brits quickly changed the recipe for cordite soon after it was introduced to tame it down a bit. A nitrocellulose cartridge was first trialed in 1894, but the winning quality of cordite over nitro is that it is stable in all temperatures.
The Empire use of the 303 cartridge could be in the tropics or it could be in the frozen tundra, same ammo.
You might have noticed some Lee Speed rifles with 'for cordite only' marked on them. This is from early days where another smokeless powder was in vogue, 'Rifleite'. Rifles were failing. Rifleite is a true single base nitrocellulose propellant, clean and relatively cool burning, but not stable in high temperatures. There were some instances of commercial rifles were bursting and litigation ensued. The problem was traced to Rifleite loaded cartridges sitting in the sun. BSA were sued for damages and had to fight in court, hence the disclaimer added to commercial barrels to cover their butt.
Cordite does not generate high pressure spikes but it does generate excessive heat which is what erodes the throat of a rifle. Yes it quickly destroyed Metford barrels but it also did much the same to barrels with Enfield rifling, same amount of erosion, it just took the effects a bit longer to destroy the bore with the deeper square shouldered rifling.
The 'Traveling View' of those days where an armourer road show would travel around the Empire and gauge each and every single rifle in service, every year, and would focus on the amount of erosion in the barrel. If need be, the rifle would be tagged for barrel replacement. A common thing with the Lee Enfield, cordite quickly wore the barrels out.
It still does, I simply will not put surplus cordite ammo through any of my toys. That coupled with corrosive mercuric primers, the ammo is nasty stuff.
Accuracy tests by the School of Musketry proved that a rifle that was shot exclusively with nitro cartridges retained accuracy over a given period, however, with only relatively few cordite cartridges through it, that barrel's accuracy would be ruined and start to drop off measurably as the lead grew towards the muzzle.
I'll dig it out the refs, but basically the note was that rifles firing cordite should not be used with nitro and vice versa.
The impulse from the milder nitro ammo used in a cordite barrel just did not kick the base of the bullet hard enough for it to obturate into the worn lead of the rifling.
If a barrel had been used for cordite, cordite should continue to be used. Barrels using nitro were to continue to use nitro, the two should not be mixed.
So what I believe Geraldtao is eluding to, is that if the rifle has been shot with cordite ammunition (which it no doubt has), modern commercial factory ammo should not be used. It is an accuracy, not a safety thing.
Hand load with a soft lead flat based bullet and a stout load to give the bullet a good kick in the arse to fill the lead.
It is often the reason that some Lee Enfields, even though appear to be in very good condition and are well bedded, for some reason they shoot like crap with factory ammo. The lead is most likely worn. There is a good chance that it would shoot just fine with cordite ammo.
I see people trying a bullet in the muzzle to see if the bore is worn. Yeah, that is a good ad hoc test, but the critical wear in the barrel is at the chamber end. If you can insert a lead gauge more than 1/4 inch up the spout, the barrel is done.
Some sage shooters will tell you that Lee enfields will only shoot well with flat based bullets. Often that is true. They need that flat base to obturate.
But if you find a rifle with a barrel that has shot only nitro, you can shoot boat tailed bullets all day long with good results.
The nitrocellulose loaded Mk.VII cartridge was introduced in 1916.
I stick with Canadian made ammo. Nitrocellulose and non of it is corrosive.