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According to the NY State Penal law "Large capacity ammunition feeding devices" are illegal to possess.
What defines a "large capacity" ammunition feeding device:
Any magazine, belt, drum, feed strip, or similar device that can hold more than 10 rounds of ammunition. One exception to this is a magazine that was manufactured before September 13, 1994. Another exception is an "attached tubular device designed to accept, and capable of operating only with .22 caliber rimfire ammunition". This means a tube similar to the magazine on a shotgun but on a .22 rimfire firearm.
Common Questions:
Q. Can I use a pre-ban magazine in a post-ban firearm?
A. Yes, the law exempts pre-ban magazines that hold more than 10 rounds no matter what firearm they are used in.
Q. How can I tell if a magazine is pre-ban?
A. See our blog post on
http://www.nyfirearms.com/blog/2008/10/preban-ar-15-magazine-identification/ . This applies to USGI AR-15 magazines which are the most asked about. As for other types of magazines there is no sure way to tell for most of them. If a magazine has a LEO or Military use only marking it is not pre-ban. If it is for a firearm that was not made before 1994 it is not pre-ban. Some manufacturers made small changes to their designs which can help determine whether or not it is pre-ban. If you have questions about a specific type of magazine please post a question in the forum and someone may be able to help you.
Q. Can I convert a higher capacity magazine to accept only 10 rounds?
A. Yes, as long as the method is permanent and not readily restorable to accept more than 10 rounds of ammunition.
Q. Can I rebuild a worn or damaged magazine with new parts?
A. Although there is no NY State ruling on the subject it is the accepted understanding among many shooters that the pre-ban body of a magazine is what makes it comply with the law. That means new springs and followers can be installed in one. Note that floorplates are the only part of many magazines with distinguishing marks so changing a floor plate to a new production part may be a risk even if the body is pre-ban. If a body is damaged and must be replaced it may be in your best interest to dispose of the magazine to avoid potential issues.
Q. Can I use a post ban magazine that holds more than 10 rounds in a pre-ban firearm?
A. No, post-ban magazines that hold more than 10 rounds are illegal to possess no matter what firearm you own.
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-Jeff
http://www.nyfirearms.com/blog/2008/10/preban-ar-15-magazine-identification/