I had in mind some specifications that I wanted to meet that would address specific concerns in regards to the shorter barrel length. The first question to be answered when considering a SBR is naturally the barrel length where I clearly want to go shorter than the legal requirement of 16" for the benefit of being a bit faster moving and shooting around obstacles but without sacrificing the reliability and accuracy as well as muzzle velocity and in the end the effectiveness of the 5.56mm round. I chose 11.5" as anything shorter seems to compromise dwell time which is the amount of time the bolt hesitates in the buffer tube before cycling back to close the bolt as it loads another round. The dwell time is affected at a number of locations starting with when the bullet travels down the barrel. The way an AR-15's direct gas impingement system (DI for short) works is that as the bullet travels down the barrel, it passes the gas block where the path of least resistance will have the gases from the fired round travel up the gas block and into the gas tube which gets fed back into the receiver where the gas impinges on to the bolt carrier's gas key which pushes the bolt back to cycle it (I found an excellent video on YouTube that illustrates what I just explained quite clearly). What changes in a SBR is that the bullet leaves the barrel sooner compared to a 16" barrel which means the path of least resistance for the gas will change to going straight out of the front of the barrel as it is a linear path as well as a larger diameter hole compared to the gas block's port and there will be less gas pushing the bolt carrier back. The lower gas creates a choppier running AR and may create reliability issues as a result starting with dwell time from the choppy cycling of the BCG (Bolt Carrier Group). I was deciding between a 10.5" barrel and a 11.5" barrel where the four percent difference in length gave it an astounding 40% more dwell time with the one inch longer 11.5" barrel and this is where my minimum length came from.
The next issue is muzzle velocity and kinetic energy where after crunching some numbers have come to show that there certainly is less muzzle velocity and kinetic energy delivered out of the end of the barrel between a 16" and SBR barrel lengths but when comparing the increments of SBR barrel lengths from 10.5" out to 14.5" I have discovered that until I got out to 14.5" that there was not a significant enough differences between 10.5" to 12.5" and decided to go with the shortest barrel that I could go without affecting reliability and made my final decision that a 11.5" barrel length is right for me.
Now that I have decided on a 11.5" barrel I wanted to be able to stabilize the bullet well considering the barrel length and I decided on a 1:7 (1 revolution in 7 inches) twist rate which is a fast twist rate capable of stabilizing up to a 77 grain projectile and enough distance within the 11.5" to put a good spin on the projectile from the rifling. I also wanted it to be chrome lined with a heavier profile to extend barrel life as well as to aid in accuracy and manage heat better. The Stag Arms barrel is made of 4150 steel which is harder than 4140 which many of the cheaper barrels are made of and resists wear, warping, and heat better in comparison between the two. 4150 is used for applications such full-auto receivers which would introduce more wear and heat which goes to show the better wear and heat management compared to 4140. I looked around to see who makes a 11.5" barrel with a 1:7 twist rate and discovered that the list is a short one, Bravo Company Mfg, Daniel Defense, and Spike's tactical to name a few and they all make nice barrels and uppers but at a cost. Looking at how a barrel is manufactured, you can only make a barrel so many ways and comparing the Stag barrel to the others I would say it is on par and in some case better than other manufactures and thus; going with Stag Arms upper which also offers me a better price.
Next comes the upper receiver and what is actually important in selecting the receiver: that the military specifications dimensions are met so that the upper receiver will mate with the lower receiver as expected where if out of specification it will affect fit and finish and it should be made with quality materials, the end. Anything beyond the necessary should be per user's use case and should take into consideration what needs the particular use will create and how those needs will be met. As for the ever so slangy buzz word that is 'mil-spec', the specifications are set by the military in regards to the M4 and M16 rifles as a minimum requirement of what the rifle should be comprised of which covers things like what the barrel material should be made of, that the dimensions should be within a given tolerance as to permit across the board parts commonality no matter who the vendor is, the flat top picatinny rails are with in a known dimension, what the buffer tube diameter and thread pitch should be, the buffer weight, and so no to ensure that when a rifle is built, the build is consistent rifle to rifle and that the parts in the armory will be applicable to all of them save for select special purpose rifles which fall under a different set up specifications and under a different weapon designation. There are very cool uppers made such as the Vltor MUR which offers a heavier duty upper, and the Lewis Machine & Tool's monolithic uppers receivers, ADCOR's B.E.A.R uppers with user changeable reciprocating handle, but I wanted to stick to the standard DI AR design for parts commonality and the Stag Arms upper meets all of my requirements without an inflated cost.
