Before you can start building up your new receiver, you need to strip the old one of a few pieces. There is no need to remove any of the gas system or barrel components, but you can if it makes working easier on your bench. Start by removing the two screws securing the butt-pad. They are concealed behind two small openings but can be seen partially backed out in the image to the right. The screws are actually Pozidrive, however if you do not have a bit a Phillips driver will work. With the pad removed, now assemble a 6mm socket driver and 12' extension.
There is a single bolt that secures the upper to the lower. Using the 6mm socket driver, unscrew the bolt 5 turns. Next, remove the wrench and tap the extension and bit with a mallet. You will not start to see the upper and lower start to separate. Snap the wrench back onto the extension, and continue the remove the bolt in the normal fashion. The bolt and plate will fall free and can be placed to the side. The upper will now rotate forward freely and separate so you can continue to scavenge parts from it.
The bolt hold back latch comes off in the familiar fashion. A few taps from a 1/16" punch and the pin will fall free and the latch can be lifted out. The SAS site instructs the hole to be drilled out to 0.093". I actually used a 0.088 bit and found the fit to be a little better. Either one will drill very quick as you are only opening the hole up a few thousandths. In a pinch a hand drill will do the job, just make sure to wear gloves.
For those of you running three gun with your MKA 1919, you have probably considered upgrading your polymer lower to an aluminum one. There are two popular choices out there, the Firebird MKA lower and the SAS lower. The main difference between the two is that the Firebird is a two piece lower and slightly heavier, while the SAS lower is one piece and lacks the floating feed ramps. We will talk more about that later in the article. For this review, we will cover the SAS MKA 1919 aluminum lower and the basic installation. Upon receiving your lower, you will find a few things in the box. The SAS lower itself, along with the various hardware that cannot be recycled from the old receiver. What you will not find is instructions. Thankfully the MKA builds up pretty much the same way any other M4 / AR15 platform would. The receiver itself is very high quality in finish. The anodizing is above par, and even with close inspection no machining lines were visible. The bolt catch pin is a threaded one which is a nice touch. I have seen a lot of amateur "gunsmiths" mar the hell out of a lower trying to set the bolt catch pin. The increased pic size will however require you to drill it out ever so slightly. Strangely the buffer tube adapter is a separate piece. I am not sure what that is about, but when all put together the lines of the weapon still flow nicely.
Removing the magazine release catch is a little different on the MKA 1919 than it is on the M4. Instead of pressing the button all the way in and un-screwing it from the right side, it is simple screwed on. On this particular rifle there was an insane amount of torque needed to remove this screw. Having said that, you will want to use a quality Flathead to avoid stripping the head out.
The hammer and trigger pins need to be driven out from left to right. For me these were in stupid tight. I had to start them with a 3/16' punch before driving them out with a 5/32". Not sure if this is normal, but be prepared to be patient here. If you are not sure that you are driving them the correct way, double check to make sure you are hammering on the rounded side... I know...
At this point, everything goes back into the new receiver in the same fashion as any other M4 build. There are instructions on the site should you need to look up anything. So with that out of the way, how does the new receiver preform? Well on mine I had a few issues and concerns. First was the fit of magazines. My ten round mags would fit in but very tight. Actually took took people to pull them back out. The OEM 5rd mags that came with the gun would not fit at all. After some searching around the net, there were some other people with the same issue. That said the ten round magazines would probably wear in and be OK yet never drop free. The five round mags would have been a lost cause. I contacted the supplier but as of writing this I have not heard back. My solution was fix it myself. I marked up the mag well with Dykem and inserted the magazines as far as I could. The center ridge at the back of the magazine turned out to be the issue. Using a full flute end-mill bit I started removing material slowly. By the end I had to remove a fair amount of material. That is to say, enough material that these would have never "worn in". For the machinist out there, the depth was 0.086. At that point neither the ten or five round mags would drop free, but they could be pulled out with finger strength. I will let them wear for a while and remove more material if necessary. So finally... how did it preform? The short answer is great for me. Using a standard stock I was able to get the platform back to the correct LOP for me. In 75 rounds not malfunctions. Keep in mind this was on a previously broken in gun. While it should have been obvious, the purpose of the floating feed-ramps hit me when I got back to the bench to strip and clean. With this receiver, you have to separate the upper and lower if you want to clean the bolt. Based on the PITA UBR II, I can say this is something I will not be doing often. If you are OCD, this may be a plus for the Firebird Precision lower.
All in all I would recommend this upgrade if you are using the platform for 3G, or you need a shorter LOP. I feel like a good deal of the pros and cons of this upgrade are subjective. For that reason please chime in at the SAS lower discussion topic if you feel differently.
Edited by VS762