rcmodel
Member in memoriam
CCI #400 is about all I have used in AR'15s and Mini-14's for going on 44 years.
That ain't the problem, unless you smash them in.
rc
AR15 slam fire
- Thread starter edfardos
- Start date
- Status
That ain't the problem, unless you smash them in.
rc
ole farmerbuck
Member
Just shoot a couple of rounds then eject the loaded round from the chamber and see if there is a firing pin mark on the primer.
(hope nobody already said this)
(hope nobody already said this)
rcmodel
Member in memoriam
In my experience, there should be a firing pin mark on every primer that has been chambered but not fired.
Military M-16's do it with mil-spec ammo, commercial AR-15's do it with commercial & reloaded ammo too.
They all do it.
It's just the nature of the floating firing pin to ding every primer when the round slams home and the bolt locks and allows the firing pin to bounce off the primer.
It will not fire it, but it always leaves a small mark.
rc
Military M-16's do it with mil-spec ammo, commercial AR-15's do it with commercial & reloaded ammo too.
They all do it.
It's just the nature of the floating firing pin to ding every primer when the round slams home and the bolt locks and allows the firing pin to bounce off the primer.
It will not fire it, but it always leaves a small mark.
rc
ole farmerbuck
Member
I see.
243winxb
Member
CCI 400 Primers
[/URL][/IMG]
My CCI 400 primers (lot No. H05Q) flow in my Savage axis with mid-range loads. Primer sensitivity may change from lot to lot? Rem 7 1/2 show no flow.
Very True.
Yup, significant ding is in the primer after the bolt closes. Always has been. But with these CCI400 (non-mag) primers, that ding is sufficient to detonate the primer compound, especially if what RCmodel is saying, with me making the primer super-sensitive by inserting it with too much force.
Editorial: CCI might be having quality control issues too. I've seen a few thousandths difference in their cup height between lots. My first box of CCI450's took an amazing amount of force just to seat flush. Subsequent lots were easy, and measured shorter.
--edfardos
Editorial: CCI might be having quality control issues too. I've seen a few thousandths difference in their cup height between lots. My first box of CCI450's took an amazing amount of force just to seat flush. Subsequent lots were easy, and measured shorter.
--edfardos
I don't believe the idea that you can make a primer super sensitive through seating, this is something that is unsupported by data. If anything, it will crack the primer cake and make a dud primer. You will run into a lot of people who do not accept the idea that the inertial impact of firing pin could set off a primer. The AR15 actually went through troop trials and was in general issue before a slamfire pattern was fired out. As usual, the Armed Services deigned there was anything wrong with their rifle, blaming ammunition manufacturer's for "high primers", but in the end, what was found that there was something wrong with their rifle and their ammunition specification:
Report of the M16 Rifle Review Panel. Volume 5, Appendix 4. Ammunition Development Programhttp://www.dtic.mil/dtic/
Primer Sensitivity
Initial Specifications. Ammunition specifications established by the Air Force on 24 January 1963 provided for quality control against cocked, inverted, loose, and nicked primers. The specifications further provided for inspection and test of waterproofing
and the crimp of primers. However, the specifications did not provide for specific limitations on primer sensitivity for 5.56mm ammunition.-
Development. At the first meeting of the Technical Coordinating Committee on 26 March 1963,16 / the Air Force representatives submitted a list of reported ammunition deficiencies, which included "high primers" and "primers too sensitive". It was agreed that Frankford Arsenal would investigate the matter and recommend corrective action.
One of the malfunctions reported by the Air Force was the premature firing of cartridges that occurred upon initial charging of the M16 rifle with a cartridge from the magazine, or upon singleloading of a cartridge directly into the chamber, or when two rounds were fired at one trigger pull during semiautomatic fire.
This malfunction was attributed to "high" or protruding primers, although the tests did not confirm this theory..
