ATI Omni AR15 Polymer Lower – Unboxing and Initial Thoughts

I picked up a new ATI Omni AR15 polymer lower from by gun dealer yesterday, and I have some thoughts I want to share before I even head to the range.  Above is a video of the unboxing and my initial observations and concerns.  Below, I have more information and photos.


Mating an upper and lower has never been a problem for me – well, before now.  I have several AR15 rifles in my safe and have access to dozens more at work.  Regardless of manufacturer, I’ve always been able to swap uppers and lowers without any difficulty.  This ATI Omni polymer lower is the first time I have ever run into any fitting problems.

I tried to attach a DelTon 5.56 upper to the ATI Omni lower, but I just could not get it to fit.  If I attached the front and tried to pivot the rear of the upper into place, it would not get past the upper portion of the lower where the stock screws in.  If I tried to align the upper starting at the rear, the front would not align properly.  I could not get this upper to work with the ATI Omni lower.

ATI Omni AR15 lower

I next attempted to mount a Rock River 5.56 upper to the Omni lower.  This time I had a little more success.  I was able to get both halves to align (mostly), but the rear takedown pin never would completely seat.  I’m not certain, but I believe the measurements on the lower are a little on the small size, and the rear pin is angled ever-so-slightly to prevent it from going straight through.

Before I take this onto the range, I will have to solve this problem.  I don’t plan on trying to shoot a rifle with one of the pins not properly seated.

Magazine Release

If you watch the video, you will see me have two magazine-related problems with the lower:  the mag well and the magazine release button.  I tried multiple magazines with the ATI Omni lower:  Brownells, Magpul, Tapco, EMA, Tango Down and some other generic metal mags.  Some of them would not drop free, even when weighted down with loaded rounds.  I suspect that with repeated use, the interior walls of the magazine well would wear down and the mags would all drop free.

The magazine release button would occasionally be extremely difficult to depress.  You also see that on the video.  I’m not sure on what may be causing that.  Needless to say, it is an annoying issue.

ATI Omni AR15 polymer lower

Bolt Release

The standard bolt release, like the magazine release button, is a bit tough to operate.  Hitting it with your thumb isn’t always enough, and it sometimes takes multiple palm strikes to get the job done.  Is this a problem of the upper and lower not matching perfectly?  Maybe.  I hope to figure it out.

ATI Omni AR15 polymer lower

Depending on my schedule, I hope to get to the range next week with the ATI Omni AR15 polymer lower.  Maybe I can get a few of these problems worked out after a few hundred rounds of 5.56.  We will definitely see!

Let me know your thoughts on what could be causing my problems, or if you have one of these receivers yourself.

Update:  Click here for the final review of the ATI Omni Lower.

ATI Omni AR15 polymer lower unboxed

ATI Omni AR15 polymer complete lower

ATI Omni AR15 lower

complete polymer AR15 lower from ATI Omni

Omni polymer AR15 lower from ATI


ATI Omni AR15 lower magwell

ATI Omni AR15 lower magwell

ATI Omni AR15 lower

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About Richard Johnson

Richard Johnson is a gun writer, police trainer and really bad joke teller. Check out his other writing in Combat Handguns, Guns & Weapons for Law Enforcement, on The Firearm Blog and at BlueSheepdog.


  1. Very distressing news indeed. I’ve been wondering for several years when a manufacturer would come out with a polymer lower to help reduce weight, and was very excited to hear about the few who have moved that way. The ATI product seemed to have promise and potential. Hopefully you’ll be able to locate the source of problem with the upper-lower connection problem, and the other problems work themselves out with use.

  2. I have two of them. Haven’t gotten to the range yet but I tried both lowers with both of my uppers with no issues. They were a little tight going on, but otherwise fine. I have a Spikes barreled upper and a GI surplus Colt A1 barreled upper.

    • Tim,

      Thanks for the feedback. I’ve had several people contact me, and all of them saying that these lowers are really tight. Some have been able to fit an upper (like you) and some have had problems like I have had.

