I own an Olympic Arms 9mm upper for my AR-15. I love it. It shoots very accurate and I use the modified sten mags. These in my opinion are the best option. Olympic arms makes a modified sten mag loader know it costs around $15, or if you are creative you can make your own sten mag loader for these modified mags.
For the olympic arms upper you have a few options for Sten mags.
1. Use modified sten mags from olympic arms
A. You can either purchase them from olympic arms at the price of $65 a piece.
B. Or you can send them your own sten mags and have them modify them for a fee of $25 apiece.
2. You could buy some Sten mags from tapco.com for a good price. They have been haveing a deal on sten mags I think the deal I got was 10 sten mags for $20 + shipping and handling and I got a green pack to carry them in.
A. Upon purchaseing your sten mags, for $65 you can purchase a Socom Block from Socom MFG.
B. Just like was mentioned above you could alway just modify your own mags useing a wire feed welder, I suppose useing some epoxy type weld glue for steel might work also. Tubular steel is not that expensive,I think a 2 foot piece of 7/8 inch steel will run you 3-4 bucks.
There was some legal questions concerning the modified sten mags. I personally called Olympic arms and the ATF and asked about these mags.
Olympic arms assured me that the mags are perfectly legal, and the ATF said to me that the mags were perfectly legal.
3. Another option is you can buy a mag block from olympic arms that allows you to use pistol mags. You can send you mags to olympic arms for them to drill a hole in them that will allow your mags to be used with there pistol mag block. What is good about this is that you can use your pistol mags in your pistol and your AR.
You might want to check out olympic arms web site and varify what pistol mags they will modify. I think they modify Beretta mags, and glock mags.
I like the olympic arms set up. I have put through my upper around 1000 rounds so far, and have had no problems.
I personlly like the modified sten mags. I dont have to fiddle with the handle on the SOCOM block, and I dont have to worry about hitting the regular mag release on my AR and dropping the socom block and mags. The modified sten mags I have work very well, they function just like a regualr magazine in an AR.