If the Remington ultra mags are bassed on the 404 case, would the long actions be suitable for conversion to the 338 Lapua?

I'm not a smith, only a tinkerer, but the .338 L is based on a .416 Rigby case. I don't know the length of the .338, but the Rigby is too long for the M700. In addition the head is quite a bit larger, than the Ultras (they being rebated) and even than the .404. I am not sure there would be much steel left in the ring of the bolt. You also have a larger case body, which would mean feed lip modification and/or follower change out. It would seem that the .338 Ultra could be loaded to handshaking distance of the Lapua. That would certainly be easier, less expensive, and perhaps, even better.

No gunsmith, but my brother has a 338/378WBY in a 700 but it is a single feed only and throated for the 300gr Sierra Matchking at 4.100" OAL. It's pushing them at 3000 fps out of a 32" Pacnor barrel with 118gr of H870. If you would like to talk to the smith that has done more than one, his number is (907)376-5261 and ask for Dave at Nitro Gunworks.

I'm going to have him build my 700 up chambered for either 338 Lapua Improved or 338/416 Rigby Imp as soon as I get the barrel for the project, it will be single feed also.

You will need a Sako extractor installed because the bolt face has to be opened up also.

The Lapua case is .100" shorter than the 300 Ultra case. OAL is limited to 3.660" in my Ultra so the Lapua should fit ok. Feeding my need to be tuned some. Remington does have a Tactical 700 in 338 Lapua not available to the public I am told.

Good luck.

I've built a .338-378 repeater on a Rem 700, so it can be done. However, I don't really recommend it. I recently did a .338-300 Ultra for a customer. I believe that the case capacity would be awfully close to the 338 Lapua and with a longer mag box you should have enough room.