      Please let us know how they shoot after you get them out to the range.


      • I did finally get one of mine to the range with a DPMS upper half on it. The DPMS upper was tight and needed the pins tapped in but fit was certainly snug. I put 4 20rd mags through it for a function test with no malfunctions. Mag well is tight though. Mags need to be pulled free. I spoke with ATI at SHOT and they said some of teh early ones were a little tight. Recommended using sandpaper and sanding until mags drop free.

  3. Richard;

    I have a few of them and so far have fitted them onto Rock River, Daniel Defense, Stag, Norinco, Colt Canada, and Chiappa upper assemblies. The Chiappa .22 was the worst due to its polymer upper. We had to firmly tap the rear pin in. In all cases the fit was snug at least though we never had one that wouldn’t at least start the rear lug into the lower properly after the Pivot Pin was slid home. Several, especially initially, needed to be tapped into place and tapped out. After about 500 rounds of various types of ammo (.22LR, 9mm, and 5.56) it was better but still snug. I didn’t consider this to be a problem myself as the fit was really good without any of the rattles you often get with ARs. My only suggestion to ATI would be go to centered pivot and takedown pins so you can use a punch or round to knock the pins out. We did this in Canada on the C7 like AI did on the old AR10s from Holland. I’m not sure if anyone is doing it Stateside but it makes for a way more user friendly set of pins and I have had enough aluminium receiver sets need hammering apart to know this isn’t out of the realm of normal for either Military or semi auto pattern receivers. (I was lucky professionally to work in two plants that made M16s for the military.)

    Shooting the Omni gun was easy, it did everything it was supposed to do. I never had any mag issues but all mine are MilSpec from Colt, Colt Canada, FNMI or Thermold. One thing to note, if you had one of the “other guys” polymer mags with a ridge that rests up against the bottom of the mag well, they can bind in a number of non-Colt receivers as the bottom radius of the mag well is not defined closely on the M16 TDP. You can get a really wide variety of openings that all technically meet print.

    Speaking of the mag well I was able to fit and fire my Rock River 9mm adapter without issue. It went in and did what it was supposed to do and came out just fine. It was a more snug fit than a 7075 lower so I think you are right, the well is a little smaller than most aluminium receivers. If I can get some gauges I’ll check it for you. Your issues with the bolt catch are interesting. I am going to check that but I never had a problem and I recall getting shit from the cop i was with as he (correctly) pointed out you never use that in a life or death situation, you rack the charging handle. As an engineer I was interested to see if it bound up or caused enough drag to fail to chamber but it never had a problem on any of the uppers in any calibre. (My cop buddy was amused by engineers and our need to “test” stuff.) I only used normal thumb pressure and it worked fine for me.

    I have fit a Chip McCormick trigger into one and it went in fine. The small diameter pins fit through and the trigger didn’t bind on the fixed trigger guard. I also put MFT stocks on two and they fit fine and functioned as well.

    We are doing more shooting tomorrow but so far we are well over a 1000 rounds into 3 guns and they are running better and better. And for under $200 up here in Canada that is one heck of a deal! More news as we get it.


    • Hi Phil,

      Thanks for the detailed feedback! I know readers will appreciate it!


      • Phil O'Dell says:

        One thing you should look for is the new Hybrid. We got our first shipment in and have been doing some builds… they are IMPRESSIVE. All the areas that the original was shy in have been addressed. I have fond exactly one area of comment and that s when installing an ambit safety you might need to clear a little polymer if yours has the arrow on both sides. This is one awesome piece of engineering and is reputed to exceed the MilSpec test for buffer tube flex under load. Its built in a new plant and ATI has got a definite winner here.

