I've been shooting while laying prone on the ground with my 30 rd mag in the gun. Get a grip/bipod. A 20 will get you closer to the ground if you want to shoot off a sand bag or two. As propellerhead says options are what AR's are all about.20 round mags are great for shooting prone. The 30 rounders are too tall. It's good to have options.
Ruger's On-Line Store says "Product is temporarily out of stock" when I checked at 8:30 CST. :-(Guys,
Bravo has the 30 rounders, black w/no window in as of ~8 est this morning.
Also. Check ruger's website for 20 round factory mini-14 mags. They are 29.95 right now!
Good luck getting those pmags!