RUM vs Lapua
It's a bolt face issue to some degree, case length in others.
The RUM uses a standard magnum bolt face. Any bolt that can accommodate a 300 Win Mag can accommodate a RUM.
The Lapua uses a size up from the RUM. The 'standard' R700 bolt does not leave much for their 'three rings of steel' if opened up to Lapua case head size.
Both have challenges with COAL when seating the bullets the way "we" seat them for long range. Both can easily exceed the ~3.65" standard R700 magazines. This can be cured very cheap and easy with a Wyatts. That will get you to ~3.75". The Lapua seems to fit in this length most of the time. Longer RUMs sometimes will not.
If using a "custom" action, Weatherby or CZ550 then the bolt face and magazine length issues pretty much go away.
You pays you money and you takes you choices.
I use Weatherby actions.