Decided I want to get into the AR 9mm game. I've got a few extra AR-15 lowers in the safe. My question is is it better or easier to build one or just buy a complete rifle? What are the watch outs when building a 9mm AR. I do know I want to have it work for Glock mags. It will also be a rifle as well as I'm not wanting to deal with the pistol or SBR situation.

Any advice is appreciated!

As you are describing, probably buy. Here is why...

If you want it set up for Glock, regular AR lower won't work, you have to have a dedicated lower. In addition, building them requires picking the right parts and hoping everything works together. It also seemed Glock models are a little more finicky about running properly and having a last round bolt hold open. They also seem to be picky about the brand of magazines on occasion.

Due to these reasons, if I was in the market for a Glock style AR, I would just by from a good company so that you have a level of support in case there are any issues.

Supposedly there are glock magazine adapters coming out. Though we'll see if it's vapor ware or not.

http://www.thefirearmblog.com/blog/2017/01/25/torkmag-20rd-glock-mags-ar-lower-conversion-kits-shot-2017/

MSRP $70 sounds reasonable.

Look into Angstadt Arms rifles. High quality, runs like a sewing machine. Love mine. Also, Wilson combat has one and I believe Nordic has one coming out.

If you buy complete, you pay FET, but you usually get a warranty on the complete gun.

If you buy parts (full or stripped lower, complete upper or assemble your own, and so on), you don't pay the FET on a complete gun and can save some money.  You may have to do some of your own troubleshooting, but if you stick with quality parts, there is usually little to none.

A lot of people like/want a specific look or whatever, which can be hard to find in a complete gun.  Choose whatever option is best for you...

I'm in the middle of a 9mm build right now. 

Make sure you fully research before purchasing parts, or you will end up buying parts you don't need, and having to go back and pay double or triple shipping for parts you should have ordered the first time.

Here are the important things to know, that I didn't know going in:

A glock mag lower requires a special upper to trip the bolt catch when the magazine runs dry. Without the special upper, you will have to rack the charging handle every time you shoot a mag dry. A dedicated, side charging glock mag upper costs about 6-7 times as much as a stripped flat top upper. 

Some 9mm barrel manufacturers use 1/2"-36 threads on the barrel instead of the standard 1/2"-28. If you get a barrel like this, your selection of muzzle devices is going to be limited, and comparatively expensive. Expect to pay double or triple for something like an A2 flash hider with this thread pitch.

A standard carbine buffer is both too light, and too short to use in a 9mm build. You will need to use a longer, heavier buffer specifically designed for 9mm. Or, you can use an H3 carbine buffer that has 3 tungsten weights, and a spacer inside the back of the buffer tube. (Some people use a stack of quarters. I learned that yesterday.) An H3 buffer weighs a little more than 5oz, and a dedicated 9mm buffer is heavier, many in the 7oz to 8oz range depending on manufacturer.

The Shockwave blade is WAY cheaper than the Sig brace, if you want a pistol brace. I think it looks better too. Though the sig brace is probably more comfortable to use.

 Don't waste your money buying a "pistol buffer tube kit" because while the tube will work, the buffer itself is just added cost because it is too short and light.

Dedicated 9mm uppers will not fit in typical receiver vise blocks for mounting the barrel and torquing the barrel nut. So you might need to improvise a way to hold the receiver firm while torquing the nut. 

The reason you need a heavier buffer is the blowback operation needs to be delayed a bit with more inertia. The reason you need a longer buffer is so that on the last round, the bolt doesn't travel too far rearward and come crashing forward into the bolt catch with a lot of energy. 

And finally, depending on your lower, some cheaper aftermarket glock magazines might need to be sanded a little to get them to drop free. So if you are buying it at a brick and mortar store, check the fit of the magazines before purchasing. (Also a good idea to double check the muzzle brake or flash hider fits your barrel threads.)

If I had it to do over again, knowing what I know now, I would buy a complete 9mm upper and just build the lower. You do pay a little more, but a lot of parts sourcing and fitment is done for you. And this especially holds true if it is your first build and you don't have the tools you need to build ARs.

