I'm not sure of 3gun,I shoot USPSA, and in that, you can use any magazine that keeps your gun within the size limit. It only matters how many rounds are in the magazine at the start of the timer, and the division you shoot in determines that. For instance, I shoot Production Division, and I have both 10 rd mags and 16 rd mags. At the start of the timer I can only have 10 rds in each of the mags I use in the match. Meaning I can have a 16 rd mag as a Barney mag, as long as it's not used in the match. Hope this clears it up! Also here's the link to the USPSA Rulebook
https://www.uspsa.org/document_library/rules/2014/2014%20Rules%20Changes.pdf