However, analysis indicates that if high primers caused the premature firing, the firing should have occurred upon impact of the bolt face with the protruding primer. At this point in the weapon cycle, the bolt head would not. have been rotated to the locked position by action of the cam pin and carrier. Had firing occurred with the bolt in the unlocked position, it would have resulted in a blow back and would not have been undetected. No such disruptions were reported . Since premature firing occurred after bolt-locking, it must have coincided in time with the impact of the bolt carrier against the bolt head. At the instant of impact, the "free floating" firing pin is moving at the velocity of the bolt carrier. The kinetic energy of the pin must be dissipated by such frictional forces as it encounters in the forward movement, and, finally, in impact of the firing pin tip with the primer of the chambered cartridge. This premise was confirmed by the visible indentation appearing on cartridges which were chambered by the mechanism and extracted unfired.
As CCI has stated, high primers are the most common cause of misfires in the article Mysteries And Misconceptions Of The All-Important Primer http://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/ because the anvil has to be firmly seated on something and the primer cake pushed into the anvil. Not saying that something weird could happen in a tight pocket or a pocket which the crimp was not removed, certainly a shallow pocket could produce the conditions necessary for a high primer to ignite, which is a firmly seated anvil and the primer cake pushed into the anvil, but since these were not here, I don't believe these are the causes.
Slamfire deniers don't recognize that primer sensitivity varies by lot and each individual primer varies in sensitivity. To them, primers are all like the same, all act the same, and they basically ignore primer sensitivity. They are very consistent in holding that primers are a non issue and that all slamfires are caused by mechanical means and user misconduct. They learned this from an Ordnance Employee who wrote for the American Rifleman. In the article he wrote, the only allowed causes for slamfires were high primers and your worn out receiver bridge. This was back in the day when the only semi autos on the market in quantity were M1a's and M1 Garands, of course , this fails for AR15's which don't have a receiver bridge. It is interesting to note that this author was a leading participant in the AR15 slamfire investigation in the 60's but when it came to slamfires and the Ordnance Department designed M1 and M14, primer sensitivity has nothing to do with anything.
Still, at the time, the Ordnance Department calculated they wanted a primer sensitivity that gave a 1:10 million chance of a slamfire for the AR15. People win lotteries every month even though the odds are 1:100 million.
Commercial primers are on the average more sensitive than mil spec primers and you would expect the more sensitive the primer the greater the chance of a slamfire. However, and this must be understood, this weapon and any weapon with a free floating firing pin can slamfire due to firing pin inertia even with mil spec primers.
This chart came from a DTIC presentation and you can see that given a certain dryness, probability of ignition varies, and varies by impact. At certain impact energy levels the probability of ignition is close to 100%, but at other energy levels, sometimes it goes off, sometimes it does not. Incidentally this is the military primer mix.
Different primer compositions have different ignition curves. Red phosphorous is a different primer technology, but you can see how the ignition curves are different. Primer cake is a mixture and the % of components will always vary so you would expect that some batches of primers are going to be a little more sensitive or a little less sensitive. We all have had dud primers, and extra insensitive primer, but slamfire deniers will not admit that there is such a thing as an extra sensitive primer.
Federal makes the most sensitive primer composition and primers, but Federal finally reacted to the many AR15 slamfires by issuing their mil spec primer. According to this, they did not change the mix, but they made the primer cup thicker, which deadens the firing pin blow. We should encourage Federal to make their military large rifle primer line available to the public and reduce the number of slamfires in Garands and M1a's.
http://bulletin.accurateshooter.com/2011/09/new-federal-gold-medal-match-primers-for-ars/
September 11th, 2011
New Federal Gold Medal Match Primers for ARs
Federal Ammunition has released a new type of small rifle match primer optimized for AR15s and similar semi-automatic rifles. The new Gold Medal® AR Match Primers, designated GM205MAR, have harder primer cups than the popular Federal 205M match primers. The harder cup is designed to perform better in semi-automatic actions that use free-floating firing pins. A Federal spokesman said that Federal’s “normal” 205M primers were not ideal for use in firearms, such as ARs, with free floating firing pins. Hence Federal designed the new GM205MAR primers. These are available now from major vendors such as Midsouth Shooters Supply, which offers the new GM205MAR primers for $35.22 per thousand.
