Coal Dragger
member
For anyone who is interested I have decided to post up an initial thoughts review of a carbine I recently acquired: the Knight's Armament SR-15 E3 Mod 2 M-Lok. I understand that not everyone is going to be interested in a high end hard use factory gun on here, but figured some might find the topic interesting since it is not something usually found on the The High Road.
For those who don't know Knight's Armament Company has been in business since the 1980's and has focused almost exclusively on selling weapon systems, suppressors, and other cool toys to Uncle Sam for our troops to use in the pursuit of shooting savages in the face. The company has been particularly successful in introducing and selling weapons and suppressors to the US Special Operations community since those units often have a lot of acquisition discretion that the regular Army, Navy, and Marines don't enjoy. Knight's has the distinction of being the last company to employ Eugene Stoner, and his influence is apparent in the larger SR-25 as well as the refinements to the smaller platform that is the SR-15/SR-16 family. In the last few years Knight's civilian market business seems to have picked up and they're making a more distinct effort to offer high quality hard use guns to regular law abiding citizens. Not that they ever discouraged it, just that up until the last few years finding any KAC complete rifle, carbine, or upper was like finding a unicorn on the civilian market.
A few months ago I ordered an SR-15 E3 Mod 2 M-Lok through Brownell's since they were offering a pretty solid price and I had some gift certificates, and their military, LEO, retired military discount applied as well. I've had a chance to shoot it a bit, and compare it to my other AR/M4 variants and have a few observations.
First of all some of you are going to ask what is it, and how is it any different than any other AR variant? That is a fair question and at the heart of the SR-15, what makes it different is the E3 bolt and the KAC approach to the gas system. There are other nice touches and a lot of quality parts, but what makes it tick is the bolt and the changes to how the DI gas system is implemented.
So about that bolt.... it is different than a standard AR bolt. To begin with KAC implemented the SR-25 like changes to bolt lug geometry that Stoner helped refine later in his life. The lugs are rounded where they meet the bolt body, both at the rear of each lug where lateral bolt thrust loads are present, and in between the lugs on the sides where rotational thrust will be present. This is a known method of reducing the possibility of cracking, and reducing crack propagation. In the interests of reducing bolt lug cracking the bolt face is supported where the normal AR bolt is relieved for the extractor. By designing the part in this manner the bolt lugs adjacent to the extractor are much more well supported against rotational thrust loads. Another effort in the crusade against premature bolt failures is the bolt cam pin hole is smaller in diameter leaving more material in the bolt tail area and increasing strength. Finally unlike a standard AR bolt, which will usually be made from Carpenter 158 steel, the KAC E3 is made from a "proprietary" alloy. The "proprietary" alloy is rumored to be AerMet 100 which is more or less a super steel used for aerospace applications, and of the 3 AerMet alloys has the highest resistance to crack propagation. AerMet 100 is also popular in the auto racing community as a material for output shafts because it is so tough. Unfortunately it costs a lot not only for the material but to machine and heat treat it. Of course KAC is mum on what the alloy actually is, but there are no reported failures of a KAC SR-15 bolt in the wild. I was able to find one where the extractor got bent up my a case head blow out, but the bolt itself was good to go just needed a new extractor.
The first photo is a factory Colt bolt carrier group, nothing unusual:
The next photo is of the KAC E3 bolt carrier group:
Also note that the extractor is different, the KAC design utilizes double springs and a relocated pivot point to exert more leverage on the case rim. Colt on the left KAC on the right.
Here we can see the differences in cam pin hole size, Colt on the left KAC on the right:
The next image gives a look at each bolt broken down, Colt on the left KAC on the right, notice the KAC bolt face supports the case in a full 360 degrees:
Assembled bolts facing the camera for a view of the bolt lug difference, Colt on the left KAC on the right:
Now in order to accommodate the different bolt lug shape the barrel extension is also different, so the KAC E3 bolt is proprietary to the KAC barrel extension. Cleverly in an emergency the KAC barrel extension will accept a standard AR bolt though, so if you have a spare AR bolt and your KAC bolt were to break you can still run the gun with a regular bolt until you can source a replacement KAC part.
