Click the READ MORE to view this article about Magazine options for the MSAR STG-556. I have had numerous questions from viewers about this specific question that I figure I would do a video for the viewers. There has been a lot of confusion on what magazines will work on an MSAR STG-556, so please read the article and view the video that covers this topic.
MSAR Magazines for the STG-556
by Jessie Indracusin
INTRODUCTION
This is going to be a fairly short article. There has been a lot of confusion on what type of magazines you can put into an MSAR. This is confusing people a bit since there is another model "Steyr AUG" design from TPD that uses AR magazines. I hope this helps clarify this for all viewers who are looking at acquiring one of these rifles.
CAPACITY OPTIONS
Microtech Small Arms Research (MSAR) offers four different magazine sizes for the STG-556. The options are as follows: 42-Round 
Magazine, 30-Round Magazine, 20-Round Magazine and a 10-Round Magazine. The first capacity, 42-round, was kind of an odd ball size but was what the original Steyr AUGs were offered in. Originally the 42-round magazine on the Steyr AUG was for the LSW version (Light Support Weapon) but was later adopted by general military units. As you can see from the magazine on the far left, MSAR adopted the same transparent magazine design as the Steyr AUG had. You can see the magazine on the far left, the 42-round magazine, you can visually estimate the number of bullets in the clip.
NEW MAGAZINE OPTIONS
MSAR has started the release of solid black magazines. The only size I have seen so far have been the 42-round magazines in the black. 
There is also a slight modification of the standard "green" magazines as they are referred to (even though they are more of a yellow/amber than a green) which have a green magazine floor plate instead of the standard black magazine floor plate. The picture on the right is a picture of the new solid black magazines compared to the original transparent magazines.
AR MAGAZINES IN A STG-556?
The magazines that MSAR are a very solid design. However, since these are all proprietary, these magazines are very expensive. These 
42-round magazines for the MSAR run about $80.00 each. Compare this to the fact you can get a 30-round magazine down to $20.00 each, this is a considerable cost. Do I think the cost is warranted for the magazine or is it just a manufacturer trying to stick it to you with a proprietary component..you can guess what my answer is? However, NO OTHER MAGAZINE WILL WORK IN AN MSAR. There are discussions that you can modify the original Steyr AUG magazines to fit in this gun, but to be honest, with how much and how hard it is to find these older Steyr magazines it is not worth it (plus it is questionable on how reliable they would be). You can see by the photos on the left comparing it to a standard government issue magazine and the high end Heckler and Koch 416 Magazines, that the notch and shape of the magazines will not allow the AR magazines to fit in the MSAR.
CONCLUSION
I hope this clarifies a bit of the confusion viewers have had about the MSAR and magazine options for the gun. Please click the link below to view the video component.