  4. Richard…I haven’t gotten to the range with mine, either, but we had no trouble on the build. It was tight…Alan Samuel came up with a “generic” upper (OK, since we were fitting a TAC2 Discreet Carry Kit anyway), and I tossed in a bolt carrier I’ve had for a while. We tried it with the military mag and had with no problems. We’ll see at the range next week…

    Michael B

    • Hi Michael,

      I hope the range trip goes well. I’ve been getting feedback from people that have had the same problem I have. The solution each has found is to file back the rear portion of the receiver where the buffer tube mates. Once a little plastic is removed, then the uppers fit (still tight, but that’s ok).

      All of the range feedback I have gotten has been positive: once properly fitted, the guns seem to run perfectly.

      To Dremel, or not to Dremel…that is the question!


  5. I came across your site a couple weeks ago and have enjoyed your reviews, thank you for the time and effort you put into them.

    I purchased a Plum Crazy Firearms polymer lower a year ago and mated it to a DS Arms ZM-4 upper.
    The only fit problems I’ve had are with some surplus metal magazines. Some, not all, fit very tight while polymer mags fit fine. I’m hoping to put together either a 6.8 or .300 Blackout next, again with a polymer lower if possible. PCF used the Accutrigger on their lower and I’m very happy with it. I believe Plum Crazy has been aquired by New Frontier Armory in North Las Vegas, you might want to check them out.

    Good luck.


  6. Colonel Andy says:

    I also have been unable to get 2 lowers to match up with different uppers. The rear pin will not align with multiple uppers I have. I havent tried the release (can’t get upper/lower to mate to test) but havent tried the magazine release yet. I’ll keep you posted.

  7. Colonel Andy says:

    Got a reply from Jaime Ramos at ATI after asking about the tightness of these receivers:


    We have used a rubber mallet to fit the omni’s for the first time. If these lowers will not fit correctly we can get them in for inspection and repair of replacement.

    Jaime Ramos customer service rep.

    American Tactical Imports

    Phone #800-290-0065


  8. Donald Crispell says:

    I too have a fitting problem on the two lowers I have. the uppers I am using is a DPMS upper and DS Arms upper. Both have the fitting problem. I just used a dremel tool to fix the problem.

  9. Have been building AR’s for some time now and had trouble getting a stripped lower made of alum so I gave these ATI’s polymers a try. So far have been impressed with the overall quality, they have a pretty nice hard feel to them, very stroing no flex feel, not a soft plastic as I feared before the order. The the lower build kit (a PSA moe kit) installed with ease and all parts fit perfect. I installed the newly completed ATI lower on my PSA (PALMETTO STATE ARMORY) upper and it installed with a perpect fix, had a nice no wobble at all exact fit. Had to tap the take down pins in the first time, but overall am very impressed with the quality and even the color is a very good match with PSA upper alum receiver. Will buy again if I can find any more.

  10. Glad to see the comments and I appreciate the feedback.

    My company is offering two versions of the Omni in Canada since last I posted: one has a Head Down upper and we are waiting it to clear into Canada so more to follow but I also have 350 High Standard complete uppers (10.5″ to 16″) in hand and they plain LOVE each other. I am getting 3/4″ to 1/2″ 5 shot groups at 50m and the fit is very snug and NOT getting loose so far. As mentioned tap together the first 20-30 times and by then it is thumb press assemble and slight help to disassemble.

    The three trial units are all over 1000 rounds each and one did a week as a range rental and it is closer to 5000 with no problems. In fact the range bought 4 guns and 3 lowers for their store and as future renters after trying this one.

    My experience is that the mag well at the back can be snug. I think it is related to the cooling process out of the mould – i know there is a fixture but it might be a case of time variation letting things snug just a bit – as most are dropping metal mags easily but a few don’t. I have smoothed a couple to let things drop free but I suggest you use a piece of flat steel and go carefully so as not to bell things top and bottom. It doesn’t take much to fix and after that give it some light oil and touch it with lighter grit paper or even crocus cloth to remove the fine glass fibres that will be raised.

    I am really pleased with these lower assemblies to date and can also highly recommend the new High Standard AR barrels as they shoot super well.


