I first posted this in this FB discussion: https://www.facebook.com/groups/NoviceSecondary/permalink/1116146825098483
however due to it's length and detail I figured it would be a good fit here as well.
Introduction
It is often asked why is “mil-spec” better than “commercial” carbine receiver extensions. Truth is carbine receiver extensions are little more complicated than simply “commercial” vs. “mil-spec”, there really are 3 major types to consider. First so-called commercial extensions, we’ll consider this group as anything that more or less looks and works like the standard carbine receiver extensions. Next the “mil-spec” receiver extension, lets limit this group to receiver extensions conforming drawing number 9390019 - Extension, Receiver and slight variations there of e.g. 5 stock positions or extra drain holes for improved over the beach capability. The last group being dimensionally identical to the mil-spec type but otherwise not meeting or exceeding all of the specifications for the mil-spec receiver extensions, we’ll refer to these types as pseudo-mil-spec receiver extensions. Now on to what makes these types different or better than one or the other.
Why commercial and Mil-Spec versions?
Ok, so a little background on some of the minutia of the carbine receiver extension. The major day to day difference between commercial and mil-spec(pseudo types included) is the primary outside diameter of the tube portion of the extension, typically in the ⌀ 1.170” ballpark for the commercial types and between ⌀1.143”- ⌀1.149” for the mil-spec types. This is due to the manufacturing method and the 1.1875-16 thread used for attaching to the lower receiver, you will notice the thread’s major diameter(⌀1.1875”) is greater than the tube OD of the mil-spec type extensions, keep this in mind. For the most part two types of processes are used to produce the hunk of aluminum used to machine a receiver extension from: Extrusion and Impact Extrusion.
Think of extrusion as how spaghetti noodles are made, the aluminum is pushed through a die shaped like the profile of a receiver extension producing a length of aluminum with a matching profile, this is typically how commercial extensions start life.
Impact extrusion is used almost exclusively to produce mil-spec type extensions(it’s also how rifle type receiver extensions are made) imagine a 35mm film canister filled with play-doh now stick your finger down in the middle of it, that’s an impact extrusion, a slug of aluminum is loaded into a die and a punch is inserted into the slug and die extruding the aluminum up and out of the die. The big advantage of impact extrusion is instead of having a solid receiver extension shaped piece of aluminum with have a hollow tube with a closed end shaped like a receiver extension, this saves a lot of machine time and scrap material. Additionally, the impact extrusion process is capable of producing a shape with a taper or multiple diameters, allowing the tube portion to be formed at ⌀1.143”- ⌀1.149” and the thread portion to be formed at ⌀1.1875”.
This stepped diameter of an impact extrusion allows for fully formed threads to cut in to the extension satisfying the mil-spec requirement, threads cut into extruded extensions are often truncated or not fully formed as there is not enough material to do so. The truncated threads of the commercial type receiver extension are one of the primary inferiorities in comparison to mil-spec receiver extension, the fit and strength of threads are nowhere as good or strong as fully formed threads.
Why are commercial receiver extensions not just extruded at the mil-spec diameter? Well at a diameter of about 1.149” you would have at best a barely functional thread form and you would be running a reasonable chance of a RUD (Rapid Unintentional Disassembly, or as rocket guys say Rapid Unscheduled Disassembly). This is where the mythical rolled threads come in, I say mythical because in my experience I’ve never come across any real evidence of this being used on receiver extensions, anyways rolled threads are formed rather than cut. A note on thread anatomy first, if you look at cross section of thread you will see what appears to be a series of peaks and valleys, the angle of these peaks and valleys being 60°. The top of the peaks represents the Major Diameter of the thread while the bottom of the valleys represents the Minor Diameter of the thread, somewhere between these two diameters is a mostly theoretical diameter referred to as the Pitch Diameter. As I said the Pitch Diameter is mostly theoretical except when it comes to rolled threads, it’s the starting diameter(or at least close enough for this example) of the part before the threads are formed, the thread rollers form the thread by displacing the volume of the thread below the Pitch Diameter in to the volume of the thread profile above the Pitch Diameter. It just so happens the Pitch Diameter of the 1.1875-16 threads used on the receiver extension is a bit smaller than the primary tube OD of 1.149”, allowing perfect fully formed threads to made using this method. In theory a true mil-spec receiver extension can be made using a standard extrusion and this threading method, it’s bit wasteful but it can be done.
So basically the dimensional differences between commercial and mil-spec type receiver extensions come down to the thread size used and the different manufacturing processes needed to achieve that. Why anyone makes commercial type receiver extension, honestly I don’t know, other than the reason would probably cause me to sigh and put my palm to my face and shake my head. Now on to mil-spec vs pseudo-mil-spec where there is very real reason for the difference.
