I will be hunting Gonerezhou elephants in just over a months from now. I'll be using a 416 Rigby and I have my loads all set. My youngest son, but a grown man, will accompany me. He will back me up with a Win Mdl 70 Safari Express in 375 H&H. I've decided on 300 grain Barnes solids for his 375 and plan to use approximately 70 grains of RL15 powder. Any concerns, encouragement or suggestions?

I used the same load on two zebra and a giraffe this year and thought penetration was great. Of course the zebra was thru and thru but the giraffe actually stopped a couple on the off side at a range of about 200 yds. I'd guess the cartridge will work well as a back up for your son.

In '07 I used the solid on a wounded and running away zebra and got full length end to end penetration. Thats like 5' in my estimation.

Were I your son I might be tempted to "Jump the gun," and mebbe shoot just a little before you do, on accident,........"Sorry Dad,......I just got excited,...... I didn't mean to,..........".

I would have no hesitation using that load combo on elephant as a back up, but your 416Rigby shouldn't need any back up unless you fluff the shot.
What bullet are you using in your 416?
I hope it's a steel jacketed solid like the Woodleigh!

I would give a 350 grain solid a try before settling on the 300 since elephant backup is planned.