  11. i picked one up from a buddy a little while back and was skeptical about it.. but the palmetto kit fit perfect and the upper fit quite snug which is a very good thing! i was thoroughly trying to find another one!

  12. Had the same problem with fitting the lowers to any of the 4 (four) uppers I own. I contacted ATI and returned them to be fixed in October of last year. They have had them since Oct. 19th and my FFL and myself have been getting the run around since. If you are considering sending them back be prepaired to wait quite awhile. The customer service in responding to emails is fast. Actually fixing the product is non existant. Told numerous reasons/excuses as to why he receivers are not done. When asked to refund the money, we are told new receivers are going to be sent out, but here we sit 3 weeks later and no receivers. I like the construction of the weight of the receivers, but if they don’t fit how can we test them.


  13. jim cottrill says:

    I’m having a feed problem with my dpms, it won’t pick up reloads from any mag i try in it.i have been very careful to size and trim my case’s and clean them very well,i have been told i may need a slow burn power for more bolt reaction, have you ever had this problem? i have never had a feed problem with new ammo. jim cottrill

  14. Update on earlier post. Received 2 (two) NEW RECEIVERS. New serial numbers. I again tried to fit them on the same uppers I tried before. Exact same issue on both of them. The one fit one upper by driving the rear pin in with a hammer, the 2nd one does not fit at all. The rear pin hole does not line up. It is approx. 1/16 of an inch off. I am not going to go through a 3 month wait again and return these, I will try out the one that fits (tightly) on my upper. at the cost of internal parts kits now I may strip all of the parts out of the 2nd one and trash it. I am also going to advise every blog site I am a member of to steer clear of OMNI receivers.

  15. Hi thanks for the info. I have the same lower on a century arms upper my issue is the lower will shoot properly with GI mags and P mags but it will not reload while using thermold 30 round mags. Mine will drop all three types of mags out but when the thermold tries to reload it won’t. The gun will fire but won’t reload on recoil only reloads if I manually do it???

    • Josh;

      Thanks for the feed back. Can you advise what the marks are on the Thermolds? They should have a year and a cavity mark on the bodies (Hxxx is typical). I have had problems getting new polymer mags to release off the bolt catch – P-Mags and TAPCO polymer mags – for the first 5-10 times in the Omni but not feed issues. Seems they need to wear a channel in the back to work right. I am wondering if the “new” Thermolds have gone back to the original design which wasn’t as good as the Canadian ones. All my Thermolds are Canadian spec and they seem to work fine in the Omni.

      When Diemaco redesigned the Thermold for Canada they changed the back considerably to open it up as they were getting interference while coming off the bolt catch in the aluminium receivers. We also experienced failures to pick up the rounds in the Thermold (we didn’t have any others to test as we were issuing those) and it was found that the spring orientation and the connection of the spring to the follower were critical to function, especially towards the end of the mag.

      If the mag is presenting right in the lower (you can check by comparing the height of the body of the working ones with the non working ones in the lower only, they should be the same height above the top surface of the lower when properly seated on the mag catch) AND if you put it all together and manually cycle and you can see the bolt lug picking up the rounds in all of them (you might see the faulty one a bit less engaged at a guess) then you have a dynamic issue inside the magazine as the body can’t go up and down or it would fall right out so it has to be internal. Its likely a case presentation issue in the Thermold and the spring-follower just isn’t getting the back of the case up fast enough to be picked up. I’d check the spring orientation and make sure it is hooked to the follower. If that doesn’t work, that particular design series might be faulty. Sometimes the follower if not sprung right and functioning fast will push the nose up but not the base. This is then “covered up” as the bolt knocks the round flat and when you cycle manually it picks things up normally. High speed photography is a wonderful thing to capture these events. We cut away a few mags and receivers to be able to see exactly this part of the cycle and it happens for sure, hence the boatload of “anti tip” followers out there.