Now, my first thought on reading this thread was that your doubles were due to bump firing. Back in the day when everyone shot with a tight sling, you just did not have bump fires. Now most shooters are shooting using sandbags and a bench rest and bump fires are very frequent. However, your primer picture makes me think that you did have a firing pin initiated slamfire especially by the weird donut on one primer. I am seeing more and more AR15 slamfire primers with that donut shape. For some reason, Garands and M1a slamfire rounds don't have this feature, they look like a normal indention. Don't know why.
Conditions that could cause an out of battery slamfire in a AR15 is a shallow primer pocket with a properly seated primer sticking high above the case head, a broken firing pin sticking through the bolt face, or an over long firing pin sticking through the bolt face. That is about the range of possibilities that I can think of.
I have never heard or seen a busted AR15 firing pin, though not saying it could not happen, and an over long firing pin could happen, but have never seen one, so as long as you ensure that your primers are properly seated below the case head, you are not going to have an out of battery slamfire in this mechanism. Only in battery slamfires.
I think is that you have a batch of standard CCI primers that are more sensitive than usual. I would not use this batch in a semi auto even though an inbattery slamfire will not blow up the rifle or you, but, a bullet zinging down range because your rifle unexpectedly went off could hurt someone else.
Report of the M16 Rifle Review Panel. Volume 5, Appendix 4. Ammunition Development Programhttp://www.dtic.mil/dtic/
Primer Sensitivity
Initial Specifications. Ammunition specifications established by the Air Force on 24 January 1963 provided for quality control against cocked, inverted, loose, and nicked primers. The specifications further provided for inspection and test of waterproofing
and the crimp of primers. However, the specifications did not provide for specific limitations on primer sensitivity for 5.56mm ammunition.-
Development. At the first meeting of the Technical Coordinating Committee on 26 March 1963,16 / the Air Force representatives submitted a list of reported ammunition deficiencies, which included "high primers" and "primers too sensitive". It was agreed that Frankford Arsenal would investigate the matter and recommend corrective action.
One of the malfunctions reported by the Air Force was the premature firing of cartridges that occurred upon initial charging of the M16 rifle with a cartridge from the magazine, or upon singleloading of a cartridge directly into the chamber, or when two rounds were fired at one trigger pull during semiautomatic fire.
This malfunction was attributed to "high" or protruding primers, although the tests did not confirm this theory..
However, analysis indicates that if high primers caused the premature firing, the firing should have occurred upon impact of the bolt face with the protruding primer. At this point in the weapon cycle, the bolt head would not. have been rotated to the locked position by action of the cam pin and carrier. Had firing occurred with the bolt in the unlocked position, it would have resulted in a blow back and would not have been undetected. No such disruptions were reported . Since premature firing occurred after bolt-locking, it must have coincided in time with the impact of the bolt carrier against the bolt head. At the instant of impact, the "free floating" firing pin is moving at the velocity of the bolt carrier. The kinetic energy of the pin must be dissipated by such frictional forces as it encounters in the forward movement, and, finally, in impact of the firing pin tip with the primer of the chambered cartridge. This premise was confirmed by the visible indentation appearing on cartridges which were chambered by the mechanism and extracted unfired.
As CCI has stated, high primers are the most common cause of misfires in the article Mysteries And Misconceptions Of The All-Important Primer http://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/ because the anvil has to be firmly seated on something and the primer cake pushed into the anvil. Not saying that something weird could happen in a tight pocket or a pocket which the crimp was not removed, certainly a shallow pocket could produce the conditions necessary for a high primer to ignite, which is a firmly seated anvil and the primer cake pushed into the anvil, but since these were not here, I don't believe these are the causes.
Slamfire deniers don't recognize that primer sensitivity varies by lot and each individual primer varies in sensitivity. To them, primers are all like the same, all act the same, and they basically ignore primer sensitivity. They are very consistent in holding that primers are a non issue and that all slamfires are caused by mechanical means and user misconduct. They learned this from an Ordnance Employee who wrote for the American Rifleman. In the article he wrote, the only allowed causes for slamfires were high primers and your worn out receiver bridge. This was back in the day when the only semi autos on the market in quantity were M1a's and M1 Garands, of course , this fails for AR15's which don't have a receiver bridge. It is interesting to note that this author was a leading participant in the AR15 slamfire investigation in the 60's but when it came to slamfires and the Ordnance Department designed M1 and M14, primer sensitivity has nothing to do with anything.