Attention to Detail and Adequateness
Attention to detail and adequateness are the primary talking points here but both really just boil down to cost vs value and the time honored tradition of penny pinching in manufacturing. Let’s tackle some of the common differences between mil-spec and pseudo-mil-spec receiver extensions, primarily final protective finish and base material. The mil-spec receivers are to be finished with a matte(except for the thread area) black type III anodize coating, this is pretty standard stuff for M16 parts, however the mil-spec also calls out for solid film lubricant on the inside tube portion.
These are the details that start to separate the men from the boys, the solid film lubricant if difficult to apply to the military’s satisfaction they have been known to reject whole lots for the slightest lack of coverage on the inside, a common issue because of the relatively deep hole geometry of the inside of the receiver extension. The solid film lube also adds cost and manufacturing steps due to this presence of the solid film lube is one of the defining characteristics between a true mil-spec receiver extension and a pseudo-mil-spec extension; pseudo-mil-spec types almost universally omit the solid film lube. The deep hole geometry also causes all sorts of issues with anodizing, bubbles are trapped inside the tube during the anodize bath process that lead to poor to no anodize coverage in the bottom of the inside diameter. If you section a pseudo-mil-spec I can almost guaranty the lack of good anodize coverage on the closed end along with the lack of solid film lube, note however that even mil-spec receiver extensions will sometime show this defect.
The other biggie in terms of mil-spec vs pseudo-mil-spec receiver extensions is the material, the mil-spec calls for 7075-T6 aluminum, pseudo-mil-spec extensions use 6061-T6 aluminum. Honestly this is probably in of its self not a big deal, I can’t think of a realistic and relevant situation where a 6061 extension is going to fail on you were a 7075 extension isn’t going to fail, but it is symptomatic of overall cost-cutting, like 5 bucks per $1000 gun sort of cost cutting. Back real quick to commercial tubes, the material used for them typically varies from 6061 aluminum to the finest Chinese pot metal, and they suffer from the same finish deficiencies as the pseudo-mil-spec extensions often do.
TL;DL
To sum up there really 3 main types of carbine receiver extensions: commercial spec, pseudo-mil-spec, mil-spec. The commercial spec extensions are stupid and cheap, the pseudo-mil-spec extensions are just cheap, and well the mil-spec is what sets the bar.
As with everything take this all with a bit a salt, this based on my years of experience manufacturing AR-15 parts and rifles and my knowledge of the M16 and M4 TDPs, I’m sure there is someone out there that knows more and has more experience, at least I hope so… as I’d like to ask them some questions.
however due to it's length and detail I figured it would be a good fit here as well.
Introduction
It is often asked why is “mil-spec” better than “commercial” carbine receiver extensions. Truth is carbine receiver extensions are little more complicated than simply “commercial” vs. “mil-spec”, there really are 3 major types to consider. First so-called commercial extensions, we’ll consider this group as anything that more or less looks and works like the standard carbine receiver extensions. Next the “mil-spec” receiver extension, lets limit this group to receiver extensions conforming drawing number 9390019 - Extension, Receiver and slight variations there of e.g. 5 stock positions or extra drain holes for improved over the beach capability. The last group being dimensionally identical to the mil-spec type but otherwise not meeting or exceeding all of the specifications for the mil-spec receiver extensions, we’ll refer to these types as pseudo-mil-spec receiver extensions. Now on to what makes these types different or better than one or the other.
Why commercial and Mil-Spec versions?
Ok, so a little background on some of the minutia of the carbine receiver extension. The major day to day difference between commercial and mil-spec(pseudo types included) is the primary outside diameter of the tube portion of the extension, typically in the ⌀ 1.170” ballpark for the commercial types and between ⌀1.143”- ⌀1.149” for the mil-spec types. This is due to the manufacturing method and the 1.1875-16 thread used for attaching to the lower receiver, you will notice the thread’s major diameter(⌀1.1875”) is greater than the tube OD of the mil-spec type extensions, keep this in mind. For the most part two types of processes are used to produce the hunk of aluminum used to machine a receiver extension from: Extrusion and Impact Extrusion.
Think of extrusion as how spaghetti noodles are made, the aluminum is pushed through a die shaped like the profile of a receiver extension producing a length of aluminum with a matching profile, this is typically how commercial extensions start life.
Impact extrusion is used almost exclusively to produce mil-spec type extensions(it’s also how rifle type receiver extensions are made) imagine a 35mm film canister filled with play-doh now stick your finger down in the middle of it, that’s an impact extrusion, a slug of aluminum is loaded into a die and a punch is inserted into the slug and die extruding the aluminum up and out of the die. The big advantage of impact extrusion is instead of having a solid receiver extension shaped piece of aluminum with have a hollow tube with a closed end shaped like a receiver extension, this saves a lot of machine time and scrap material. Additionally, the impact extrusion process is capable of producing a shape with a taper or multiple diameters, allowing the tube portion to be formed at ⌀1.143”- ⌀1.149” and the thread portion to be formed at ⌀1.1875”.
