As Stated by LloydM we have been making detachable magazine kits for the Rem 700 for a while now. They use Accuracy Int mags. The 308 can use 5 or 10 shot mags, the 300 win or 338 Lapua have 5 rnd mags available.
Sorry Lloyd HS does not make anything bigger tan the 3 rnd mags except in 308 cal. You could switch to ours, that would give you 5.
There are photos and pricing on our website