Many moons ago just after I acquired my 404 I corresponded with an old guy who also had a 404. I can't remember all the details of the rifle now but I recall when I offered him some of the RWS 400gr solid bullets I had just imported he turned them down saying he did not like shooting jacketed in his 404 as it had a bore/bullet size of .418". I don't know the history of what he shot in his rifle other than i think he did cast for it and recall I sent him a few of my cast projectiles to try in his gun. He passed away shortly after I had last corresponded and who knows what happened to his 404 rifle.

From what I have seen in sectionalising one of my RWS bullets and from what others have said about the original Kynoch bullets, the side walls on these early projectiles were always quite thin with the idea they would swage down easily to accommodate a range of bore sizes.

Pondoro Taylor wrote about the 404 Jeffery in his "African Rifles and Cartridges" book and relates the experience of an old German hunter he knew who had problems with both German and British ammo where the bullets often broke up. Taylor himself did not experience this problem when using the 404 nor to his knowledge did any others and as he said the cartridge would not have become such a workhorse in Africa had the .423" bullets consistently misbehaved as the German hunter experienced.

I myself have always wondered if that German hunter just happened to have a rifle with .418" or smaller bore which caused the thin sidewall jackets of .423" bullets of the day to be severely swaged and likely even the jackets cut when fired thereby initiating bullet break up. This could be a logical explanation for an unusual phenomenon where ammo from different manufacturers habitually failed when used in one particular gun.