Will a 9mm lower work with a regular .556 upper???

Discussion in 'DIY Projects' started by a.Truetone, Dec 13, 2008.

  1. a.Truetone

    a.Truetone Guest

    Hi I was wondering if I could drop my m4 bushmaster upper on a 9mm lower, or do I have to have both a 9mm upper and lower? - and also just out of curiosity will a pistol sized AR upper (11.5", etc) drop on a regular lower or do you need a pistol lower as well- (legally of course w/class 2 sticker) the same goes for a 9mm short barrel (pistol sized ar upper)- would it work with a regular lower or not??? Thanks for any help on this matter!- oh and could anyone give me your best sites for buying uppers/lowers of different types/calibers and sites for cheaper accesories/flip up and reflex sights/ custom stuff, and all AR-related goodies, etc...

    And one other question- I just bought a Bushmaster M4 with a quad-rail freefloat handguard and I was told that it couldn't be removed from the gun... Is this true or not, because I really want to get one of the new RRA half quad/half alluminum freefloaters- so any help would be much appreciated!


    PS- any good sites for buying ammo-both bargain/specialized type for any calliber???
  2. Dillinger

    Dillinger New Member

    First off, welcome to the forum.

    No, a 9mm lower has a different hammer, a different buffer and, obviously, a different mag well attachment.

    Switching back and forth is not a problem, but you will need to change the hammer, for sure, and the mag well attachment. There are guys who swear by the 9mm buffer in their .223 rifles, I am not one of those guys. :rolleyes:

    Compliments of ninthstage

    No, your friend is wrong. Any fore end can be changed, free float tubes and hand grips, or anything else for that matter, but you will most likely need an Armorers tool. If it were permanent, that would defeat the purpose of putting it on there in the first place. If you aren't familiar with the process, have your gunsmith do it to avoid hassles.


    Midsouth Shooters Supply

    Surplus Ammo dot com

    Hope that helps -


  3. BillM

    BillM Well-Known Member Supporter


    Well---in some cases. I've got an American Spirit 9mm upper, and it
    works just fine with the standard hammer, buffer etc. The mag adapter
    just slips out the top of the lower, so switching to a 5.56 upper is
    pretty painless. Different brands of 9mm AR's do require a hammer change,
    and they also use different magazine adapters and magazines.

    In 5.56 pretty much any upper will fit on any lower, as long as the pin
    size is the same. The LEGALITY of some combinations is a different
    matter--get caught with a shorty upper on a rifle lower without the proper
    SBR paperwork and life will get interesting, and not in a good way.
  4. a.Truetone

    a.Truetone Guest

    thanks guys for your info- but, what if I had a 9mm lower- would it work directly with a regular 556/223 upper or would there need to be modifaction made for that as well??? I've read that a 9mm firing pin will fire a .223 round, so if that's the case then all I would need is a 9mm lower with a dedicated magwell, correct?
  5. BillM

    BillM Well-Known Member Supporter

    Most, but not all 9mm AR's start with a standard 5.56 lower, and add
    a magwell adapter. So to convert it back to use as a 5.56 lower all that
    is required is removing the adapter. Some just slip out, some are retained
    by screws. There is also at least one that is a dedicated 9mm lower
    using Glock mags that can not be converted to use with a 5.56 upper.

    Anybody know how a genuine Colt is set up? Magwell adapter or dedicated

    You may also have to change out the hammer, buffer etc. As I said before,
    my 9mm conversion (American Spirit Arms) doesn't require a special hammer
    or buffer---I can push out 2 pins, pull the magwell adapter and put a
    5.56 upper on it. YMMV
  6. infotech

    infotech New Member

    If the purpose is to be able to make range time more affordable you can buy any number of drop in 22lr adapter kits which allow you to use your existing 223/5.56 rifle without modification. The only issue I've read is that some of the magazine adapters can crack the magwell (due to the use of a set screw to hold it all in place).

    Spike's makes a .22 lower you can swap out on your stock upper, I've seen reviews of all types on it but it seems some sort of quality control issue in the earlier version was solved with a supplier change and the recent reviews are all very good.

    I've been thinking of just building a complete rifle set up for 9mm or even this "new" variant of the 7.62 which is affordable and abundant, I can't remember what the round was exactly , something about 45mm if I recall correctly.

    LaRue and some of the other high end uppers can be had in 7.62, 308 and other calibers. These are usually better suited for "sniper" variants such as the LaRue Stealth which is shipping now. All of this fed into my desire to set up a full CNC shop and get my FFL mfr license someday. I plan to build a decent collection of ARs in various calibers.