These photos, posted on AR-15.com by m1ajunkie, compare AR-15 parts to the equivalent SCAR parts. They are very interesting photos.
Bolt Carriers
Bolt Faces
SCAR carrier disassembled.
Many more photos here.
Many thanks to Jay for emailing me the link.
“Stoner made the piston part of the bolt”
No, he eliminated the piston in the design for his lightweight surivival rifle. For a general use military rifle it made for a very dirty, unreliable design.
1 
0Here is an interesting thread regarding the SCAR at militaryphotos.net:
http://www.militaryphotos.net/forums/showthread.php?t=157086&highlight=scar
0 0R.A.W
I don’t think you understand the difference between bolt and bolt carrier. In an AR DI the bolt is the piston. In an AK it has an overhead piston that is attached to the bolt carrier.
This is an AR15 bolt. It is also act as a piston.
http://www.bravocompanyusa.com/BCM-AR15-Bolt-Assembly-MPI-p/bcm%20bolt%20assemly%20mp.htm
This is the AR15 bolt carrier.
http://www.bravocompanyusa.com/BCM-Bolt-Carrier-Auto-M16-p/bcm%20carrier%20stripped.htm
Combine the two it is called Bolt Carrier Group.
http://www.bravocompanyusa.com/BCM-Bolt-Carrier-Group-MPI-Auto-M16-p/bcm%20bolt%20carrier%20group%20auto%20mp.htm
The AR15 DI action is simpler then any overhead piston design like the AK or SCAR because the piston is part of the bolt. In an AR15 DI everything is in one nice compact package.
Carl the SCAR is an overhead piston design hence the BCG will be big and heavy.
0 0Carl – at a guess, i’d say the overhanging piston bit also might house the mainspring. if they decided to build a spring housing into the bolt carrier to keep that spring from (some of the) assorted fouling and dirt of hard use, that might account for some of the size. otherwise, the part of the SCAR bolt carrier that (ehem) carries the bolt seems fairly smallish, actually.
and i have to say that that bolt face comparison just did a lot to sour me on the SCAR. the ridiculous locking lug / barrel extension part of the AR-15 is its most unappealing aspect to me, and i wish FN had decided against copying it.
0 0jdun1911 is right, folks. The AR-15 series DOES have a piston, just like nearly every other gas operated rifle. We just call it the “bolt”. The idea that the AR is “Dirty” because it allows gas to “leak” is preposterous. There isn’t a single gas operated rifle out there that doesn’t allow some gas to leak. On most AKs, you’ll notice that the piston doesn’t exactly seal off the gas tube, which has stamped in ribs that support the piston during travel. Where does that gas go when it blows past the piston? Right into the receiver. The whole notion that the problem with the AR has to do with direct gas impingement is totally unfounded.
The problem with the AR is the Johnson/Stoner bolt design that has too many lugs and a bad extractor design. The forces are not equally distributed on the bolt lugs, and they can break. The extractor springs weaken with use, and they fail to extract. The spring loaded plunger loses force, and it fails to eject during full auto. The gas port on the AR-15 does not have a way to constrict the amount of gas that enters the system, which is a problem when gas port erosion causes the gases to travel faster and faster into the gas tube with age and use, causing very high rates of fire, which causes parts breakage and failure.
Back on topic, the SCAR looks like it has an equally poorly designed bolt, which again fails to equally distribute the recoil forces on the lugs, but at least the ejector plunger isn’t in a place that will weaken a major lug, and the extractor looks a little beefier. That carrier is probably made as large as it is in order to reduce the rate of fire and to increase the inertia necessary to close the action when it gets dirty or fouled.
0 0The worst part is the reciprocating charging handle. didn’t FN have a design for a non reciprocating handle
0 0I have both and I can tell you that my AR platform performs better than my SCAR. My AR is much lighter the trigger feels smoother and is more comfortable to carry, the SCAR has better grouping and less recoil. I would check out Sig 556 as well before buying any weapon at the SCAR’s price point.
Beauty is in the eye of the beholder but practicality should always be the reason you buy a weapon system.
0 0So, why is the SCAR bolt carrier so frickin’ huge? Looks like a waste of metal and space to me.
0 0Jdun1911, you do know that in an AK the piston is part of the bolt carrier, right?
Between that, the lack of an articulated plunger ejector, and no gas rings on the bolt, I’m pretty sure that the core AK operating system has fewer parts than the core AR operating system.
0 0I still like the XCR better.
0 0Interesting that the SCAR has one less bolt lug than the AR.
0 0So… I must say I do not know what the differences mean…
From what is see able from the pictures, is the SCAR simpler, or would it be less prone to jamming, etc?
0 0The SCAR without a doubt is a modified AR18 action, similar to the G36. I don’t know how FN could said it is a new action. It clearly is not.
0 0Valhalla,
Check out the AR15 link.
Mechanically, any overhead piston design will have more parts then DI AR15. Stoner made the piston part of the bolt. What this does is increase accuracy, made it easy to manufacture, and lighter.
If you like I can explain how an DI AR15 action work. It’s very simple but most people don’t understand it.
0 0Rifles that are made out of plastic IMO are not design for military combat. They might be ok for LEA.
0 0I would love the rifle… but I really hate the idea of a plastic lower.
Any time they swtich from metal to plastic is a step backwards in my opinion.
0 0I just can’t bring myself to like this rifle. I still say it would look better in black, but I despise FDE on guns unless it’s absolutely necessary. The stock still feels flimsy and the weapon just doesn’t balance very well in my opinion…but I certainly can’t afford to buy one to take to the range so my limited handling experience should be taken with a grain of salt.