Good advice. For the BOLD part and in general I would pick up this awesome thing! I have put together 556, 22lr, and a buddies 9mm ar on it, no issues with any caliber

My glock mag lower has lrbho built into it. I believe the PSA lower does too. Mine works great. I have a 5.5 ounce buffer,carbine spring and spacer. Also I have a 1/2x36 barrel and bought a A2 flash hider for less than $20. I ended up installing an SLR ti mini comp and it works. It is a puzzle if you mix and match parts. If I had it to do over I would have bought a complete rifle and skipped all the headaches. Lot of experiance around here. Just read and read some more. It helped me out big time. Good luck

Just a few counterpoints to your comments - no disrespect intended, just wanted to offer additional info - anyone is free to take them for what they are worth or disregard them.

If one makes a mistake and have to pay an extra $5, or $10, or even $20+ for shipping, it isn't that big of a deal.  Every time one goes shooting, they are literally burning money, so what's a few bucks here or there?  I am a consummate cheap ass, but even I don't worry if I miss something on an order and have to make another order and pay separate shipping (shit happens sometimes).

The glock lower deal is kind of like the old days of the Colt/Oly comparison, some have specific part/component requirements and others don't.  All types are out there - just pick the system you want to go with and know its strengths/weaknesses.

Buffers - the AR9 works just fine with standard length buffers.  The standard weight is ~5 oz and adding quarters/spacer to take up excess bolt overtravel is a cheap solution.  The longer and heavier buffers are just alternate solutions to address known issues in a different way.  Specialty buffers are not a requirement for most builds.  If one wants to buy a "stock kit", one can always find a place that offers buffer options/substitutions (KAK and others offer this - just check around)

Upper blocks - they make different types - some clamp around the upper and will only work with uppers that fit within them - others fit up into the upper from below and pin to it so the exterior of the upper doesn't really matter.

Thread pitch - I don't personally find this to be a big deal, but I know for others, it is a seemingly a life or death type issue.  I wouldn't purposely pick a barrel because of thread pitch.  There is a decent assortment of muzzle devices in 1/2x36 and the prices aren't unreasonable.  An A1/A2 in 1/2x28 is $5-10 and in 1/2x36 it is maybe $15-20.  For 1 or 2 builds, that difference is not a big deal (see my shipping comment above).  If I were building a dozen or more uppers, then I might opt for the lower cost option.  If I wanted suppressor compatibility with a specific thread pitch or muzzle device, then a specific thread pitch might be a higher priority.

As far as cheap aftermarket mags, I'm not going to comment as some people like to buy cheap shit and tinker and if they can get it to work 90-99%, they are okay with it.  If one wants to mess with any cheap shit and mod said parts or their guns to get things to work, more power to them.  I'd rather have fewer quality items rather than more cheap/shitty items, but that's just me.

PSA males two different glock lowers. The original didn't have lrbho, the 2nd gen one does

I bought one when they first started making and selling them on ebay

In my case, I'm going to end up paying probably $400 more than I really needed to, because of lack of research on my part. Just trying to save other folks the frustration. 

The shipping in my case was also because tracking down the parts I wanted required shopping at three different online dealers (and having to order twice from one because I forgot to add something to my cart). 

I ordered a side charging, LRBHO upper after already buying a cheap flat top upper. It will probably get used on another 5.56 build some day. I've been wanting a lightweight pencil barrel upper so while it was money I would rather not have spent, it won't go to waste. I had 3 H buffers lying around, and assembled an H3 by taking the tungsten weights out of them and combining them in one. But decided to order a 9mm specific long buffer anyway. 

I also learned that the SIG brace needs a 1" to 1.2" buffer tube O.D. The buffer tube I bought has a 1.25" O.D.  It will fit, but it will be like the first time with my high school girlfriend. 

For the upper, I'll just clamp it in the vise with some wooden blocks on the top rail and the bottom, with the plastic filler inside. No reason that shouldn't work with a flat top upper. I've done builds like that before I had vise blocks and didn't get any warping or distortion. 

I would have preferred to have 1/2"-28 threads, just because the HPA might pass and it would be fun to play with suppressors without having to special order adapters. I do have a lathe, but single point cutting internal threads in a 1/2" bore isn't something I'm equipped to do. Looking at 1/2"-36 taps, they are more costly than just buying an adapter. 