Still, at the time, the Ordnance Department calculated they wanted a primer sensitivity that gave a 1:10 million chance of a slamfire for the AR15. People win lotteries every month even though the odds are 1:100 million.
Commercial primers are on the average more sensitive than mil spec primers and you would expect the more sensitive the primer the greater the chance of a slamfire. However, and this must be understood, this weapon and any weapon with a free floating firing pin can slamfire due to firing pin inertia even with mil spec primers.
This chart came from a DTIC presentation and you can see that given a certain dryness, probability of ignition varies, and varies by impact. At certain impact energy levels the probability of ignition is close to 100%, but at other energy levels, sometimes it goes off, sometimes it does not. Incidentally this is the military primer mix.
Different primer compositions have different ignition curves. Red phosphorous is a different primer technology, but you can see how the ignition curves are different. Primer cake is a mixture and the % of components will always vary so you would expect that some batches of primers are going to be a little more sensitive or a little less sensitive. We all have had dud primers, and extra insensitive primer, but slamfire deniers will not admit that there is such a thing as an extra sensitive primer.
Federal makes the most sensitive primer composition and primers, but Federal finally reacted to the many AR15 slamfires by issuing their mil spec primer. According to this, they did not change the mix, but they made the primer cup thicker, which deadens the firing pin blow. We should encourage Federal to make their military large rifle primer line available to the public and reduce the number of slamfires in Garands and M1a's.
http://bulletin.accurateshooter.com/2011/09/new-federal-gold-medal-match-primers-for-ars/
September 11th, 2011
New Federal Gold Medal Match Primers for ARs
Federal Ammunition has released a new type of small rifle match primer optimized for AR15s and similar semi-automatic rifles. The new Gold Medal® AR Match Primers, designated GM205MAR, have harder primer cups than the popular Federal 205M match primers. The harder cup is designed to perform better in semi-automatic actions that use free-floating firing pins. A Federal spokesman said that Federal’s “normal” 205M primers were not ideal for use in firearms, such as ARs, with free floating firing pins. Hence Federal designed the new GM205MAR primers. These are available now from major vendors such as Midsouth Shooters Supply, which offers the new GM205MAR primers for $35.22 per thousand.
Now, my first thought on reading this thread was that your doubles were due to bump firing. Back in the day when everyone shot with a tight sling, you just did not have bump fires. Now most shooters are shooting using sandbags and a bench rest and bump fires are very frequent. However, your primer picture makes me think that you did have a firing pin initiated slamfire especially by the weird donut on one primer. I am seeing more and more AR15 slamfire primers with that donut shape. For some reason, Garands and M1a slamfire rounds don't have this feature, they look like a normal indention. Don't know why.
Conditions that could cause an out of battery slamfire in a AR15 is a shallow primer pocket with a properly seated primer sticking high above the case head, a broken firing pin sticking through the bolt face, or an over long firing pin sticking through the bolt face. That is about the range of possibilities that I can think of.
I have never heard or seen a busted AR15 firing pin, though not saying it could not happen, and an over long firing pin could happen, but have never seen one, so as long as you ensure that your primers are properly seated below the case head, you are not going to have an out of battery slamfire in this mechanism. Only in battery slamfires.
I think is that you have a batch of standard CCI primers that are more sensitive than usual. I would not use this batch in a semi auto even though an inbattery slamfire will not blow up the rifle or you, but, a bullet zinging down range because your rifle unexpectedly went off could hurt someone else.
SlamFire1, wow, just wow. You've obviously chosen the right nicname on this forum as well. I agree with all your points. My biggest concern was an out of battery slam fire, but as you, and others have indicated, that's highly unlikely. I tend to agree i have a batch of sensitive primers. I'll likely fire my remaining live rounds and sell my unused primers to bolt-action friends. I'll be switching back to CCI450 (mag) primers when I can find'm. While I'm sure CCI400's have worked in many AR15's for years, there's a clear indication I have a sensitive batch. I emailed CCI to this effect as well. I also use a deserted area of the National forest, far from anyone and anything, but to be honest that second shot is right behind the first, it happens that fast.
