Building your first AR15 pt 3
Part 3, The Upper Receiver
The receiver itself is a lot more simple to build on the upper half, an upper receiver parts kit will really only consist of 9 parts total. The receiver, the charging handle, the forward assist pawl, forward assist spring and retaining pin, the dust cover, dust cover spring, dust cover pin and a "C" clip for it. There are some upper receivers that are referred to as "sport" or "slick sides"
Models that do away with the dust cover and the forward assist altogether. Some even remove the shell deflector. These can be found in a few models, A2, which is the classic Vietnam Era look with the large carry handle and rear iron sight as part of the receiver. The A3, or flat top, which has a picatinny rail running the length of the top to enable different optics to be attached.
As with the Lower, they can be had as forged or billet. Most receivers these days come with M4 feed ramps, which aid in rapid fire reliability. I can't see any reason not to have these.
This device is used to help push the bolt into battery if the buffer spring does not complete the action. This is one of those things that will normally never be used, but if the gun becomes very dirty it can sometimes fail to fully chamber a round. It can also be useful for people hunting because you can use the charging handle and "ride the bolt" slowly forward to chamber a round silently and then use the forward assist to push the bolt into battery, This will be much quieter than the normal chambering procedure. I personally have had to do this within about 75 of a buck and it enabled the kill.
This is a spring loaded flap that can be pushed closed when the weapon is not in use to keep your receiver, bolt and chamber clean, upon firing or cycling the bolt it will snap open out of the way. I have it on my builds but admit to rarely remembering to close it.
This is what charges the weapon, pretty simple. There are a million variations, most changes include an oversized latch or adding a latch to to opposite side to help left handed shooters.
Some receivers and bolts are made to be side charging, like a Hi Point Carbine, this would also be fine yet they are considerably more expensive.
The Bolt Assembly or Bolt Carrier Group (BCG):
This is made of several parts and can be found in many materials and coating options.
The mil-spec carrier is made from 8620 steel and has a manganese phosphate finish
They work fine. Other materials include 9310 which is claimed to be even stronger.
Try to find a chrome lined carrier, as well as gas key. This makes them very tough and easier to clean.
The bolt itself is usually made from Carpenter 158 steel, and also has the manganese-phosphate finish, although lately many different options are popping up. Metallurgy has come a long way since the 1960's. These days there are several different coatings and treatments available for all of these parts. The most common are nickel boron, nickel teflon and black nitride or Melonite. These all claim to be the slickest snot out there.
You definitely want to buy your BCG from a well known company. The quality of the BCG will affect the accuracy and overall reliability of your weapon.
This is also a critical part of your rifle, a good AR15 barrel is capable of 2-3" groups at 100 yards while a great barrel can do less than 1 inch.
As with the BCG, there are lots of options when it comes to material and finish. The Mil-Spec is a Chrome-Moly steel or also called Chrome-Moly Vanadium (CMV), either 4140 or 4150 with a chrome lined chamber and bore. 4150 is reported to be able to withstand about 100 degrees more than 4140 before starting to degrade, but we are not talking the difference between 200 deg and 300 deg here! Odds are you will never get your barrel close to the temps needed for this to show up.
The chrome lining aids in corrosion resistance and aids in keeping it clean. Chrome lined barrels can last for 10's of thousands of rounds before getting worn out. Chrome lining can negatively affect accuracy though, the common claim is by as much as ⅓.
On the other side you have Stainless Steel, most commonly 416R, this is chosen for superior rust and corrosion resistance as well as ease of cleaning. Stainless barrels are chosen for incredible accuracy, however they are much more susceptible to abuse, extended rapid fire can degrade these quicker than a CMV barrel.
Barrel length also affects accuracy and velocity. As does contour. Keep in mind to stay within your decided use for the gun, you don't want a 24", 3 lb barrel on a home defence weapon!
Barrel twist rate: This determines the speed of the spin imparted on your bullet.
The numbers you see refer to the inches of barrel it takes to spin the bullet 1 time.
For example, 1:7 will spin the bullet 1 time in 7 inches. 1:9 will spin it once in 9 inches.
In general you will find either 1 in 7, 8 or 9 for an AR 15. Choosing one is dependent on what type of ammo you will be shooting. A small lightweight bullet will require less spin to stabilize.
A long heavy bullet needs more spin. For short distances a short bbl with a 1:9 will work great. This will let you fire projectiles from about 40 gr up to about 69 gr. (most of what you will find)
For a longer bbl and longer distances a 1:7 will let you fire all the way up to monster 80gr projectiles (provided they are loaded short enough to load in your magazine!)
A 1:8 is a good compromise which will fire the 55 gr and 62 gr milsurp 5.56 ammo fine and let you get up into the mid 70's for longer ranges.
The AR 15 is normally a direct impingement gas system, which means that hot gas from the fired round is directed back into the receiver via a tube which unlocks the bolt from the chamber by rotating the locking lugs of the bolt out of the lugs of the chamber, then using the gas to cycle the bolt. There is also a piston gas system which uses a rod to push on the bolt. There are eternal arguments for either system and you will have to decide which is best for your use.
There are also different lengths of gas systems, pistol is the shortest and normally goes with very short barrels. Then carbine, mid length and the rifle. The length of the gas system (length from the receiver to the gas port) should increase as the barrel length increases. The reason has to do with "dwell time" the length of time that the bullet is in the barrel after the shot is fired.
On a longer barrel, the dwell time will be longer, because the bullet has to travel a longer distance. Specifically, there is a tiny amount of time when the bullet is traveling through the barrel and it is past the gas port, but it's still in the barrel. During this time, there is hot gas traveling through the gas tube into the receiver. As soon as the bullet leaves the muzzle, the gas stops flowing. If there is too much barrel length after the gas port, then too much gas flows into the receiver and it can cause issues with excessive recoil and wear on the rifle.
If there is not enough barrel length after the gas port, then too little gas flows into the receiver and the rifle may not cycle or may jam.
In general, 7 to 12" will have pistol, 14.5 and 16" bbls will come with a carbine length, 16 to 18" will be mid or rifle and 20" or more will be rifle. Sometimes different calibers will require different gas systems than the norm. For instance my 16" bbl in 300 blackout has a pistol length gas system.
It's worth mentioning that there are factors that can affect the gas system, beyond the length of the system.
Adjustable gas blocks some gas blocks available that allow you to throttle down the amount of gas that travels through them. For example you can constrict the gas to the point that the rifle is single shot. The gas block is what attaches to the barrel and redirects the gas backwards down the gas tube into the gas key on the bolt carrier. Gas blocks come in a few varieties, those with a front sight already attached, normally called A2 gas blocks
those with a rail on top to attach a sight,
or those designed to fit under a free floated handguard.
Buffer weight the weight of the buffer can have a big effect on how the AR15 cycles. A heavier buffer will generally require more gas pressure to cycle, so it is possible to moderate the effect of too much gas pressure and smooth out the action by using a heavier buffer, or vice versa with a lighter buffer. Buffer springs can also have an effect.
Ammo type The weight of the bullet and the powder in the cartridge can make a big difference. A heavier bullet may travel more slowly and cause a longer dwell time, whereas a low-power cartridge may not build enough gas pressure to cycle the action properly.
Building an AR15 is an extremely rewarding decision that will make you proud and really make you very familiar with the firearm. Good luck and have fun!