Bolt Carrier Group, or BCG is responsible for exactly what the name implies: the bolt; but additionally it creates the lock up against the chamber, loads new rounds and extracts and ejects spent shells, allows the firing pin to travel within to strike the primer for ignition, manipulates the hammer to the ready to fire position, provides a gas key as a surface for the gas to impinge upon, and is a contributing factor to dwell time. The dwell time is affected by the BCG by density and the heavier the bolt, the better it is as far as a SBR is concerned. The additional mass aids in BCG dwell simply by being heavier and creating more mass for the buffer spring to overcome before pushing it back into battery. The weight also helps in minimizing bolt bounce which affects follow up shot times, accuracy, and reliability as well as preventing a possibility for accidental bump fire. The heavier bolts are often called full-auto or M16 bolts and the name comes from the BCG used in full auto M16 and M4 rifles having a heavier mass to accommodate the full-auto BCG cycling of the rifle as well as manipulation of the full-auto sear. The full-auto bolt is not an automatic part and will not turn your semi-auto into a full auto rifle as commonly misunderstood, it just means that it's bigger and heavier as seen below.
The Stag Arms upper I chose comes included with the M16 BCG and a mil-spec compliant charging handle where other companies such as Bravo Company Mfg. does not include the BCG or charging handle in the upper and must be purchased separately, or in case of other manufactures the included BCG is of the AR-15 variety where I prefer the M16 BCG for the increased mass to aid in the dwell time and overall operation and reliability of the SBR.
My next requirement was to have a low profile gas block but Stag Arm's site shows the SBR uppers only with the Front Sight Block (FSB), oh noes. I called Stag up and asked if it would be possible to get the FSB swapped out with a low profile block at the factory to accommodate after market handguards that would go over the gas block and not only were they very accommodating to the request but that it would also be done free of charge and will be tested before it is shipped....well that's awesome. All of these reasons are why I chose Stag Arms over the competitors and have ordered the upper from Operation Parts and Op Parts owner Charlie is an awesome guy helping me source all of the parts for my SBR and highly recommend buying your next AR needs from him and form my experience, if it is not listed on his site; just email and ask him and he more than likely can get it in for you at am awesome price.
I wondered in an earlier post what stock to buy and I decided on a stock and a buffer tube system, the Vltor A5 buffer tube and IMOD stock. The A5 buffer tube is a mil-spec diameter tube that is longer than a carbine tube but not as long as a full length rifle tube where the extra space is the final piece in aiding in dwell time, the longer length means more space the BCG can compress the spring down and the further it can travel back. Add to this the A5H2 buffer which is heavier than a standard H2 buffer and create a complete solution to SBR dwell time. The other thing that the A5 aids in preventing or out right eliminating is bolt bounce where the bolt bounces back from the chamber lock up before seating into battery or at worse, preventing lock up and not seating into battery at all and at the hazardous end, accidental bump fire/negligent discharge. AlphaOps has a deal on a combo of the IMOD and A5 and not only is it an amazing deal, the customer service is superb and everything shipped quickly. The other stock I considered was the Magpul STR which came out recently to rave reviews but I could not find the color that I wanted in stock but the IMOD was my first choice and it's awesome. The IMOD is comfortable to shoulder and the clubfoot version is comfortable to shoot prone, is lighter that it's predecessors with the same reliable construction you expect from Vltor with a shorter length compared to the EMOD.
As I mentioned earlier that I wanted the low profile gas block to allow a handguard to cover over it and my requirements were to have a continuous rail up top from handguard to upper receiver, with it being thin and light as possible and the handguard I chose was the recently released Troy Industry's Bravo BattleRail. The Bravo BattleRail does away with the complex mounting of it's previous iteration and takes cues from the Alpha rail resulting in a thinner and lighter rail handguard. The 11" version weighs in at 13.35oz. with the width at 2.2" and height of 2.44" and will cover all of the barrel and allows me to mount sights as far out as possible to give it the longest sight radius on the 11.5" barrel length. I also considered the Rainier Arms/Samson Evolution which is a small diameter handguard with options to mount rails as needed but decided that I did not want to buy nor mount the rails separately and adhered to the KISS principle (Keep It Simple Stupid).
Last but certainly not least is the muzzle device. The Stag upper comes with an A2 but that is not what I intend to keep on it, I will be testing out a two muzzle devices from Griffin Armament, the M4-SD Tactical Compensator and the soon to be released flash suppressing compensator. Austin over at Griffin Armament has been kind enough to invite me to test the muzzle devices and I will be publishing my findings here as well as in various forums.
Now to wait for all of my parts to come in...