I have not heard this but it would not suprise me to hear BH offering a heavy MK bullet for the 300 RUM.
It would suprise me a little for them to use the 240 gr version over teh 200 or 220 gr which will work in all factory rifles with a 1-10 twist.
From a commercial point of view, the 240 gr version may limit their sales numbers a bit compared to the 200 and 220 gr versions.
BH's tends to be pretty good at giving shooters what they want and they make fine ammo as well. Everything I have ever tested or shot in custom rifles performed very well.
You have to have your hat on straight to load ammo better then BH's does.
Good Price too,