I would try to photograph the gas manifold but all attempts resulted in mediocre photos that didn't really show what is going on very well. So I'll explain it as best I can. First of all the gas system is longer than a mid length, but shorter than a full rifle length on this 16" barrel so we'll call it an intermediate length gas system. The second change is the attachment method of the gas manifold/gas block, instead of dimples in the barrel and set screws or a roll pin, or taper pin, KAC has threaded the outside portion of the barrel ahead of the gas block. On this threaded area a castle nut is torqued down against the KAC gas block, the inside diameter of the block must be tapered a bit, and so must the outside diameter of the barrel where it mounts; the castle nut being torqued down creates a very tight secure seal as the tapered ID of block is forced onto the cone OD of the barrel.
On top of the gas manifold where the gas tube mounts there is another change, instead of the gas tube inserting into the gas block the end of the tube is flanged to fit over a nipple shaped outlet on the gas block with a threaded area behind it; over which a nut is torqued down forcing a tight leak proof seal here as well. All of these changes allow a relatively small gas port to be utilized for the gas system length, resulting in a very very smooth shooting carbine. Even with the stock 3 prong flash hider, which does nothing to mitigate recoil or muzzle climb, the gun stays pretty flat, and recoil is negligible. Brass is ejected into a nice consistent pile to the 3:00 with 5.56 NATO and to about 4:00 using 75gr .223 match ammo. I haven't tried any remanufactured 55gr .223 ball which is often loaded light so I can't comment on it. Winchester M855, IMI M193, Black Hills MK 262 Mod 1, Hornady 75gr 5.56 Superformance, and Hornady 75gr Steel Match are all it has been fed so far. No failures to feed, extract, or eject. One failure to lock back on the last round using an ancient Colt 20 round magazine with a spring so weak I was surprised it didn't have any other issues, this mag will often not hold open the bolt on my other AR's so it's time for a new magazine spring. All other magazines have functioned 100%.
Accuracy is in the 1" range for 5 shot groups at 100 yards, I've only shot it using the iron sights and a Nightforce NXS 1-4X24 with the FC-3G reticle in a Nightforce Uni-Mount. Not an ideal optic for shooting groups since the center dot obscures quite a bit of target and is tough for me to consistently place on target the same every time. I suspect that if I mount my 3.5-15X50 NXS I'll get better results, and might do that for fun, otherwise the little 1-4X24 is a light compact little scope and it gets useful hits on reasonably sized targets just fine and makes more sense on carbine that weighs 6.5lbs.
So what else do you get for all your hard earned $$$ (make no mistake it costs some $$$ too...)?
Well for starters the carbine has a 16" cold hammer forged, chrome lined barrel. KAC doesn't want to share with everyone what the alloy is, but Ballistic Radio put 20,000 rounds through one and never cleaned it and got it really hot... and it was still turning in 1" groups at 100 yards at the end; so whatever it is it's pretty tough. The contour is not heavy but it's not quite a pencil contour either, there is no Government Profile stupidity going on before the gas block. Just a gentle taper until the barrel flares out a bit for the gas block to mount against, and then a light contour out to the muzzle. The muzzle appears nicely crowned and the chrome lining looks good. Inside by either luck or design the gas port on my barrel is in one of the grooves, so happy meal for me. The chamber area chrome lining looks like a mirror, so KAC did a good job of polishing the chamber and fired brass shows it, no dings, no scratches, no tool marks. Looks like it came out of a bolt gun. Out front KAC mounts their 3 prong flash hider, which is also a suppressor mount for their QDC line of suppressors, it looks nice and probably does a good job of killing muzzle flash. I will probably replace it with a hybrid device until I get a suppressor. The rest of the upper is also pretty nice, with the KAC URX M-Lok rail which is 14.5" long and doubles as the barrel nut making the assembly light and very very stiff. There are built in QD cups for a sling at the 9:00 and 3:00 on the rail ahead of the upper. Included are KAC folding iron sigths, the front is adjustable for elevation with no tools. The rear sight is adjustable for windage, and has a calibrated elevation drum from 200-600M.
