Builder on a Budget: Building a GPR Upper for the Low

This Blog is for entertainment purposes only. The thoughts and objectives of this Blog are my own and in no way reflective of Anderson Manufacturing's build processes or procedures. Always consult a licensed gunsmith when modifying any firearm. Enjoy.

Builder on a Budget: Building a General Purpose Upper for the Low (Sub 500)

Take this journey with me as we dive into the general purpose rifle build

Once I set my mind to a general-purpose build, I am truly looking to build a rifle that can check-off as many tactical applications possible. In this blog, I am going to focus on the upper. With that being said, I tend to steer away from any limiting factors when getting my parts list compiled, for instance, I wouldn't throw a heavy profile 20'' barrel on a GPR (General Purpose Rifle). As accurate and soft-shooting as an HBAR (Heavy Barrel) can be, this would make the rifle too heavy and long which would definitely hold a negative impact in the area of mobility.

For the purpose of checking the boxes of as many tactical applications possible, I have pieced together a build-list for an upper that will offer the versatility demanded by a GPR and I've made it a mission to keep this upper below $500. Let's see what I can make happen. Along the lines of how I did the Kiger Frame Blog, 'The Eye of the Kiger', I am going to use a minimalist tool set that includes: Bench Vice w/ Reaction Rod for 5.56, AR-15 Armorer's Tool, Ball Peen Hammer, a Hex Key Set, a 5/64'' Starter and Finishing Punch. As I pour through the build-list, I will also document the assembly of my GPR Upper.

My first choice for the build-list is the AM-15 Anodized Black Stripped Upper Receiver NOFA (No Forward Assist). I chose this upper receiver because it's lightweight, I would have undoubtedly gone with the Elite Lightweight AM-15 Upper had it been available as a stand-alone item, but for now, it is only available as a set with the AM-15 Lightweight Elite Lower. I have nothing against the forward-assist, as a matter of fact, I used to spend a lot of time working on my 'tap, rack, bang' drills in my time on active-duty, but to support the theory of making this a lightweight upper, I figured I would shave off weight where I can. I still have some OCD when it comes to the dust cover, so I opted to keep that option in-tact.

If you are doing a complete rifle build, I highly recommend checking out the AM-15 Lightweight Elite Receiver Set , It comes in white (no coating), giving the builder full discretion on pattern/design and shaves off a ton of extra weight while still bolstering the Forward Assist and Ejection Port Cover.

Now, we go to the Ejection Port Cover (EPC). For full clarity on this one- I did not use the same EPC as recommended in the build list, that's strictly due to the fact that I am trying to make this as budget-friendly as possible. Post-build I realized that the AM-15 Don't Tread EPC Kit gives you everything you need at the best price.

Installing your EPC can be tricky if you've never experienced it, reference the visuals below for additional insight as to what I'm talking about. First, I take the Ejection Port Cover Rod and inspect it to ensure that there is a c-clamp attached to it (this will restrict the rod from walking out and maintain proper tension on the Ejection Port Cover Spring). Now, I keep the c-clamp end of my rod facing the direction of the muzzle end of my upper and insert the opposite end into the first EPC hole, I don't push it past or through the hole. I grab my Ejection Port Door, place it between my two EPC holes located on my stripped upper. The Port Door has a detent installed that locks into your upper, when I place this door, I ensure that the detent side is facing upward and upside down (once placed the detent should be facing you). Once I have the door in place, I continue to slide the rod through, stopping at the opening in the middle of my port door. Here's where the tricky part comes in, the spring. On the spring, I notice one side of the spring is taller, I hold my spring with the tall part facing the muzzle end of my upper. Now, I hold the spring with both hands, keeping my left hand on the 'little' end, I twist the 'tall' end one half turn (to make the 'tall' end face downward -or- toward me) with my right hand. I hold the spring tension and place the spring in that opening mid-port door while simultaneously sliding my rod through the rest of its path (your rod should go through your spring, the other half of the door and the other hole on your upper.

Thankfully, this is one of the easiest parts to function check. I simply close the EPC to ensure that it locks into its respective place on the upper, then I reach inside the upper and push the EPC open. To test the spring tension, I flip the door from the open position. If my EPC doesn't bounce back to completely open then I would be lacking spring tension and would have to back my rod out and revisit the spring install.

