Here's a company that will modify 7400/7600 rifles with muzzle brakes, rail systems, cryo treated bbls etc Custom Remington 7400 7600 750 Rifles Accuracy Systems Gunsmith Gunsmithing

I thought that they used to offer custom 10 and possibly even 20rd mags, but I only see 5rd mags listed. Though some of the custom rifles pictured seem to have a 10rd mag in place.