I have a S&W mp15t on order. Is there some way to convert it to 9mm for plinking and still be able to switch it back to 5.56 when im done? If so what would I need? Im assuming I would need a different upper but can you use the same lower? Please educate me. If it would be cheaper to just buy a 9mm ar please tell me. Thank you.

You need a new upper, including a 9mm bolt carrier group. Because the 9mm mags are skinnier, you would also have to have a mag block. You will also have to have a DPMS hammer, and you will need to have your 9mm bolt ramped.
I almost did this setup, but when I realized how expensive it was all going to be, I realized I could build a suppressed SBR dedicated .22. So...that is what I have now. I figured "do I really NEED a 9mm?" MY answer was no. But damn, the 9mm's sure look good don't they?
If you are looking for a caliber for "plinking" just buy a CMMG .22 bolt and black dog magazine, all of which will run you less than $200.00.
Just my $0.2

It's more sensible to buy a 9mm complete. Why not buy a 22 and plink for next to nothing.
Check this link: http://www.ar15.com/forums/topic.html?b=3&f=15&t=441890

Spec

You can make your .223 AR convert to 9mm and them back. However, you may find its not so cheap as you might wish for.

Certainly you need separate complete uppers and these run about $500 give or take. You need a good mag well adapter for the 9mm (about $175) and then there is the hammer and buffer. There is debate on just how to do this, either a dedicated 9mm hammer or the use of certain .223 hammers with a ramped bolt on your 9mm. The bugger should probably be changed, another $25 or so.

From that is looks like you need $200 $250  in parts to convert you lower to 9mm. Oh, then the magazines, you will want a few of these and they are $20 to $40. So, if you start with a .223 AR you should plan on spending at least $800 to $900 more to convert back and forth. Also there is the work, changing out the magwell adapter and the buffer and hammer each time you want to switch.

If you shop around you can get a complete new 9mm CMMG or RRA for not much more. Do this and you will have two complete rifles. Its a better deal in  my book, but then I like more rifles. I built a RRA 9mm only because I wanted a matching set of rifles with consecutive  serial numbers and had the parts to do so. If I had it to do all over I would just buy a 9mm rifle and be done with it.

9mm AR's are the Cat's Ass and sweet to shoot and I have no regrets. I have  a 9mm, a .22, and a few 5.56/.223  ARs and love them all. If you can afford it, just go for a complete 9mm factory set-up.

1 lower, multiple uppers (556 and 9mm) -

With the Olympic 9mm system, all you need is a mag block and their upper/bolt....



With the Colt/RRA system, you need to change the hammer, too....



 

I went to the Olympic system for my 9mm and .45acp AR's, and it's definately the way to go.  There's no swapping hammers, and they are 100% reliable in both semi and full-auto, unlike the Colts I've owned in the past.

And, you don't need a magwell block if you convert some cheap STEN mags over.  Just weld a piece of square tubing to the back and mill them for the standard .223 mag catch.  The problem I had with magwell adapters is that I kept trying to drop the mag with the standard mag release, since that's how I've done it thousands of times before.  If you have a magwell adapter which is held in place by the .223 mag catch, both your mag and adapter are hitting the ground.

Just get a no-notch hammer, I have them in all 3 of my ARs. 9mm, .22, and 5.56. They cost nothing too if you don't already have one.


I, like several others have trimmed the mag relase button to keep this from happening. I understand that it's not convenient if you want to switch over, but by the time you buy a 9mm upper, mag block, and buffer, you can easily save up and fork over another $225 for another lower. It really is just easier setting up another rifle for dedicated 9mm and getting it to work the way you want rather than trying to save money by welding up magazines that may or may not work depending on how well they were welded. You also have to take into account how high you mount the mag on the built in block. Plus, no pouches for them if you want pouches for them.

Take my advice for what it's worth, but who wouldn't want a sexy dedicated beast like this in their safe?

If you go with an Oly 9mm upper,
Sten mags, and a mag block, you
can use a bone stock lower and
switch back to your 5.56 upper
in less than a minute.

John

Easiest way would be to use a VM Hytech mag block.

+1

While the cost of unmodified Uzi mags is undoubtedly cheaper than the Colt style mag offerings, any bottom loading block will suffice if one is after "quick change" capabilities from a caliber standpoint.

Some iterations of the VM HyTech blocks have also been finicky with certain types of Uzi mags.

In the end, the cost of lowers is relatively minimal in the grand scheme of things ($100 lower, $50-60 LPK, $20-100 stock).  Most users will eventually migrate to having a lower for every upper (except for those that require tax stamps for their lowers, of course) because that is truly the EASIEST way.

Using a Top Load Magblock is pretty quick too since it just pops out while you switch uppers.

I run a RRA ramped bolt from Adco, and a DPMS "sporter" hammer. I can pull my Hahn topload block and switch back to a 5.56 upper in about two minutes or less. I was already running a 9mm buffer with my 7.5" 5.56 upper so it's not an issue.



ETA: another 30 seconds to drop in a CMMG 22 kit and that's three calibers with no real effort.

The way my 9mm is setup, I can switch to 5.56 in a matter of seconds.

I have a VM Hy-Tech mag block (never had any issues with mag fit and I've got quite a few Uzi mags)
I run a DPMS "no-notch" hammer
I've got an Ace ARFX rifle length skeleton stock with a rifle buffer (almost the same weight as a 9mm buffer)

So let's say I want to switch from 9mm to 5.56.................

-Push out take-down pins
-Remove 9mm upper
-Remove VM Hy-Tech mag block
-Put on 5.56 upper
-Insert 5.56 mag
-Enjoy!

It doesn't get much easier than this.  I did swap back and forth at the range once.  Just to make sure everything functioned as it should when swapping.  It did.

So while I don't swap calibers (my 9mm just stays as a 9mm), it's nice to know I can if I have/want to.  

It's also nice to know that when I get out of NY, and get an SBR stamp for one of my lowers, that I can get a 5.56 SBR upper and 9mm SBR upper and run them on the same registered SBR lower.