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Analysis of the Rock River Arms 9mm System

There are two other major types of AR-15 9mm setups (Colt and Olympic), and eventually I hope to also obtain samples of these types of uppers for side-by-side comparison.

Bolt Carrier Differences

From a top side view, the absence of a gas impingement system is a very obvious difference. The 223 (above) has a 'cup' up top for the gas to push against, while the 9mm only has the vestigal bump in order to help prevent the carrier from tilting.
Side by side, a substantial difference with the 9mm carrier (left) is that the bolt and bolt carrier are one piece. On the 223 (right), the rotating bolt is a very large difference.
Top view. Notice how the Rock River carrier has a shorter slot? Just more examples of having more material in order to have a heavier bolt carrier.
A diagonal view. You'll notice the slot on the 223 bolt carrier (on the right) to allow the bolt to retract and rotate as it slams into the forward position. As noted earlier, the 9mm has no counterpart of this, and is a straight blowback action with no rotating/locking bolt. However, you will notice that the pins which keep the firing pin in place are identical between the Rock River and Stag bolt carriers.
The under view of both bolts shows (if you look quite closely - there's a closeup later, though) that the 9mm bolt has a deep groove on the left side of the bolt, for the extraction hook on the magazine well adapter.
Notice the roll pin in the back. This pin penetrates through both sides of the bolt carrier, and holds the rear carrier weight in place.
Notice the difference between the rears of the two bolt carriers. There's a substantial weight difference, caused by the insertion of what appears to be a steel weight into the back of the 9mm bolt carrier.
This is both bolt carriers stripped. You'll notice that the firing pins are visibly different between the two. As has been noted, the rotating bolt on the 223 adds quite a number of additional moving parts to the 223 bolt carrier.
Closeup on the disassembled 223 bolt carrier. Notice how the firing pin is fairly long? Unlike the 9mm firing pin, it isn't spring-loaded. It also happens to be longer than the 9mm firing pin.
Closeup on the disassembled 9mm bolt carrier and firing pin. As mentioned above, the 9mm firing pin is spring-loaded. This is necessary because in the 223 design, the bolt slows its travel near the end of its recoil cycle, and the firing pin isn't subjected to as great of an impulse as it locks into battery. In the 9mm system, the bolt carrier slams into battery and stops abruptly. Without a spring to counteract that impulse, it would be possible for light primers to result in slamfires or out-of-battery explosions.
Side-by-side view of the 223 and 9mm firing pins. You'll notice that the 9mm pin is actually shorter than the 223 pin. The shaft is almost the same length, but most of the reduced length comes from shortening the rear striking surface.
Right-side view of both bolt carriers. Notice the notches on the 223 carrier? Those are for the forward assist to gain traction when pushing the carrier into battery. The 9mm bolt carrier doesn't have them.
Front view of both bolt carriers. The bolt has been removed from the 223 carrier, so that it's possible to pay more attention to the carrier itself. Notice that the extractor slot is machined into the side of the 9mm bolt/bolt carrier because it's one piece. The 223 bolt (not pictured here) has its extractor on the bolt face.
Close-up detail shot of the forward right side on both bolt carriers. The grooves for the forward assist are quite apparent for the 223, but not so much on the 9mm. Notice that the side cutaway is actually smaller on the 9mm than on the 223. However, whereas the 223 has a couple of small holes drilled in the side, the 9mm only has an oval insert of some sort. I have a hunch that the two holes on the 223 are to allow gas to vent after the bolt has gotten past a certain point. Also, farther down there are another two holes drilled in the side of the 223 bolt carrier, whereas the 9mm only has one (which is identified as being for the fiting pin retention pin.
Close-up of the underside of the two bolt carriers. I'll have to come back to this one later, when I've had the time to identify exactly which pin is which in this picture.
Close-up on the left side of both bolt carriers. This side looks more uniform between the two; the holes are for the firing pin retention pins.
The underside of both bolt carriers. Notice that the 223 (right) has a smoother, longer slope leading up to the bolt area. The 9mm is more angular, and has a deep groove towards the right side in the picture. The groove is on the left side of the bolt when it's in a normal orientation, for reference. At any rate, that groove is what the magazine adapter's extraction hook slides along. More on that later, of course.
A far better view of the groove on the 9mm bolt carrier. Notice also the contoured curve to the left side of the 9mm carrier (right side of the pic) and the divot immediately behind it.
A top-down detail shot of the front part of both bolt carriers. Notice the hole drilled immediately after the 'hump' towards the front of the 9mm carrier? I don't know what that does. Also, notice the 9mm's scooped out area right behind the front of the bolt carrier, and the narrower channel compared to the 223 carrier to the right.
A detail shot emphasizing how much open area there is in a 223 bolt carrier compared to the 9mm.
Notice how the 223 bolt carrier has a ridge just before the gas tube riser, while the 9mm doesn't? I'm not sure what that ridge does, although I have a hunch it may be some kind of failsafe headspacing measure.
Slightly different angle for the same items mentioned above. This one really brings out that ridge I'm unsure of.
The 9mm bolt carrier (right) has a nice clear view of the claw which locks onto the rim of the 9mm round. To the left in the picture, you'll find the deep ridge I've mentioned a few times.
Notice, the 9mm bolt carrier has a contoured ramp at the rear of the grooved area, while the 223 just ends. I suspect that the contouring may be to help stop recoil, in case the blowback action actually imparts a sharper recoil impulse to the mechanism than the gas impingement action. I strongly suspect this is the case, given that 9mm is typically loaded with faster-burning powders.
Notice how the weight at the end of the bolt carrier has an integral ramp as well.
The 9mm carrier is on the left, and the 223 carrier is on the right. Notice how the hammer slot is the same diameter, but there's a lot of material milled out of the 223 that's left in the 9mm? However, the hammer slot widens considerably farther down in the 9mm, compared with the slot on the 223.
A comparison of the 223 and 9mm bolt faces. Notice the teeth on the 223 bolt, which lock into the corresponding bits on the barrel. Notice also that the 9mm bolt face looks a little bit deeper than its 223 counterpart.
Notice the scooped-out part of the 223 bolt carrier (in the foreground)? The 9mm bolt carrier's got far more material than the 223 here.
A close-up showing only the right side of the 9mm bolt carrier.
A more distant shot showing only the right side of the 9mm bolt carrier.
A shot of the underside of the 9mm bolt carrier, revealing the open part behind the firing pin.
Closeup on the front left side of the 9mm bolt carrier.
Full-length shot of the top of the 9mm bolt carrier.
Close-up on the front of the 9mm bolt face. Notice how visibly larger the groove is on the left side. Take particular note that it actually intrudes upon the left side of the bolt face. This does mean that when the round is being fired, the very base of the case is unsupported on the lower left side. This could pose a problem for brass longevity. The actual reason for the groove, is that the magazine block has a hook on it that goes along the groove. When the bolt carrier travels rearward, the hook hits the left rear part of the case and kicks it towards the ejection port during the rearward travel.

The Upper Receiver

The Magblock

-- SeanNewton - 18 Sep 2008

Thanks to Gregory Ruiz-Ade for pointing out what one of the relief cuts in the uppers was for (in order to enable to bolt assembly's cam pin to rotate).