I had the opportunity to see one of the so-called “gold” blemish models up close at a local show recently, and it didn’t change my opinion any. Gun’s still ugly.
0 0
The worst part is the reciprocating charging handle. didn’t FN have a design for a non reciprocating handle
I have both and I can tell you that my AR platform performs better than my SCAR. My AR is much lighter the trigger feels smoother and is more comfortable to carry, the SCAR has better grouping and less recoil. I would check out Sig 556 as well before buying any weapon at the SCAR’s price point.
Beauty is in the eye of the beholder but practicality should always be the reason you buy a weapon system.
jdun1911 is right, folks. The AR-15 series DOES have a piston, just like nearly every other gas operated rifle. We just call it the “bolt”. The idea that the AR is “Dirty” because it allows gas to “leak” is preposterous. There isn’t a single gas operated rifle out there that doesn’t allow some gas to leak. On most AKs, you’ll notice that the piston doesn’t exactly seal off the gas tube, which has stamped in ribs that support the piston during travel. Where does that gas go when it blows past the piston? Right into the receiver. The whole notion that the problem with the AR has to do with direct gas impingement is totally unfounded.
The problem with the AR is the Johnson/Stoner bolt design that has too many lugs and a bad extractor design. The forces are not equally distributed on the bolt lugs, and they can break. The extractor springs weaken with use, and they fail to extract. The spring loaded plunger loses force, and it fails to eject during full auto. The gas port on the AR-15 does not have a way to constrict the amount of gas that enters the system, which is a problem when gas port erosion causes the gases to travel faster and faster into the gas tube with age and use, causing very high rates of fire, which causes parts breakage and failure.
Back on topic, the SCAR looks like it has an equally poorly designed bolt, which again fails to equally distribute the recoil forces on the lugs, but at least the ejector plunger isn’t in a place that will weaken a major lug, and the extractor looks a little beefier. That carrier is probably made as large as it is in order to reduce the rate of fire and to increase the inertia necessary to close the action when it gets dirty or fouled.
Carl – at a guess, i’d say the overhanging piston bit also might house the mainspring. if they decided to build a spring housing into the bolt carrier to keep that spring from (some of the) assorted fouling and dirt of hard use, that might account for some of the size. otherwise, the part of the SCAR bolt carrier that (ehem) carries the bolt seems fairly smallish, actually.
and i have to say that that bolt face comparison just did a lot to sour me on the SCAR. the ridiculous locking lug / barrel extension part of the AR-15 is its most unappealing aspect to me, and i wish FN had decided against copying it.
Here is an interesting thread regarding the SCAR at militaryphotos.net:
http://www.militaryphotos.net/forums/showthread.php?t=157086&highlight=scar
R.A.W
I don’t think you understand the difference between bolt and bolt carrier. In an AR DI the bolt is the piston. In an AK it has an overhead piston that is attached to the bolt carrier.
This is an AR15 bolt. It is also act as a piston.
http://www.bravocompanyusa.com/BCM-AR15-Bolt-Assembly-MPI-p/bcm%20bolt%20assemly%20mp.htm
This is the AR15 bolt carrier.
http://www.bravocompanyusa.com/BCM-Bolt-Carrier-Auto-M16-p/bcm%20carrier%20stripped.htm
Combine the two it is called Bolt Carrier Group.
http://www.bravocompanyusa.com/BCM-Bolt-Carrier-Group-MPI-Auto-M16-p/bcm%20bolt%20carrier%20group%20auto%20mp.htm
The AR15 DI action is simpler then any overhead piston design like the AK or SCAR because the piston is part of the bolt. In an AR15 DI everything is in one nice compact package.
Carl the SCAR is an overhead piston design hence the BCG will be big and heavy.
“Stoner made the piston part of the bolt”
No, he eliminated the piston in the design for his lightweight surivival rifle. For a general use military rifle it made for a very dirty, unreliable design.
So, why is the SCAR bolt carrier so frickin’ huge? Looks like a waste of metal and space to me.
Jdun1911, you do know that in an AK the piston is part of the bolt carrier, right?
Between that, the lack of an articulated plunger ejector, and no gas rings on the bolt, I’m pretty sure that the core AK operating system has fewer parts than the core AR operating system.
Rifles that are made out of plastic IMO are not design for military combat. They might be ok for LEA.
I would love the rifle… but I really hate the idea of a plastic lower.
Any time they swtich from metal to plastic is a step backwards in my opinion.
Valhalla,
Check out the AR15 link.
Mechanically, any overhead piston design will have more parts then DI AR15. Stoner made the piston part of the bolt. What this does is increase accuracy, made it easy to manufacture, and lighter.
If you like I can explain how an DI AR15 action work. It’s very simple but most people don’t understand it.
The SCAR without a doubt is a modified AR18 action, similar to the G36. I don’t know how FN could said it is a new action. It clearly is not.
I still like the XCR better.
Interesting that the SCAR has one less bolt lug than the AR.
So… I must say I do not know what the differences mean…
From what is see able from the pictures, is the SCAR simpler, or would it be less prone to jamming, etc?
I just can’t bring myself to like this rifle. I still say it would look better in black, but I despise FDE on guns unless it’s absolutely necessary. The stock still feels flimsy and the weapon just doesn’t balance very well in my opinion…but I certainly can’t afford to buy one to take to the range so my limited handling experience should be taken with a grain of salt.
I had the opportunity to see one of the so-called “gold” blemish models up close at a local show recently, and it didn’t change my opinion any. Gun’s still ugly.