You also get a fully ambidextrous lower receiver with the safety, mag release, and bolt release all being lefty friendly. The furniture is Magpul and I see no reason to change it. The trigger guard is a KAC aluminum piece that fills in the gap between the pistol grip and the lower, very nice touch. The receiver extension is a 6 position, nothing fancy just functional, KAC has put on an end plate with built in QD cups on both sides though. Inside you will find the KAC two stage match trigger, they claim a 4.5lb total trigger pull weight and that is probably accurate. It is a similar trigger to a Geissele SSA not only in feel, but in how the sere surfaces are relocated vs a GI trigger. One nice touch is all the trigger group internals appear to be nickel plated or NP3 plated. I'm a huge Geissele trigger fan boy, but the KAC trigger will stay where it is, no need to change it.
The carbine also ships with:
1 hilariously detailed owners manual with fold out schematics.
1 30 round Gen 3 PMag with window.
1 box of 6 MagPul M-Lok rail grip panels.
1 KAC 3" 5 slot aluminum M-Lok Picatinny rail adapter.
1 KAC locking device that inserts into the mag well, and then via roller locks, locks into the upper making it impossible to separate the upper from the lower, or do anything with the weapon. it is also designed to make it very difficult to get a set of bolt cutters into the area where the pad lock locks it up.
1 Pad lock and keys for the above mentioned KAC solution to preventing unauthorized users.
2 QD sling swivels for the owner to mount his or her sling with in the provided QD cups on the carbine.
1 SR-15 sticker, cause you gotta represent yo!
1 KAC branded hard case.
Ultimately for a not insignificant amount of money you get a very refined light carbine that shoots well, will probably last longer than a "lesser" carbine under a heavy firing schedule, and that is ready to go out of the box. Just add a sling, an optic if you want, and whatever else you think you need bolted onto the outside but the rifle itself needs basically no upgrades at all.
Hope some of you found this interesting.
For those who don't know Knight's Armament Company has been in business since the 1980's and has focused almost exclusively on selling weapon systems, suppressors, and other cool toys to Uncle Sam for our troops to use in the pursuit of shooting savages in the face. The company has been particularly successful in introducing and selling weapons and suppressors to the US Special Operations community since those units often have a lot of acquisition discretion that the regular Army, Navy, and Marines don't enjoy. Knight's has the distinction of being the last company to employ Eugene Stoner, and his influence is apparent in the larger SR-25 as well as the refinements to the smaller platform that is the SR-15/SR-16 family. In the last few years Knight's civilian market business seems to have picked up and they're making a more distinct effort to offer high quality hard use guns to regular law abiding citizens. Not that they ever discouraged it, just that up until the last few years finding any KAC complete rifle, carbine, or upper was like finding a unicorn on the civilian market.
A few months ago I ordered an SR-15 E3 Mod 2 M-Lok through Brownell's since they were offering a pretty solid price and I had some gift certificates, and their military, LEO, retired military discount applied as well. I've had a chance to shoot it a bit, and compare it to my other AR/M4 variants and have a few observations.
First of all some of you are going to ask what is it, and how is it any different than any other AR variant? That is a fair question and at the heart of the SR-15, what makes it different is the E3 bolt and the KAC approach to the gas system. There are other nice touches and a lot of quality parts, but what makes it tick is the bolt and the changes to how the DI gas system is implemented.
