My new little Ruger Mini-14 came with one magazine, a five-rounder, that
fits flush and completely inside the bottom of the rifle. I kind of like
that for a couple of reasons:

1. it tends to slow down the pace of shooting, at least for me, and
slower shooting tends to be more accurate, at least for me.

2. with no magazine protruding from the bottom of the rifle, when I bag
rest the firearm, there are many places along the stock that can be used
for that purpose without the bag and a magazine banging into each other.

Ruger apparently makes 5, 10, 20, and 30 round mags for these rifles.
The 10-rounders, I am told, are simply 20-rounders with a "stop" welded
inside so they cannot be converted to 20-rounders, and that makes the
10-rounders legal for sale in states where larger cap mags are
restricted (such as Maryland).

http://tinyurl.com/nrhurzr

I have a pair of 20-round mags for the rifle I legally bought at a
Virginia gun shop. Such are the vagaries of our state's laws...you can't
buy a 20-rounder here, but you can buy one out of state, bring it into
Maryland, and use it here. But if I buy any more mags, they'll be the
five-rounders. The way I see it, if I can't drop an attacking,
brain-dead, right-wing zombie with five rounds, 20 rounds won't do the
job, either.