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Considerations When Building an AR-15: Part 1

Updated: Mar 25

Let's face it, one of the many attractions that draw people to the AR-15 is the absurd amount of modularity offered by the platform. You can go from a simple A1 style carry handle rifle that would look right at home in a Vietnam documentary or you can trick the whole package out to something that would rival an episode of "Pimp My Ride".

While this can be what excites many builders it can also lead to headaches and even money lost due to incompatible parts or even a build that lost focus and now doesn't do what the owner wanted it to in the first place.

The PLAN...

As with any project we need to have a solid outline with a clear end goal in mind. This is where we want to be very specific and shoot for the stars while understanding that we will need to carve out a hierarchy of wants or needs in our end goal (There is no free lunch in nature and this holds true when building an AR-15).

So, let's say we list our top 5 things we need our AR to do and do well:

  1. Be Reliable - Truthfully, this should always be #1

  2. Be Lightweight

  3. Be Accurate

  4. Be Cheap to Shoot

  5. Be Able to Hunt Deer

Now the seasoned builder will very quickly see that we are destined for a rough road ahead with this list of priorities. Several of these items are bound to fight against each other and, depending on where compromises are made, this could end up with a rifle that does none of the above well. This would surely lead to a sour experience for the owner and likely money wasted.

If the rifle is to be cheap to shoot then we are talking about something is 22LR, 9mm, or 223/5.56 and most of these are either illegal (state dependent) or subpar to hunt deer with. A rifle with a heavy match grade barrel may print tiny groups but it will not be lightweight and, depending on how tight the chamber is, it could affect reliability or need match grade ammo and thus not cheap to shoot either.

Now 80% of the build is going to be done in this planning stage and will result in the owner knowing EXACTLY what they intend to end up with before purchasing the first part or accessory.

For our theoretical rifle lets say the following:

  1. Reliable with factory hunting or match ammo - Handloading is unnecessary

  2. Able to harvest eastern whitetail deer - State law - minimum 23 caliber

  3. Optic capable - Flattop upper receiver

  4. A comfortable weight to carry - less than 9lbs with optic and loaded mag

  5. Packable length for a 4wheeler front/rear rack - Collapsible stock or shorter barrel

This is our "wish list" for our theoretical rifle build, and while some of these items may have to flex to fit into our ideal rifle's actual reality it is a focused identity for the role this rifle will fill.

Where to start?

Since we have a state law to contend with it makes sense to start there. We must have a chambering of 23 caliber or greater. We know our intended role is that of a packable hunting rifle so we can now define our area of operations (AO) or where we are going to hunt. Does it offer longer distance shots to 400 yards along with the real potential for strong winds or is it more likely to be limited by thicker vegetation with almost no wind to contend with? These two AO's push the caliber needle in very different directions for our hunting rifle. If we assume the worst case scenario of "Why not both!?" then we can assume the rifle has to contend with longer shots in higher winds and potentially thick vegetation shorter range shots between deer stands.

This will immediately push out certain short range, hard-hitting calibers such as the 458 SOCOM or the 450 Bushmaster due to the longer distances along with other chamberings that don't support higher BC bullets to keep from being bullied by the wind at distance (where a bullet that drifts can result in unethically wounding and losing an animal). That removes the 350 Legend, 300 Blackout, the 300 HAM'R, and 7.62x40 WT.

We just thinned the pool of available calibers rather quickly which helps us ensure the rifle will stay on track to meet the intended goal. While it might seem like we should settle on a caliber right here we have another item to consider which will affect our choice and that item is barrel length.

How Short is Too Short?

Since we need our rifle to fit on the 4-wheeler or ATV, we need to be considerate of overall length of the design. since barrel length is one of the major features that negatively affects packability of a rifle it makes sense for us to choose a barrel that is just long enough to get the job done without adding any unnecessary length. The downside to this is that shortening the barrel is going to negatively effect our cartridge's velocity and therefore cause the bullet to hit less hard and drift in the wind more (no free lunches here). As a general rule of thumb we can expect a 25-35 fps loss in velocity for every inch of barrel we cut down from normal or ideal lengths (typically 18-22 inches for most cartridges). This "rule", however, shows a lower velocity loss when utilizing larger bore diameters with faster burning powders. This can be seen with certain calibers that tend to excel in shorter barrels (part of the reason the 300 blackout doesn't need much over 9.5" of barrel length to achieve its top velocities).

Lets say we want to err on the side of portability vs all out capability at distance because we may or may not get a clean shot at 400 yards but we will definitely be toting this rifle a lot. in that case we will go as short as we legally can without applying for a tax stamp and we will run a pinned and welded 14.5" barrel (this gets us to the minimum 16" barrel length per the ATF - don't get me started on this one). Now that we have our package size outlined let's see what cartridge fits into it best while keeping bore size into consideration.

Our available options are:

  • 6x45

  • 6mm ARC

  • 25-45 Sharps

  • 6.5 Grendel

  • 277 Wolverine

  • 6.8 SPC

Since we are not looking to handload for this rifle we are going to eliminate the 6x45 and 277 Wolverine as they are handload only affairs at the moment. The 25-45 Sharps is a neat round and hits hard but it just doesn't have enough "oomph" in our shorter barrel for those longer shots. This leaves us with the 6mm ARC, 6.5 Grendel and the 6.8 SPC.

If we break out each of these cartridges by their velocity and energy with commonly available ammunition we find they all tend to punch in the same weight class with the 6.5 Grendel and 6mm ARC edging out the 6.8 SPC in energy and wind drift.

6.5 Grendel vs 6mm ARC vs 6.8 SPC
All 3 cartridges are capable of getting the job done.

6.8 SPC wind drift, 6.5 Grendel wind drift, 6mm ARC Wind drift
Although the 6.8 SPC will drift more than the others it is still within the capable performance window.

