I have an SP1 M16 clone waiting for a stamp, and a 6920.

I would like a 9mm upper for both.  But I've heard that they tend to chew up your lower.  Or that they egg out the holes.  How can that be avoided?

I generally tend to go with Colt brand, but I'm open to good quality brands.

I have a crapload of UZI mags, so using them would be a plus.

I have had an AR15 of one kind or the other for 20+ years I guess, but I've never really customized or done any gunsmithing.  So my knowledge level on that aspect is poor.

Thanks

I just saw there is a pistol caliber forum.  Derp.

You'll want a dedicated lower for the 9mm. You can't just swap uppers, too many differences.

Yes, you can do a straight swap with the following 2 options:
1.  CMMG Banshee RDB (Radially Delayed Blowback) upper
2.  MEAN Arms Bearing Delay Blowback upper

I own both and in my experience they both have failure to eject issues.  Option #1 is the lowest cost if you just buy the bolt / BCG combo and if you just keep an eye on the ejector spring health, it is fine.
I have this documented on my site here: https://c3junkie.com/?page_id=221

Two more options that also use a delayed action w/ a more robust ejector in the upper....but use a compact action so you must use the provided picatinny back plate on your lower installed to keep the recoil spring guts from falling out are:
1.  Angstadt Arms Roller Delayed Blowback upper - I tested one of these till one of the proprietary rollers sheared apart and damaged the proprietary locking piece.
2.  CMMG Dissent RDB - this one works very well but not tune-able anymore unless you do custom machining like I did.  The custom machining I did also allows it to drop right on a standard lower.  https://c3junkie.com/?page_id=1733

Fortunately, that isn't the case anymore.  I egged out the hammer pin holes on a full auto transferrable Colt SP1 back in the day shooting straight blowback 9mm.
I have all the current full auto 9mm M16 options on my website here:
https://c3junkie.com/?page_id=361
Option2 (Maxim roller delayed buffer) at my link above may work for the OP if he wants to keep the 6920 stock but he would have to deal with using a mag block with the fixed ejector or as discussed on my site use the MEAN Arms mags which have the ejector on the mag which I do NOT like that at all since you must leave the mag in the gun to clear a live round.

Regarding UZI mags, the old VM Hytech block uses unmodified UZI mags but is no longer made: https://thehebrewhammer.net/2015/02/02/vm-hy-tech-ar-15-uzi-mag-adapter-review/

Regarding the CMMG RDB, I was never impressed with it in it's stock form.
High speed video I took of a stock setup:


High speed video I did of my OLD tuned CMMG RDB below:
Details of that build here: https://c3junkie.com/?page_id=280


I haven't done a high speed video of my current hybrid CMMG Dissent build yet but it is smoother than my OLD CMMG RDB config.  I discuss details of my hybrid build in my previous post.
Video of my Phase I and Phase II of my hybrid build in full auto:


I would have no concerns running my OLD tuned CMMG RDB or obviously my current build on a full auto transferrable lower.

Maxim Defense delayed buffer is the answer.

For a Colt setup just use a STD lower with mag adapter. Many adapters out. Mine ran fine with no issues. I regret selling the adapter and mags.

I built a dedicated 16" 9mm race gun, which is great, but my SBR lower was fun shooting 9mm out of a 5".

Since you're starting from scratch, I'd recommend avoiding a straight blowback setup. There are many articles and posts from guys who painstakingly developed some special formula to produce a 9mm AR that runs as it should. It doesn't need to be that hard.

Of course, I am an sample of one, but I completely abandoned the idea of a straight blowback 9mm. Having to mess with ultra heavy buffers and springs when all you want to do is swap uppers is a PITA. Besides, a blowback setup will never run as smooth as a delayed setup.

My goal was to have a 9mm upper that I could literally throw on top of any lower that I had and run it. I found a CMMG radial delayed blowback barrel and bolt on sale for $325. I threw that into a spare upper that I had and used Endomag inserts. It runs very well with a standard spring and either an H or standard buffer. It's literally an upper swap when I want to run it.  This was the best way I found for a smooth and easy swap to 9mm without going to a dedicated lower.

I'd be OK with semi-dedicating the FA lower to 9mm if I could also use it for 22LR.

I actually only bought the converted SP1 because someone let me shoot their suppressed FA 10-22 and it was a hoot.  I looked at the prices for FA 10/22s and it's a joke.

I read through the answers and the links everyone provided here and it seems like the AR is way less friendly to 9mm than I thought it would be. But not impossible to achieve.

The nice thing is how much energy that people put into modding the AR platform.  There are 1 million experts out there and an infinity of parts and gizmos.   (Actually, the infinity of parts is kind of slowing me down.   When I went to ask about a 22 upper, I found that basically every single guy in the USA is running some different variation.  That is confusing.)

I’m currently assembling this recipe and plan on using standard AR mags with the CMMG adapters.
Link

I built a 9mm SBR back in 2007 and used a standard AR lower with a mag well adapter. Like others have said, it's hell on the lower. I used KNS pins from the start so as far as I can tell I don't have egged out pin holes. But it will beat the hell out of lower parts, especially the disconnector. And when the disconnector gets worn it does things you don't want it to do. BBBBBRRRRRRTTTTTT. Scared the hell out of me the first time it did it.

At first it was terribly unreliable and would malfunction multiple times each magazine. I did some reading here and determined that the bolt needed to be ramped, and once I got that done it's been 100% reliable.

Other than that, I don't regret a 9mm AR at all. It's one of the most fun ARs I have, and it's more accurate at 100 yards than I thought a 9mm could be.

Damn double tap.

Heavy buffer, KNS pins, ramped bolt, and Colt SMG drop-in mag block should do it.
I think a lot of the lower wear was from light buffers, and especially the unramped bolts.