So about that bolt.... it is different than a standard AR bolt. To begin with KAC implemented the SR-25 like changes to bolt lug geometry that Stoner helped refine later in his life. The lugs are rounded where they meet the bolt body, both at the rear of each lug where lateral bolt thrust loads are present, and in between the lugs on the sides where rotational thrust will be present. This is a known method of reducing the possibility of cracking, and reducing crack propagation. In the interests of reducing bolt lug cracking the bolt face is supported where the normal AR bolt is relieved for the extractor. By designing the part in this manner the bolt lugs adjacent to the extractor are much more well supported against rotational thrust loads. Another effort in the crusade against premature bolt failures is the bolt cam pin hole is smaller in diameter leaving more material in the bolt tail area and increasing strength. Finally unlike a standard AR bolt, which will usually be made from Carpenter 158 steel, the KAC E3 is made from a "proprietary" alloy. The "proprietary" alloy is rumored to be AerMet 100 which is more or less a super steel used for aerospace applications, and of the 3 AerMet alloys has the highest resistance to crack propagation. AerMet 100 is also popular in the auto racing community as a material for output shafts because it is so tough. Unfortunately it costs a lot not only for the material but to machine and heat treat it. Of course KAC is mum on what the alloy actually is, but there are no reported failures of a KAC SR-15 bolt in the wild. I was able to find one where the extractor got bent up my a case head blow out, but the bolt itself was good to go just needed a new extractor.
The first photo is a factory Colt bolt carrier group, nothing unusual:
The next photo is of the KAC E3 bolt carrier group:
Also note that the extractor is different, the KAC design utilizes double springs and a relocated pivot point to exert more leverage on the case rim. Colt on the left KAC on the right.
Here we can see the differences in cam pin hole size, Colt on the left KAC on the right:
The next image gives a look at each bolt broken down, Colt on the left KAC on the right, notice the KAC bolt face supports the case in a full 360 degrees:
Assembled bolts facing the camera for a view of the bolt lug difference, Colt on the left KAC on the right:
Now in order to accommodate the different bolt lug shape the barrel extension is also different, so the KAC E3 bolt is proprietary to the KAC barrel extension. Cleverly in an emergency the KAC barrel extension will accept a standard AR bolt though, so if you have a spare AR bolt and your KAC bolt were to break you can still run the gun with a regular bolt until you can source a replacement KAC part.
I would try to photograph the gas manifold but all attempts resulted in mediocre photos that didn't really show what is going on very well. So I'll explain it as best I can. First of all the gas system is longer than a mid length, but shorter than a full rifle length on this 16" barrel so we'll call it an intermediate length gas system. The second change is the attachment method of the gas manifold/gas block, instead of dimples in the barrel and set screws or a roll pin, or taper pin, KAC has threaded the outside portion of the barrel ahead of the gas block. On this threaded area a castle nut is torqued down against the KAC gas block, the inside diameter of the block must be tapered a bit, and so must the outside diameter of the barrel where it mounts; the castle nut being torqued down creates a very tight secure seal as the tapered ID of block is forced onto the cone OD of the barrel.
On top of the gas manifold where the gas tube mounts there is another change, instead of the gas tube inserting into the gas block the end of the tube is flanged to fit over a nipple shaped outlet on the gas block with a threaded area behind it; over which a nut is torqued down forcing a tight leak proof seal here as well. All of these changes allow a relatively small gas port to be utilized for the gas system length, resulting in a very very smooth shooting carbine. Even with the stock 3 prong flash hider, which does nothing to mitigate recoil or muzzle climb, the gun stays pretty flat, and recoil is negligible. Brass is ejected into a nice consistent pile to the 3:00 with 5.56 NATO and to about 4:00 using 75gr .223 match ammo. I haven't tried any remanufactured 55gr .223 ball which is often loaded light so I can't comment on it. Winchester M855, IMI M193, Black Hills MK 262 Mod 1, Hornady 75gr 5.56 Superformance, and Hornady 75gr Steel Match are all it has been fed so far. No failures to feed, extract, or eject. One failure to lock back on the last round using an ancient Colt 20 round magazine with a spring so weak I was surprised it didn't have any other issues, this mag will often not hold open the bolt on my other AR's so it's time for a new magazine spring. All other magazines have functioned 100%.
