The
RND 2500 in .408 CheyTac, Alexander Arms
Ulfberht in .338 Lapua Magnum and
CheyTac M400* in .408 CheyTac sound like interesting base weapons to apply modifiers to for an entry carbine suited for ST 13+ monster hunters facing supernatural horrors.
The
VR1 rifle might also be worth looking into, though a 5-rd magazine is obviously not enough.
*I don't think that one ever went into production, though.