For now we will say that any of these calibers will work but we will revisit this again before nailing one down.


Since we are working with calibers that were never even a thought when the platform was invented we have to see what other changes must be made to the AR-15 to support these calibers. All three of these cartridges require a barrel change, magazine change and a bolt change at the minimum and could also require gas and buffer tuning but we will leave that for another discussion.

When we look at magazines we find that they are the heart of the system's reliability and a bad magazine can shut down any semi auto system. Some magazines for these cartridges, when loaded to the advertised capacity, can swell and cause feeding or magazine insertion issues. Ensuring that the magazines you choose have a proven track record and will function reliably is paramount to producing a rifle that will support the plan.

The bolts, while designed to operate in a standard AR-15 carrier, have to accommodate the larger case head diameter and therefore are generally considered to be weaker than their 5.56 counterparts. Ensuring that the bolt has been Magnetic Particle Inspected (MPI) and High Pressure tested (HP) should be considered a minimum and choosing the right bolt can be the difference between years of reliable service or a rifle that fails rather spectacularly early in its life.

Moving to the barrels for these cartridges we have a single thing to be mindful of for all of the cartridges (with the exception of the 6.8 SPC) and that is barrel twist. We need to make sure that the twist rate will support the bullet weights we intend on shooting. It would do no good to have our rifle that needs to sling heavier bullets setup with a slow twist rate that is really only conducive for shooting lighter weight bullets (1:10 is slower than 1:7). The 6.8 has another consideration and that is the chamber depth. The 6.8 SPC has either a Type I or a Type II chamber and the bolt must match the chamber to headspace correctly.

Since we could, in theory, get by with any of these cartridges to complete our plan let's assume that we can get a good deal on ammo loaded with the bullet we want to shoot and select that caliber - 6mm ARC for this example.


Since we intend on carrying this rifle we want to make sure that the components we choose are going to work well with a lighter weight rifle without hindering any of the other parts of our plan.

Selecting a shorter barrel will help with our overall weight, but we can still make the wrong choice even with the shorter barrel by choosing a heavier contour than what is necessary for our end goal. Since we will be carrying the rifle a good bit more than we will be shooting it we can go with a lightweight contour which won't sacrifice any accuracy. Heavier barrels will heat up much slower than thinner barrels but that comes at a drastic weight penalty. Since we will only be taking a couple of shots on game we don't need to worry about heat soaking the barrel and can use the lightest weight contour available to us.

The scope and mount choice can also add up with many low power variable optic setups (LPVO) running 22-30 ounces while larger scopes can run as heavy as 40+ ounces. This added weight, while carried between the hands, can work against us and our end goal of a lightweight rifle. It sure would look bad for us to save all that weight on the barrel contour and length while mounting a telescope to the top of the receiver. Since our longest range we are looking at is around 400 yards and we are looking to use this rifle for deer we will keep the magnification modest and select a 1-6x power LPVO and lightweight mount to keep the additional weight down toward 22 ounces.

Another component that can make or break our weight is the handguard. While a short barrel and a long handguard can make for a sexy looking rifle, it also adds a good bit more weight. If we don't intend on adding accessories to the rifle, which would add even more weight, then we should select a handguard that provides enough material for our support hand to grasp while also protecting the barrel should we need to brace against a tree or other hard object during a hasty shot. Aluminum gets cold during hunting season while polymer isn't as harsh on the hands during cold weather so this should be kept in mind as well. Remember, we want to be able to carry this rifle as well. Free floating the barrel can enhance the rifle's accuracy but most free float handguards tend to be heavier that handguards of the polymer variety so we want to strike a balance of weight with acceptable accuracy for our intended game. Since deer have a fairly large vital area we will say a 1-1/2 MOA rifle would be more than adequate so we don't need a free float handguard and will opt to use a polymer one instead.

How did we do?

Now that we have worked through our plan let's look back and see how we did.

  1. Reliable with factory hunting or match ammo - Handloading is unnecessary

    1. We will run the 6 ARC as it hits hard and has factory ammo offerings in the bullet weights and types we wish to use.

    2. We have selected reliable magazines meant expressly for the 6 ARC

    3. Our bolt is MPI and pressure tested to mitigate failure points

  2. Able to harvest eastern whitetail deer - State law - minimum 23 caliber

    1. The 6 ARC meets the state minimum and is adequate for eastern sized whitetail deer

  3. Optic capable - Flattop upper receiver

    1. We will run an LPVO 1-6x optic with a lightweight mount on our rifle

  4. A comfortable weight to carry - less than 9lbs with optic and loaded mag

    1. Keeping the weight down with a lighter LPVO and lightweight mount

    2. Opting for a polymer handguard over an aluminum free float one

    3. Selecting a lightweight pencil contour for our 14.5" pinned and welded barrel

  5. Packable length for a 4wheeler front/rear rack - Collapsible stock or shorter barrel

    1. Utilizing a 14.5" pinned and welded barrel

    2. Selecting a collapsible stock setup for a shorter collapsed length

Overall it looks like our plan will give us a rifle that will meet all of our criteria and perform as intended. When we start from our end goal and break out the rifle into a plan of bite-sized chunks we start to look at the rifle as more of a system of parts that are all working towards the same end goal. This is what makes a truly great and enjoyable rifle, one that is boringly reliable and will seem to be an extension of the shooter when it is put into action. It doesn't have to look fancy or end up on the cover of a magazine to get the job done and be money well spent.

In our next part we will look into putting our plan to work for a home/civil defense carbine and the additional considerations that come with that platform. Until then, get out there and burn some powder!

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