That's a pretty big nut.  You can do what I do when that happens.  Save all your spare parts to use for another build or whatever - when you get the itch to do the next build, order more parts - while assembling them, remember that you already had some of the parts you just received - check spare parts bin to confirm - make a mental note to definitely use those spare parts on next build...

Yep. It will all get used eventually on a new build, or it will go into the "big box of shit to bring to the next local match to see if someone wants to buy it". 

I have several big quad free float rails that have been made pretty much worthless by the new trend towards slim keymod and M lock tubes. But the new rails are cheap enough, I guess if you don't care about having a specific brand. I just haven't had the time or inclination to change them out for the latest fads. I definitely prefer the slimmer, lighter ones. 

I might just do a 5.56 pistol with the leftovers from this. I have a regular lower and LPK lying around, the flat top upper, buffer spring and buffer, a few hand grips, and a BCG. All I would need to complete it would be a barrel, pistol buffer tube, castle nut and another slim free float tube. Then just whatever sights and optic I want on it. 

I'm slooowwwly building my 9mm ar-15.  Starting with a QC 10 colt style lower.  I've got other lower parts to almost complete the lower but my problem is that I keep falling in love with expensive crap ( jp silent capture spring, factory QC 10 bolt, side charging upper, pdw stock, etc, etc).
Sometimes I think I could've gotten off way cheaper buying a cz skorpion or sig mpx.  My only other pro to doing a build is that I can buy a part here and there when it's on sale or when I scrounge up a few bucks.

Right now you can buy complete much much cheaper than you can build your own.  9mm gets pretty expensive building from scratch.  I've seen complete 9mm pistols sub $500 lately (PSA) and my attempt at a cheap build this past Black Friday still ended up at $1K plus

Magazines in the long run would be a big factor for me. Also factor in a shockwave brace.
NFA Side Charging Upper 235
New Frontier 5" Rail $110
Ballistic Advantage 5.5" Barrel $105
KAK Flash Can Micro $20
Recoil Guns Glock lower $150 before FFL Fee.
Strike Industries Takedown Pins $15
Quality Milspec Trigger $40
Trigger Pins $5
Detents/Springs $2 Tops
Castle Nut $3
DSG Arms QD End Plate $10
Kaw Valley BCG $130
Kaw Valley 7.5" Buffer $40
Kaw Valley Buffer Spring $5
Shockwave Tube $25
Shockwave Brace $50

Yeah overall it can come out to about $945. Personally I think the rail is meh, and I would of got something else if I was going for a 7" or longer.

If you already have glock magazines though its well worth it.

Obviously though it can be made cheaper with other parts.

I don't feel so bad then if someone else ran close to $1k on a build. 

Here's my price break down before I knew WTF I was getting into:

9mm barrel $99
Upper receiver $55 won't need this
Upper parts kit $25 won't need this
9mm BCG $120
2 ct 17rd. mags $30
1 ct 33rd mag $25
FF handguard $56
Buffer tube kit $40
Billet glock mag lower $166
Retail tax $51

Then I ordered the following after some research:

9mm buffer + S&H $47
YHM flash hider 1/2-36 threads + S&H $46
LPK + S&H $34
Side charging LRBHO upper + S&H $239

I already had a hand grip so that cost me nothing. And I had a charging handle already, so that cost me nothing (if I was going to use the standard upper receiver).

The total I spent to build it the way I want it so far is $953 plus or minus some change. And this is without buying "good" parts like a nice LPK, for example.

That's not including the parts I bought but didn't need to. I spent another $80 on the standard upper receiver and parts kit (which I will use to finish a build one day, so they won't go to waste). And I still need to buy sights and/or an optic and might do an arm brace. Or I could spend another $200 and wait forever to SBR it. I sure hope it runs reliably after all that expense!  

I definitely should have just bought a complete 9mm pistol. I got in the mindset that I wanted to do a build because I had a stripped lower already, but then I learned that I would be better off buying a dedicated lower since I wanted to use glock mags, so I bought a glock mag lower and then I just started buying parts I thought I would need with no real research into it. I should have priced complete pistols compared to the parts to build one.

The most expensive bit is getting LRBHO. If you can live without that, you can build it cheaper. But still probably not as cheap as a complete pistol with LRBHO. 

My build experience seems to have differed quite a bit from others in this thread. I wanted to put the gun together to see what I did and didn't like before spending a ton on parts. I wasn't sure if I wanted to go side charger, what barrel length, etc.

