Several years back, I built a 9mm AR upper. In that time, I've learned a bunch about some of the quirks with them. Ramped bolts very heavy buffers and springs Receiver pin holes egging, and pins breaking. In short, I've learned several things that helps the guns do better that I'm going to share to maybe help someone else. I admit that I don't have any hands on with the new glock lower variant and while interested, I'm pretty content with the setup that I am running now and see no real reason to change anything. Especially since I don't own a glock and I have a few uzi mags since I have 3 guns that share mags. The bolts on the glock kits have a more narrow bottom shelf to fit between the glock magazine feedlips. This much I know. And to my knowledge, there are a few "hybrid" bolts that will work with either style upper, but I'm not getting into the critical details and differences of the 3 major types of 9mm AR's. (Colt style, Olympic Arms variant, and the newer Glock style). But for this topic, I'm going to concentrate on what I have done to my Colt style conversion to maybe help someone who's considering building one to help them save a lot of headache to give a few options they may want to consider. The main reason why I went with a Colt style was it was the most common at the time. I appreciated the fact that I can use several different kind of magazines in it. And as we all know, a magazine will make or break the gun and can mean the difference between one working like it's supposed to or a jam-o-matic. MAGAZINES There are Colts/Metalform magazines (which have proven themselves as good quality and are generally held in high regard). Second up, is the IMI 9mm Tavor magazines. I think these are just as good of quality as the Colts, and they are about $8-$10 cheaper. IMI stuff is all good stuff. The Israeli's don't screw around about the quality of their stuff. And like the Colts magazines, have a bolt hold open follower. Next are modified uzi magazines. These are also well known good quality but don't lock the bolt back on the last round. And standard uzi mags will also need a notch milled in them to accommodate the AR mag catch. * The IMI Tavor 9mm mags I mentioned earlier do have a bolt hold open follower and are ready to go in an AR9 out of the box and why I mentioned them first. And there are other numerous "average" quality mags out there too. ASC, Cproducts, Promag and I'm sure there are probably some others too that have slipped my mind over the years. RAMPED BOLTS These are the most ideal due to reducing the amount of force exerted on the hammer because 9mm blowback is a lot worse in an AR than 556 and other rifle calibers. Reducing the initial pressure on the hammer also reduces the pressure exerted on the hammer pin, the receiver pin holes (egging out the aluminum) and feels better when shooting it too. It is not too uncommon to hear of a broken hammer pin, especially with incompatible parts combos. KNS PINS I like to use Anti-rotational pins in my 9mm lower. For one, they are stainless steel, and for the other reason, the pressure is distributed on both the hammer and trigger pin holes rather than the brunt of the force being on the hammer only. BUFFERS and SPRINGS There are several different combinations that you can use. I'm not going to pretend that one is "the best" over the others. But here is why I am listing these two first. The 9mm AR bolt doesn't have to cycle as far rearward as a rifle due to the length of the cartridge case itself. So, to reduce the amount of travel that the bolt has to go, and also to reduce the possibility of the bolt breaking the bolt catch because it has a massive run-n-go that isn't necessary, I recommend the Spikes Tactical ST or the New Frontier Armory Extra Heavy buffer Both of the buffers I mentioned above are longer than a regular 9mm carbine buffer which reduces the amount of unnecessary movement that I mentioned earlier, and is heavier than a standard 9mm buffer. I think it's pretty evident why I think it's the best option for anyone who is just starting their build. For those of us who don't want change their buffers, it's best to use the heavy 9mm buffers that exist, but in order to reduce the travel, you can add enough quarters in the rear of the buffer tube to take up the unnecessary space in there behind the buffer spring, or you can install an aftermarket buffer like the New Frontier Buffer Spacer or Spikes ST-9BS. Another option for those of us running standard 9mm buffers and buffer spacers is to add a +power buffer spring from Wolffe or Springco Naturally, those options all take will all need tuning to your setup. MAGAZINE BLOCKS In order to use 9mm mags in a standard AR15 lower, you have to use a magazine block. Or, a dedicated 9mm lower receiver. But since I'm using a 556 lower, obviously I had to use a conversion magazine block. Prices range from ~$50 for the plastic promag mag block up to ~$200 for the Hahn mag blocks. There are several other brands that are priced in the $100-$150 range. The magazine block that I have was made by AMM, but to my knowledge, they are no longer selling them. Mine was a top load model, and would occasionally want to "move around" inside my magwell with use or external pressure. Especially when "smacking" the mags in making sure they're seated properly. So, I went a step further and "affixed" it to my lower via a long roll pin that went through the standard magwell and through the mag block itself and also used a set screw that I put in the front of my magwell that is hidden behind the front pivot pin lug. In order for it to come out now, the upper has to be off of the lower to access it because the lug would prevent it from moving while assembled. I just never fully trusted the tensioning worm screws would never move at the worst possible time. Especially since they did. This was more for reassurance that everything is going to remain exactly the same place with no risk that the mag block is going to change positions, which could cause feeding problems. This is just one of the ways that I go above and beyond "commercial standards" and when I make something that I may have to depend on for life or death, I don't want any shortcuts in my stuff. MAG CATCH Another common occurrence with 9mm conversions in a standard lower, is often times the magazine catch is often "too short" to hold some 9mm mags in the gun. They'll occasionally just drop out with no rhyme or reason. There are a few things that you may do to remedy that. Screwing the mag catch and the mag catch button as far together as you can. That may help a little. You may can weld the tab of the mag catch and file it down so it doesn't grab on the follower or spring as you're shooting to give it more surface area, or Spikes Tactical and RRA do offer a 9mm mag catch that is profiled a little differently than a standard AR mag catch and the thinner profile allows them to protrude a little deeper in the mag body. Since I have already typed more than I had planned to, I've not added any specific photos of commercial products, but I did highlight some of the key words for you to be able to research for yourself if you're interested.