Im going to reload 300 grain barnes x bullets for my 416 remington for practice and am wondering if anybody has any experience with it and reloder 15.

I don't have the 416 rem mag but when working up loads for my 416 taylor with 300 gr bullets. I just took my 350 gr load and went from there.

Use a chrony and watch the vel and pressure signs and you well do just fine. If you get to what should be the proper vel. with out pressure signs I stop. A 300 gr some where over 2700 to 2800 maybe a bit more should wack just about anything. I was shooting 2640s in my taylor.

Killed wis deer just fine.

Don't know what you are calling "experience" but:
RL-15 From 86.0 grains to 93.0 grains
Remington 9-1/2M Primer

And it was my powder of choice with 300's.

Was thinking of starting up with 79.5 grains and working up .5 at a time. I shoot 350 grain bullets with 84 grains and they act ok with no pressure signs.