They’re working on reliable 30 round drums for it now. I’d suggest that you have a GripPod or other bipod for it at that point because it starts being kind of heavy.
I actually like the 12 round drums that are out for it now better than the 20s. The weapon balance is better.
Here’s a 30 round mag for the Saiga 12.
I have a 12 round mag and it works well but it is kind of awkward because of it’s length. I’ve yet to try my 20 round drum.