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For the most parts the upper / lowers are designed to be easily swapped without much fuss. Most upper receivers will fit the std mil spec lower receivers . Key terms to look at "mil-spec" - or forged. There are some other styles like billet which tend to be best matched with the same mfg. I have found that Mfg like Rock River tend to be "tighter" than most and may require the application of a dremel or light touch of a file for best fit for other manufactures "tighter" receiver part. A matched upper / lower combo will provide better looks and less worry on "will it fit".

which model?

I suggest if in doubt that you contact the manufacturer and ask their advice

The parts you will need for a 9mm build are listed below. LOWER PARTS stripped 9mm lower (or AR15 lower w/ magwell adapter) Lower Parts Kit (9mm hammer required if using non-ramped bolt buffer tube/spring/buffer (9mm dedicated buffer may be needed) buttstock Bullet button or build featureless. UPPER PARTS Barrel (any 9mm AR type barrel with 9mm extension) Upper receiver (any upper receiver should work) I like slick side uppers. Bolt (ramped bolt for standard hammer---9mm hammer for non-ramped bolt) AND...bolts used for Glock mags will need to be shaved for the magazine lips. Lone wolf and others offer this service. Handguard w/ barrel nut muzzle device (most 9mm barrel threads are 1/2 x 36) NO GAS BLOCK/GAS TUBE ARE NEEDED. It is a blow-back type of operation. THERES ALOT OF GREAT INFO HERE-- http://www.calguns.net/calgunforum/showthread.php?t=1098494

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