This stepped diameter of an impact extrusion allows for fully formed threads to cut in to the extension satisfying the mil-spec requirement, threads cut into extruded extensions are often truncated or not fully formed as there is not enough material to do so. The truncated threads of the commercial type receiver extension are one of the primary inferiorities in comparison to mil-spec receiver extension, the fit and strength of threads are nowhere as good or strong as fully formed threads.
Why are commercial receiver extensions not just extruded at the mil-spec diameter? Well at a diameter of about 1.149” you would have at best a barely functional thread form and you would be running a reasonable chance of a RUD (Rapid Unintentional Disassembly, or as rocket guys say Rapid Unscheduled Disassembly). This is where the mythical rolled threads come in, I say mythical because in my experience I’ve never come across any real evidence of this being used on receiver extensions, anyways rolled threads are formed rather than cut. A note on thread anatomy first, if you look at cross section of thread you will see what appears to be a series of peaks and valleys, the angle of these peaks and valleys being 60°. The top of the peaks represents the Major Diameter of the thread while the bottom of the valleys represents the Minor Diameter of the thread, somewhere between these two diameters is a mostly theoretical diameter referred to as the Pitch Diameter. As I said the Pitch Diameter is mostly theoretical except when it comes to rolled threads, it’s the starting diameter(or at least close enough for this example) of the part before the threads are formed, the thread rollers form the thread by displacing the volume of the thread below the Pitch Diameter in to the volume of the thread profile above the Pitch Diameter. It just so happens the Pitch Diameter of the 1.1875-16 threads used on the receiver extension is a bit smaller than the primary tube OD of 1.149”, allowing perfect fully formed threads to made using this method. In theory a true mil-spec receiver extension can be made using a standard extrusion and this threading method, it’s bit wasteful but it can be done.
So basically the dimensional differences between commercial and mil-spec type receiver extensions come down to the thread size used and the different manufacturing processes needed to achieve that. Why anyone makes commercial type receiver extension, honestly I don’t know, other than the reason would probably cause me to sigh and put my palm to my face and shake my head. Now on to mil-spec vs pseudo-mil-spec where there is very real reason for the difference.
Attention to Detail and Adequateness
Attention to detail and adequateness are the primary talking points here but both really just boil down to cost vs value and the time honored tradition of penny pinching in manufacturing. Let’s tackle some of the common differences between mil-spec and pseudo-mil-spec receiver extensions, primarily final protective finish and base material. The mil-spec receivers are to be finished with a matte(except for the thread area) black type III anodize coating, this is pretty standard stuff for M16 parts, however the mil-spec also calls out for solid film lubricant on the inside tube portion.
These are the details that start to separate the men from the boys, the solid film lubricant if difficult to apply to the military’s satisfaction they have been known to reject whole lots for the slightest lack of coverage on the inside, a common issue because of the relatively deep hole geometry of the inside of the receiver extension. The solid film lube also adds cost and manufacturing steps due to this presence of the solid film lube is one of the defining characteristics between a true mil-spec receiver extension and a pseudo-mil-spec extension; pseudo-mil-spec types almost universally omit the solid film lube. The deep hole geometry also causes all sorts of issues with anodizing, bubbles are trapped inside the tube during the anodize bath process that lead to poor to no anodize coverage in the bottom of the inside diameter. If you section a pseudo-mil-spec I can almost guaranty the lack of good anodize coverage on the closed end along with the lack of solid film lube, note however that even mil-spec receiver extensions will sometime show this defect.
The other biggie in terms of mil-spec vs pseudo-mil-spec receiver extensions is the material, the mil-spec calls for 7075-T6 aluminum, pseudo-mil-spec extensions use 6061-T6 aluminum. Honestly this is probably in of its self not a big deal, I can’t think of a realistic and relevant situation where a 6061 extension is going to fail on you were a 7075 extension isn’t going to fail, but it is symptomatic of overall cost-cutting, like 5 bucks per $1000 gun sort of cost cutting. Back real quick to commercial tubes, the material used for them typically varies from 6061 aluminum to the finest Chinese pot metal, and they suffer from the same finish deficiencies as the pseudo-mil-spec extensions often do.
TL;DL
To sum up there really 3 main types of carbine receiver extensions: commercial spec, pseudo-mil-spec, mil-spec. The commercial spec extensions are stupid and cheap, the pseudo-mil-spec extensions are just cheap, and well the mil-spec is what sets the bar.
As with everything take this all with a bit a salt, this based on my years of experience manufacturing AR-15 parts and rifles and my knowledge of the M16 and M4 TDPs, I’m sure there is someone out there that knows more and has more experience, at least I hope so… as I’d like to ask them some questions.