Accuracy is in the 1" range for 5 shot groups at 100 yards, I've only shot it using the iron sights and a Nightforce NXS 1-4X24 with the FC-3G reticle in a Nightforce Uni-Mount. Not an ideal optic for shooting groups since the center dot obscures quite a bit of target and is tough for me to consistently place on target the same every time. I suspect that if I mount my 3.5-15X50 NXS I'll get better results, and might do that for fun, otherwise the little 1-4X24 is a light compact little scope and it gets useful hits on reasonably sized targets just fine and makes more sense on carbine that weighs 6.5lbs.
So what else do you get for all your hard earned $$$ (make no mistake it costs some $$$ too...)?
Well for starters the carbine has a 16" cold hammer forged, chrome lined barrel. KAC doesn't want to share with everyone what the alloy is, but Ballistic Radio put 20,000 rounds through one and never cleaned it and got it really hot... and it was still turning in 1" groups at 100 yards at the end; so whatever it is it's pretty tough. The contour is not heavy but it's not quite a pencil contour either, there is no Government Profile stupidity going on before the gas block. Just a gentle taper until the barrel flares out a bit for the gas block to mount against, and then a light contour out to the muzzle. The muzzle appears nicely crowned and the chrome lining looks good. Inside by either luck or design the gas port on my barrel is in one of the grooves, so happy meal for me. The chamber area chrome lining looks like a mirror, so KAC did a good job of polishing the chamber and fired brass shows it, no dings, no scratches, no tool marks. Looks like it came out of a bolt gun. Out front KAC mounts their 3 prong flash hider, which is also a suppressor mount for their QDC line of suppressors, it looks nice and probably does a good job of killing muzzle flash. I will probably replace it with a hybrid device until I get a suppressor. The rest of the upper is also pretty nice, with the KAC URX M-Lok rail which is 14.5" long and doubles as the barrel nut making the assembly light and very very stiff. There are built in QD cups for a sling at the 9:00 and 3:00 on the rail ahead of the upper. Included are KAC folding iron sigths, the front is adjustable for elevation with no tools. The rear sight is adjustable for windage, and has a calibrated elevation drum from 200-600M.
You also get a fully ambidextrous lower receiver with the safety, mag release, and bolt release all being lefty friendly. The furniture is Magpul and I see no reason to change it. The trigger guard is a KAC aluminum piece that fills in the gap between the pistol grip and the lower, very nice touch. The receiver extension is a 6 position, nothing fancy just functional, KAC has put on an end plate with built in QD cups on both sides though. Inside you will find the KAC two stage match trigger, they claim a 4.5lb total trigger pull weight and that is probably accurate. It is a similar trigger to a Geissele SSA not only in feel, but in how the sere surfaces are relocated vs a GI trigger. One nice touch is all the trigger group internals appear to be nickel plated or NP3 plated. I'm a huge Geissele trigger fan boy, but the KAC trigger will stay where it is, no need to change it.
The carbine also ships with:
1 hilariously detailed owners manual with fold out schematics.
1 30 round Gen 3 PMag with window.
1 box of 6 MagPul M-Lok rail grip panels.
1 KAC 3" 5 slot aluminum M-Lok Picatinny rail adapter.
1 KAC locking device that inserts into the mag well, and then via roller locks, locks into the upper making it impossible to separate the upper from the lower, or do anything with the weapon. it is also designed to make it very difficult to get a set of bolt cutters into the area where the pad lock locks it up.
1 Pad lock and keys for the above mentioned KAC solution to preventing unauthorized users.
2 QD sling swivels for the owner to mount his or her sling with in the provided QD cups on the carbine.
1 SR-15 sticker, cause you gotta represent yo!
1 KAC branded hard case.
Ultimately for a not insignificant amount of money you get a very refined light carbine that shoots well, will probably last longer than a "lesser" carbine under a heavy firing schedule, and that is ready to go out of the box. Just add a sling, an optic if you want, and whatever else you think you need bolted onto the outside but the rifle itself needs basically no upgrades at all.
Hope some of you found this interesting.
Last edited:
