5+1 primary magazines is plenty. You should be aware, however, that Blacksheep doesn't allow the M27 to be used as a support weapon. As far as I'm aware, the only support weapons allowed by BS are belt-fed models like the M249, M240, M60, etc. This may have changed in the last few months.
At the two BS events that I've attended, we spent a considerable amount of time holding buildings and not being particularly mobile. We had lots of time to re-load magazines, eat and rest up. My advice would be to run as light a rig as you can. If you absolutely must carry extra stuff, put it in an assault pack that you can drop when you set up in a building.