      I am assuming your gun is relatively clean and cycling properly since it goes far enough back to function with PMags and GI issue. Please let me know what you find out as these are interesting engineering problems for all of us to be aware of.

      Phil O’Dell

  16. My sons had this gun explode on them last week the gun was bought new and had only 10 rounds shot through it so be aware of it.

    • Sabrina,

      Can you advise what upper receiver assembly he had mated to the Omni Lower and what ammunition he was using? On an AR/M16 platform the entire pressure containment is in the barrel, barrel extension and bolt. Any photos of the entire gun after the incident would be appreciated.

      I have investigated a few overpressure events in AR/M16 designs and I’ve never had occasion to blame a lower yet. (There’s a first for everything in this business.) Any pictures or info on the upper receiver/barrel/bolt & carrier assembly would be greatly appreciated.

      The majority of over pressure or pressure related events I have ever seen occur outside of intentional things done by engineers are bore obstructions (sand and snow mostly but mud/water sometimes) and faulty bolt heat treat.

      The community would be very appreciative if you have actual photographs of the gun immediately post event and the information above on the manufacture of the upper half parts.

      Phil O’Dell, P.Eng.
      O’Dell Engineering Ltd.

      • Matthew says:

        I have one of these ATI lowers and the buffer tube threads were stripped causing failure after three rounds.
        What’s worse is I cant get ATI to replace it with a new lower because im in Canada and the distributor (O’Dell Eng.) Will NOT return my calls.

        • Phil O'Dell says:


          I have no messages from you. How are you trying to contact us? I have no emails from you. Any issues we have had with these guns we have resolved immediately for our customers. Our issue rate is less than 1% and that’s lower than any other AR/Modern Sporting Rifle line we deal with.

          As it is we are bringing out our LMSR2 shortly and if you have an issue and actually call me I will upgrade you to the LMSR2. Assuming it is a legit issue, and I have no reason to suspect otherwise, you will get that upgrade free.

          Where are you located? We are in Ontario.

          Phil O’Dell

          • Matthew says:

            Hi Phil,
            is the number listed on your website not good?

            I’ve also attempted your email from both my work and home emails.

          • Phil O'Dell says:


            There was a package insert in your rifle that gives the contact number and email. I am not sure what you got off the web site but we are at 519 654 9600. The office is usually attended 9:30-5:30 or you can email me directly at anytime to arrange for warranty work. We take considerable pride in our warranty and support to our customers so this is a very high priority to us to solve.


          • Matthew says:

            Email has been sent today. Please advise of receipt.

          • Phil O'Dell says:


            Received and responding now with info to do the exchange.


          • Hi everyone;
            I just wanted to follow up here to let everyone know how my experience has gone since first contact.

            Today, Phil DROVE TO MY HOUSE and handed me a replacement. Let me say that again DROVE TO MY HOUSE. If thats not customer service, I do not know what is. We chatted for a bit and Phil is a great guy and cares about the product and the people who purchase it. I can’t say many more great things about my interactions here.

            The new Hybrid lower looks great and functions pretty good dry and I appreciate your hard work in helping make this right, Phil. Top notch guy and company everyone.

  17. Can a 9mm upper work with this lower

    • Phil O'Dell says:


      Thanks for your note. I have fitted the Rock River 9mm upper and magazine well adapter to my original Omni and a new Hybrid and they work in both of those. I can’t speck to others but I would guess the old Olympic Arms conversion with the full width magazines should work also.

      What brand of 9mm upper were you looking at?



  1. [...] couple of weeks ago, I unboxed and gave you my initial impressions on the lower.  My first impression was a bit mixed, as the assembled lower I received had a few quirks [...]

  2. [...]  I’ve got an unboxing here and a full review of the ATI Omni lower here.  Ultimately, this seems to be a pretty good AR lower [...]

  3. […] them.  Phil O’Dell of O’Dell Engineering in Ontario, Canada has provided a lot of feedback on the lowers and rifles here at GHG.  It is worth reading through his […]

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