I rebarreled a Remington 798 action from a 375 H&H to a 375 Weatherby. This action has been described to me as a modified Mauser action.

The problem I'm having is that on occasion, the rounds that are in the action pivot up from the rear when I open the bolt. A gunsmith called it "porpoiseing".

He also said because of the Weatherby case design, that the rounds may not be stacking properly. BTW he said he didn't know a proper fix.

Does anyone have a suggestion as to what can be done to fix the problem?

I had a couple MKXs(same action) in 375 H&H I found they needed a touch of work to perfectly feed the H&H. The Wby case has far less taper raising more issues. He is probably right on. The factory box for the 06, normal magnums and even the 375 are basically the same. The 375 has a spacer in the end. I would "guess" the action doesn't have enough of a lip to hold the larger dia cases. Ideas that come to mind are a new after market bottom metal or altering the current box to allow more overlap.

Your cases are binding at the shoulder.
The magazine box is tapered and the Weatherby case has very little taper.

You might be able to experiment with an old 98K magazine box. Find a rusted clunker of a magazine box and wedge it open at the approximated shoulder location. Once wedged open enough the cases should start to stack properly at the base and quit popping out.

There is a guy here name Duane Wiebe that can tell you a lot more.
He has a sticky on feeding at the top of the forum.

Check this thread

ramrod340 and SR4759,

Thank you both for taking the time to respond.

Not a problem.
I should have been more clear. When I say more overlap basically it is expanding the box to allow the cases to spread.

SR4759 gave you a great link. Duane breaks it down in detail.

Work on the box first it is a lot easier to replace if screwed up than to repair a messed up rail.

Thanks Paul