First Step of installing my Ejection Port Rod (C-clamp to the right)

Getting ready to place my Ejection Port Door

Preparing to place my Ejection Port Cover Spring

Keeping tension on my spring while sliding the EPC rod into place

I made it past the spring while retaining the tension

Moving on, my next step is going to be my barrel. As I mentioned, I want this GPR to have versatility in its use, therefore I decided to go with a 16'' 5.56, Mid-Length Gas, Government Profile Barrel. I like the government profile for this GPR because it is beefed up where it needs to be, in comparison to a pencil barrel. It also holds more frontal weight which would reduce muzzle rise to an extent to help with follow on shots or rapid fire. The mid-length gas system is going to pay dividends in reduced felt recoil and added dwell-time with reduced gas port pressure- which inevitably equates to slower moving parts, this reduces the chance of a malfunction and minimizes some of the wear and tear on your BCG. These are things that you have to think about on a GPR, not only do you want this rifle to be versatile, you also want longevity of use. Although the speed of the function probably won't be noticed by the naked eye, these are some minute details that can have a large impact. The 1:7 twist rate is well suited for any 5.56 projectile weight but really shines when you get into heavier ones, like the 77 grain.

The first thing I do is inspect my barrel. I do a once over of the barrel's extension and ensure that there is no 'gunk' built up on my feed ramps, that the extension sits flush against my barrel and that the extension is pinned properly. This is also the perfect opportunity to give your barrel a good, thorough cleaning. Then, I go to the other end and check my threads for any type of build up or corrosion. I, then hold my barrel up, extension to my eye and point the muzzle toward a light source allowing me to see that there is rifling and that there are no obstructions/blockages nor internal corrosion. Once I have inspected my barrel, I am going to plug it into my upper receiver. I take a look at the muzzle end of my upper, I see a notch for the pin of my barrel's extension. It's that easy, I ensure that my barrel's extension pin is aligned with the notch in the muzzle end of my upper and push my barrel in (see below for visual). I can get my barrel in using hand strength but if for any reason I could not, I would ensure that there is nothing blocking my barrel from pairing with my upper and that I have the notch aligned with the pin. If it's still giving me fits, it is ok to get the barrel started and lightly tap the backside of the upper with a rubber mallet until that pin seeds all the way into that notch and the barrel appears to sit flush against the receiver. I will now place my 'barreled-upper' on my reaction rod and work from the bench vice/reaction rod set-up.

Ensuring that my barrel is seated all the way into the upper

Alright, I have my barrel in place, now I need to secure it. I will achieve this with my barrel nut. My barrel nut came with my Utility Low-Mass Handguard Kit, this is nice because it tells me that my barrel nut and handguard system are compatible (which, not all barrel nuts work with all handguards, so it's a very nice feature). Military Specifications call for 30-80 Ft./Lbs. of torque when applying your barrel nut. I grab my barrel nut, thread side facing my upper, and slide it down my barrel until I meet the threads of my upper receiver. I hand tighten the barrel nut onto the threads, once I reach the point to where I can no longer hand tighten, I will transition to my reaction rod and utilize my Armorer's Tool to finish tightening it down. You can use a multitude of household tools to include a crescent wrench. it really just depends on the type of barrel nut you are working with. I have a torque wrench at our shop here in Hebron, Kentucky so I took the opportunity to double check that I met the minimum of 30 Ft./Lbs. and I did. I completed my barrel nut with an Armorer's Tool because I like to take a minimalist approach at projects. I do this because I know that I won't always have the proper tooling at my disposal- this is not in anyway to deter anyone from using the proper tools, this is just my weird way of ensuring that I can complete tasks in alternative methods should I find myself in a situation without the appropriate tool set.

I like to keep my reaction rod clamped tight but not so tight that I can't adjust on the fly

My barrel is now secure, thanks to the barrel nut, my next objective is the gas system installation. For this step, I am going to open up my Adjustable .750 Steel Low Pro Gas Block and ensure that it has the appropriate hardware: (2) mounting screws, (1) gas block pin, and (1) hex key. I place all the hardware to the side, grab my gas block and mid-length gas tube. As I do with all of my parts, my first step is inspect, I observe the gas block and where the gas hole lies. Then, I check out my gas tube, I ensure that it has 3 holes on the closed end, one for gas and two for the gas block pin to hold it in place, the opposite end should come to an open end where the gas will flow to the gas key of the BCG (Bolt Carrier Group). Once I have completed my inspection, I pair the gas tube and gas block utilizing the closed end of my gas tube. You want to match the gas hole on your tube with the one on your block which should leave you with the gas hole (of your gas tube) facing downward and the pin holes on the right and left flank. I am going to push my gas tube in until the pin holes align with the ones on my gas block. I observe the gas block set screws that came with my gas block to ensure that they have been treated with thread-lock and I always add just a dash of my own, then insert my gas block set screws just enough to hold them in place. I'm going to hold off on pinning it, for now, and slide this sub-assembly over my barrel while trying to not misalign the holes but so long as the tube stays in place; that's what really counts, I can adjust as needed.

