The .303 British, military fmj bullet has an inside aluminum tip, in front of the lead, to make it unstable and tumble during this particular bullets terminal ballisticks on human tissue in order for it to creat servere or fatal wounds.
I cannot in any way see how exposing any of that aluminum by filing or cutting a cross on this 303 British fmj bullet with aluminum tip can in any way endanger the gun or the shooter, more than for example using aluminum tipped Winchester Sivertips ?
However, I can imagine if the tip of a non-bonded fmj bullet is cut to far back on the bullet that it might create a seperation problem between the jacket and the lead, especially in military chambers with generous freebore were the jacket will get stuck when hitting the rifling, and the lead bullet core continuing through the bore.
Just exposing a tiny bit of the fmj bullets lead by some gentle filing on top of the bullet tip, or cutting a small cross at the fmj bullets tip will not, in my experience, seperate the jacket from the lead when firing, since, among other things, the curved rifling will force the bullets jacket into the core lead bullet and thus "lock" the jacket and the lead together.