Your advice with the following will be appreciated.

Is possible to convert a Weatherby Accumart in 338-378 to allow magazine insert?
I want the magazine to hold five rounds. Is it possible to easily toggle between a magazine insert and the use of the original floor plate?

What are some candidate magazines available for such a project?

Would this project require extensive modifications to the rifle?

Ok, in order of your questions, yes, good luck on the 5 rounds, not easily done, sako TRG-S has probably the only easily available mags, and yes. It would be a great deal of work, but anything is possible. - Dan

Dan,

Thanks for your response.

I took a closer look at my rifle and noticed that, in fact, the upper portion of a magazine exists and is sandwiched between the bottom of the action and the top of the floorplate triggerguard assembly.

It seems to me that one could use a file to thin out or widen the opening of the floorplate and thus allow a standalone magazine to be inserted.

To hold the magazine in place the existing floorplate�s locking mechanism could be reused. This would require that a piece of metal be added to the back of the magazine (at the appropriate height and flush to the stock). I am not sure if that would be enough to hold the magazine in place or would a similar piece of metal need to be added to the front of the magazine such that its shape would make use of the void left by the floorplate hinge?

I think this approach would leave the rifle's appearance clean looking; although the magazine itself would have two relatively small projections making them somewhat cumbersome in your pocket.

What are some of the pros and cons to taking this approach?

I went over to a friend's house and looked at his Accumark, I think you can do this. You would probably be best served by re-using the magazine box that came with your rifle, modifying it, and modifying the floorplate and locking mechanism to work. This should work for a flush magazine, which will give you a two round capacity. Any more than that will most likely stick out the bottom of the rifle. Good luck. - Dan