As I am sliding the gas system sub-assembly down the barrel, I take notice of a small metallic dimple on the bottom side of my barrel. This will be where my rear gas block set screw will land. The dimple is in place to prevent the gas system from walking out on you or getting shifted left or right; a shift or walk-out could move the gas block away from your barrel's gas pad and throw the entire functionality of this project off. Once I have my gas system as far down as the barrel allows, I ensure that my gas block sits flush against the contour of the barrel. Once I am sure it is flush, I grab my hex key with my left hand and tighten the rear set screw as far as I can without securing the gas block to the barrel so that I can still adjust. Now, I focus on centering my gas block in the middle of my upper, I like to adjust my reaction rod so that I can get a square look at it and ensure that I am as centered as possible. Once I have my gas block dead center and completely flush on the backside with the contour of the barrel, I tighten the set screw as tight as I can get it utilizing my hex key. I now examine once more to ensure that my gas block is properly placed, once I am sure, I lock in the second set screw.

Dimpled barrel helps secure the gas block

Ensuring that my gas block sits flush against the contour of my barrel

Ensuring proper alignment of the gas block, with set screw and hex key at the ready

There are many tips and tricks of the trade to centering your gas block: uncooked spaghetti, string method, chalk it out; just to name a few

Now, it's time to address the gas tube. While setting the gas block, I am real careful not to move my gas tube out place, but if it did for whatever reason misalign, it's an easy fix; I would just maneuver the gas tube however I need to in order to get it properly aligned with the gas hole facing the floor and the pin holes on the flanks. Once I have that alignment (i.e- I can see through the gas block pin holes) I take my starter punch and my gas block pin and get the pin started from either side (I always go in from the left, but either side is fine). Once it is started I use my finishing punch to drive it in place, ensuring that the pin does not protrude in any way. I give the gas tube a tug to ensure it is pinned into place and move on.

The next move for me is to open and inspect my Stallion Muzzle Device to ensure that there is no damage or blockages and also that my crush washer was included. Everything checks out, so I take the crush washer and observe that it has a bowl-like shape to it, I place the crush washer on my threads so that when I look at it I can see the bowl opening toward me. I then grab my muzzle device and hand-tighten it until I can't. Once I reach that point, I grab my armorer's tool and use the muzzle device notch by placing it on the bottom side. Now, I want to give my muzzle device one full turn and then continue until the muzzle device is completely squared with my upper and gas block. In this instance, I have the four vents at the top of my muzzle device to use as guidelines to capture the center of the upper in the center of those four vents.

Time for some good ol' test-fire action. To further support this build being general purpose, I thought that an adjustable gas block would be optimal so that I can tune the gas if I'm using a suppressor or any instance where the buffer weight may change, etc.- I can tune to it. With that being said, I have my muzzle device installed, I know that I'm using the Low Mass BCG, I also know what lower I plan to use this on so I am going to optimally tune my gas block so that it cycles and I want to ensure that this GPR upper has the lightest felt recoil as possible. So, I start by opening the limiter, utilizing the hex key that comes with it, 10 clicks (this adjustable gas block bolsters 20 settings) and install my Low Mass BCG and charging handle. I test fire, fires one round, does not pick up the second. I continue to go click by click, test firing in between until I find where the upper cycles flawlessly. Once I find this, I fine-tune, paying attention to the most subtle changes in recoil and ejection pattern. I have found my optimal setting.

Installing Charging Handle prior to test-fire

BCG Installed, ready to roll into test fire

We got the pew-pew out of the way, for now, let's take the upper back to the reaction rod and tidy up the rest of this build. Next up, the handguard. I take my handguard with the top picatinny rail facing the muzzle and slide it over my barrel and gas system until the handguard sits flush against my upper. I notice that there is a drop-off ramp that needs to align with my upper's picatinny, this makes it super easy to line up and super-noticeable if it's not done properly. Once I have the handguard lined up, I grab the two screws that came with my handguard kit and place them on the table, then I grab a towel and place a couple drips of thread lock on it. Using my hex key, I pick up the first screw and roll the threads through the thread lock and hold the handguard in place using my dominant hand and tighten the screw with my other. I repeat this process for the second screw, then I revisit both screws with my hex key to get them as tight as I can without stripping them while also making sure that they are both equally tight.

Securing Handguard while maintaining alignment

Once my handguard install is complete, my GPR upper is complete! I appreciate you for taking the time join me on this journey to complete my upper and I hope the build list inspires you or enables you to put your own spin on it and make it even better! I feel like my next journey will be a lower built that I can designate to this upper, until next time...