The product in this review: Rainier Arms UltraMatch Billet Upper and Ambi Lower Combo Set - MOD 3

There once was a time, as hard as it is to believe, when AR receivers were generally the same. By this, I mean as long as they worked, largely they all looked and functioned, the same. Then we started to see innovation on the platform. Eventually we started to see ambidextrous safety selectors. More and more trigger offerings came about. Eventually we saw all sorts of extra add on products to make various functions of the gun ambidextrous. Left handed shooters could finally rejoice, sort of. These new products had some hope of making the controls somewhat friendlier to your plight. Over the last few years however, some companies have put out some innovative products. One of the more innovative is the Rainier Arms Ultramatch Billet Upper and Ambi Lower Set.


The Rainier Arms Billet Upper Receiver is, well, a typical upper receiver, well sort of. It is considerably more attractive than the plain old boring forged uppers we’re all used to seeing. Now, that said, one detail I noticed right from the start was scalloping in the recesses of the picatinny rail. This does a nice job of removing the sharp edges that are so common to the various uppers and rail systems on the market. In addition to the scallops, the cleanly finished upper receiver has all the other edges cleanly deburred resulting. I am impressed.

Keep in mind that this upper is not assembled. This means you have a clean slate for your wishes. To add a forward assist or not to add a forward assist, that is up to you.


Now we turn our attention to Rainier Arms’s Ultramatch Billet Ambi Lower… and oh what a lower it is! My initial thought had been that uh ok, it’s another lower, big deal. That initial impression changed as I looked it over. I immediately noticed it had sling swivel attachments machined right into the lower on both sides! Convenient was my first thought. Right after that, I noticed the generously flared magazine well. The longer I looked at it, more details started to jump out at me. It became obvious that a lot of thought had gone into this lower receiver.

The obvious part of this lower was that it has a left side magazine release and a right side bolt release in addition to the traditional units on their respective sides. Shouldered set screws retain both levers. Additionally, a threaded hole replaces the bolt catch pin hole and includes the appropriate screw. As I looked at the rear of the receiver I noticed the takedown pin detent hole was missing. Upon closer inspection, I found Rainier Arms had relocated the detent hole ninety degrees downward to a location covered by the top surface of the grip. A further detail on this modification is a threaded hole. This allows for the use of a set screw to help prevent launching the detent and spring across their work space. I suspect there are three of them somewhere in my shop.


Rainier Arms added another threaded hole. Inside this threaded hole is polymer tipped set screw to tighten against rear upper receiver lug. This one is located above the grip and directly beneath the rear takedown pin location. Near this, they have also located another hole for a trigger reset adjustment screw for the folks running triggers that can benefit from it.

The integral trigger guard is more user friendly than the traditional separate piece trigger gurads. Rainier Arms made this trigger guard wider and beveled the sides somewhat. Ari, at Rainier Arms, explained to me that this design change came about due to shooting three gun matches and having to frequently hold the gun one handed. Adding this extra material helps to spread the weight out and removes the edges and gap traditionally found on AR lowers.

Safety selector markings are pictograms with the safe, semi, and wishful full auto marking (we can all dream, right???). The semi auto pictogram is also has an arc from 45 degrees to 90 degrees to accommodate short throw selector as well as standard.


The lower that I have for this post is actually a blemished unit. I looked it over and didn’t really see what the issue was, so I asked. It was revealed to me that the issue was that the quick release hole was only very lightly anodized if at all. Once it was pointed out, I could see it. This doesn’t meet Rainier Arms’ quality control standards but don’t affect the functionality of the lower so they elected to sell them as blem units.


Constructed of billet 7075-T6 aluminum and finished in black hard anodize, the Rainier Arms Ultramatch Billet Upper and Ambi Lower set is an attractive and well designed lower pair. The design improvements Rainier Arms made to the traditional lower receiver make for a more versatile unit that will be a great base for your build. Additionally, if you’re looking to have your build refinished, you can keep an eye out for the blem sets and save yourself some money. Happy building!


Cameron is a gun writer, blogger, photographer, and USPSA shooter. He has been fortunate to receive firearms training from Graham Combat, Haley Strategic, Follow Through Consulting, Firearms Academy of Seattle, and Massad Ayoob. His education has been focused on defensive use of the pistol but has also included carbine use as well as employment of both in and around vehicles and buildings. In addition, Cameron has spent time managing an indoor firing range and worked on the retail side of the gun industry. His writings can be found frequently in The Blue Press, here, and at his blog

The product in this review: Rainier Arms UltraMatch Billet Upper & Ambi Lower Combo Set - MOD 3