i am new and sure this has been addressed and debated many times but just for kicks can yall do it again. i have own many guns and right now my favorite is my ak but i have been drooling over some ar's for awhile now. i have a bit of a problem i want the .223 but also my beloved 7.62x39, then there is the ever cheap .22 lr, but i like the 308, the 9mm, of course the 50 bmg, etc, etc. i think yall understand. i was really thinkin one lower and alot of separate uppers, hey a couple pins and boom a whole new animal, but this mgi qcb, setup has me thinkin. i like the idea but it still seems that your pretty much putting on a new upper, but it cost more. i understand staying with the .223 it's nice to easy move from a 10" barrel to a 16" or 20" but they say to switch to a 7.62 for example is a matter of new barrel, bolt, mag well, etc. hmm that dont sound as quick as two pins. but then i ask myself if i do a bunch of different uppers, how do i feed the different ammo, and where do i find the mags and uppers? so if yall can give me some advice, i don't mind spending some money but i am by no means rich and will probly go piece by piece, but i don't want junk or substander stuff either. so go with all mgi, different uppers and speacial mags, or mgi lower and different uppers?

You can have 1 lower.

And then seperate "complete" uppers for just about anything you can dream of.  

When you have another COMPLETE upper including iron sights or scope you simply push 2 pins out, swap uppers, push 2 pins in, grab relative mag and load up.  As long as you sighted that new COMPLETE uppen in it will still be zeroed.

Just a few of the AR calibers off the top of my head:
17.......
22lr
204 ruger
221 swift
223/5.56
243
6.8
300 whisper
300 fireball
308
50 bmg
9mm
40
45
50 beowulf

Just to name a few...

....and bfreek17's favorite, 7.62 x 39....  

http://www.del-ton.com/AR_15_Barrel_p/cv1033.htm

Quoted: If it was me I would go with 1 lower and seperate uppers/mags. <cut>

What he said.  At least to start if you've got the different calibers bug.  In the long run I highly recommend more than one lower.  At least that way if something in your lower goes down you're not stuck with a bunch of non functional uppers.

Maybe 1 lower per two-three uppers if you must.

Personally I like a complete rifle for each.

I'd run out of $ before getting all that done!  Hard to keep up with ammo, reloading supplies, and dies for all that stuff!  I like the idea though.....

Don't some conversions require a different hammer?  I thought 9mm's require a hammer change, for instance.

thats kinda what i was thinking.
i am unsure how the different mags will fit in the lower.
mag conversions? also would i have to use special mags or could i use the mags i have for my ak or 10/22, etc. in the ar?
what are some good places to find the different uppers? and
i would love to have spare lowers but if i were to have a lower for every upper it kinda defeats the point of interchangeable uppers. i am just thinking it would be great to have a large briefcase with several different setups and be able to sit at the range and just swap them out. my ak would be collecting dust then.

The problem with your logic is that uppers are like rabbits.  After just a little while sitting next to each other in a dark & romantic gunsafe - bam - they miraculously grow lowers.

i am sure they do. i am sure mine will be no different, but i am getting what i can, a lower receiver and a few lower kits and as many uppers as i can before bush is out cause i bet if we  
get a democrat we can kiss our "assault" rifles goodbye. but anyways...

i am just looking for some good places to get different caliber uppers and where to get the different caliber mags to fit the ar or how to make them fit.

Quoted: Quoted: You can have 1 lower. And then seperate "complete" uppers for just about anything you can dream of.   When you have another COMPLETE upper including iron sights or scope you simply push 2 pins out, swap uppers, push 2 pins in, grab relative mag and load up.  As long as you sighted that new COMPLETE uppen in it will still be zeroed. Just a few of the AR calibers off the top of my head: 17....... 22lr 204 ruger 221 swift 223/5.56 243 6.8 300 whisper 300 fireball 308 50 bmg 9mm 40 45 50 beowulf Just to name a few... And a few more.. 5.45 6.5 Grendal 450 Bushmaster 458 Socom 499 Litnerwise

7.62x25 Tok
.338 Spectre

Quoted: Don't some conversions require a different hammer?  I thought 9mm's require a hammer change, for instance.

The colt style 9mm uppers do not function with a notched hammer (though ramping the 9mm bolt may correct that).  The standard DPMS AR hammer will work with the colt style 9mm uppers, since the DPMS hammer does not have the notch.

now do thses converstion, complete uppers, fit any standard ar lower? also i don't see anything about a 308 or a 30-06. my co-worker is willing to sell me a bushmaster, the model escapes me but it is like a 1200-1300 rifle new, and he has a nice scope and uprdade stock and what not with only 500 rounds or so through it for $700. now that is the cheapest i've seen. anyway this lower should fit all the cal. conversions mention previsouly in this post?

.308Win and .30-06 are too long for the AR magwell.  There are a few .308Win rifles similar to the AR-15, but they use a larger upper and lower receiver (won't work with standard AR lower).  

The .50BMG upper (the only one that I've seen, at least) is a single shot bolt action upper, so the size of the magwell is not an issue.

.45acp uppers can be a bit of a problem with finding a magazine that will work with a standard AR lower.  The easiest option is probably a CAV MkII lower with the Hahn adapter to use M-3 Greasegun mags in that lower.

As far as I know, the other calibers listed will work with a standard AR lower, but some will require an adapter for the required mag to fit the AR magwell.