I bought the PSA complete Glock lower as a blem for $229.00
BA 11" barrel for $125
Spinta Precision 9mm bolt (blem) $109.00

I used an upper I had laying around along with a delta ring assembly, carbine handguards and end cap, CH,  and a FSB that was drilled for set screw mounting. I drilled an A2 flash hinder out for 9mm ( this would basically just be a thread protector since its usually suppressed)

Since I built it I've decided against going with a side charging upper so I will probably order the Spinta billet slickside upper for $100

Once I figure out what barrel length I want I will either keep the one I have as-is, get it cut down, or sell and buy another.

I settled on keeping the PSA buffer that came with the lower and using a delrin spacer. I'm using a Tubb .308 flatwire spring.

Once I buy the Spinta upper I'll have around $600 in it. $800 if you count the tax stamp

ETA: forgot I put a POF trigger in it. I forget what that cost but it works really nice

I think some of you guys may be overthinking the AR9 build vs buy argument.

1. If a bottom of the barrel clearance/blem deal for $5-600 is exactly what you want, then buy that.
2. If you can build the same thing for the same price, than build it.
3. If you are going to build the exact same thing, but it will cost you more than $5-600, than see 1 or 2.
4. If you want to build something specific that is not offered as a complete upper, complete lower, or complete gun, than you pretty much have 2 remaining options
4a. buy something complete and modify it
4b. build exactly what you want
5. if you don't like any of the above, there is always the option to build/buy something other than an AR9

There are so many variables that can drive cost when building an AR9.  On average, I have $750-1,000 into each of mine.  I like RRA 9mm uppper and they tend to cost more than std uppers.  I like better furniture which tends to cost more than std furniture.  I prefer Hahn mag blocks which costs more than a dedicated upper.  If I started to add $100-300+ custom triggers, $200-400 high end rails, $100 charging handles, $200 BUIS, $50-100 sling, or other expensive parts, it would be easy to get them up to the $1,500-2,000 range.

From a cost only perspective, the last thing I would do though is compare what I built to a $500 clearance/blem gun that isn't even close to being the same thing.  That would be like comparing a <$200 hipoint to a $1,000 HK45 and getting upset that I overpaid for the HK, because they are both pistols and basically the same thing.

I ended up making some last minute orders today.

I decided to go with a two-tone look. I ordered an FDE Mlock handguard, FDE MBUS sights, FDE MOE+ grip, and an FDE Shockwave.

I also ordered a Holosun sight. Hopefully it will hold up well (for range toy use).

I'll have enough leftovers to start another pistol build (probably in 5.56) and put some upgrade parts on some existing rifles. So even though I way overspent, I will make use of the extras. 

It looks like I am a little late but here is my two cents worth.

If you want a good inexpensive rifle you should build one.  Here is what I did:

PSA dedicated COLT lower receiver (blem if you can get one).  
Colt feed ramp, Colt style ejector and LRBHO latch (all available from CMMG)
9mm buffer and carbine spring
Remaining lower parts can be any AR lower parts.
I used a Timney single stage trigger (some aftermarket triggers will not work, Mil Spec triggers do work)
PSA G-9 Hybrid 9mm bolt (uses CMMG firing pin and extractor)
Any upper (I used a slick sided Aero Precision)
Double Star Free Float tube (about $40)
Any 16" 9mm barrel (I used a PSA - only $99)

Most 9mm barrel manufacturers are switching to 1/2-28 threads.  There are devices out there for 1/2-36 threads.

You should be able to build one for less than $700 using a Mil Spec trigger.

I chose the Colt route because they feed better (the Glock pattern does not use a feed ramp) and the steel stick mags are inexpensive.  The Colt LRBHO is way simpler than the Glock.  The PSA Hybrid G-9 bolt works very well.  I had to tweak the extractor and the ejector a bit.

I just finished converting mine to a left side charger.  I put a C-More on it and will use it for Steel Challenge this year.  USPSA also has a Pistol Caliber Carbine Class this year.

That's a slick looking build!

A lot of good information on here. I also was looking into building an AR-9, this answered a lot of questions and unknown possible mistakes of building vs. buying complete firearm.

That's a great looking build. Exactly what I'm going for and for the same use.