Remington XCR 257 Weatherby.

[Wm.Traynor]

So what do you put 'massive' copper fouling down to?

If ever there was a gun that would melt copper, that thing would be it

[Baz460]

Apparently it was the Barnes projectiles, the guys at the Barn said they are notorious for copper fouling, and pushing them out at around 3800 fps, and me not being thorough enough with my cleaning, even though I put the rod through every 20 shots I only anti fouled it twice, and I have fired around 80 rounds.
So lesson learnt.

[Monty]

The TSX does indeed have a reputation for bad fouling.

Interestingly competitors like the Nosler E-Tip and Hornady GMX don't suffer from this.

Maybe Barnes could do with tweaking their alloy recipe.