Sterling made 20, 30, and 40 round AR15/AR180 magazines. These mags are quite popular with AR180 owners, so they won't have to cut a mag catch into their USGI mags.
The 40 rounders are in the largest demand, since they are generally the only reliable 40rnd magazines. As Kevin stated, usually $65.00 +, often $85.00 + for NIW.
The 20 and 30rnd mags are fairly rare, and generally bring a premium, $50 - $60+ Often times higher if they are NIW.
We have a 20 Sterling for $40.00 with over 90% finish. Only catch is it has an incorrect floor plate. Says "Armalite AR180" on it, rather than "Sterling of England". I pulled the floor plate off of it to mate it with my Sterling 40rnder.
Shoot me an email if you're interested. Hope this helps.
Thanks!
Cody @ Magazine Specialties
[email protected]