Great science guys - this is a huge help.
--edfardos
Great science guys - this is a huge help.
--edfardos
blarby
Member
And thats why I was waiting for his response
genebofunk
Member
- Joined
- Aug 20, 2011
- Messages
- 40
I had a batch of CCI primers that were sensitive too. I had several slam fires out of one lot. One happened when I inserted a magazine and hit the bolt release. It put a decent sized hole in the concrete 2 feet in front of me. It happens. As a result I always keep the barrel pointed at the berm/backstop when I charge the first round.
GW Staar
Member
- Joined
- Feb 26, 2009
- Messages
- 3,695
If you are going to prime using any progressive press, you have to make sure you've not only got the crimp out, but that you have a decent presentation to the pocket. That means a rounded or chamfered entry that won't catch an edge and crumple the primer going in. There's nothing worse when loading progressively than having stoppages due to primers not feeding right....and no press is immune when primer pockets are suspect.
For trouble-free feeding either ream with a reamer that rounds the edge or swage and then touching the swaged pocket to a reamer for a half second in a Trim Mate (while doing uniforming and flash hole deburring operations)
If you look at commercial brass, you'll notice they all have rounded entry.
Since the O.P. uses a Pro 2000, I'd suggest uniforming pockets so that you can use the depth stop....that's the only way you can be ensured of pockets set the same depth using the press.....it makes up for the lack of feel you can only get from a hand primer. I still use a hand primer, for odd balls I don't load progressively, and I use one to prime a first case to the proper depth....put that case in the press in #2 and use it to set the depth screw to make quick work of the rest of the batch.
I had one slamfire 2 years ago (Remington R25/DPMS AR10 clone), and that was before I got decrimping military brass to be used on the progressive figured out. (in the middle of the learning curve with a new press) The reason for that one was a high primer....one of two in a batch of 100. The high primer was caused by my swager shearing off a thin ring of brass in the crimp and depositing it into the bottom of the primer hole......Then when I uniformed the hole the ring of brass just spun preventing it from cutting to the target depth.
I found a similar ring of brass in the pocket of the 2nd high primer in the batch. (that round was un-shot but taken apart for inspection) Moral of the story is if you uniform and it doesn't seem to be cutting, check for a brass ring.
One more note for the O.P. if he uses a swager. Using a Trim Mate, (or drill) you can have trouble with the pocket catching on the uniformer and wanting to yank it out of your hand. That's another sign of too sharp a presentation to the pocket. Touch the pocket to the reamer first, then uniform the pocket with no trouble at all.
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GWStaar, I disable the depth stop and seat by feel on the rcbspro2000. The variances in rim thickness will allow high primers even as the press hits the stop. I use the stop in 40S&W, because it's easy to crush/deform small pistol primers, and for some reason 40's seem to seat deep easily. I think RCmodel is right about making sensitive primers by using too much force, but my force hasn't changed for the last 2000 rounds and I'm seeing slamfires with a new lot of CCI400's. Looking at my loading logs, it would appear I used CCI400 long ago, w/o problems. I'm still thinking I have a bad lot. The only other variable is this FC brass, but I can't make a connection to that at this time.
1300 CCI400 primers for sale! $45, Auburn/Sacramento area, bolt action users only please
.thanks again for all the info. I feel more comfortable shooting the rest of these since the AR15 can't really slam-fire out-of-battery unless the primer is high (which I'll be double checking before I go to the range).
cool,
-edfardos
GW Staar
Member
- Joined
- Feb 26, 2009
- Messages
- 3,695
Rims don't vary that much. If you set the stop to seat .007 with a "thin" rim and they vary .002, worse case is .005 below flush. Still plenty deep. But if you can "feel" them in with the press, go for it.
I can't...too old I guess.
ArchAngelCD
Member.
That statement is completely incorrect. The NATO rated CCI#41 primers are magnum strength primers without a doubt and that is directly from CCI, not a guess. The information is at the bottom of the linked page and other spots on their site too.
http://cci-ammunition.com/products/primers/primer_chart.htm
I have used a handful of different primers in AR ammo and all have worked for me without incident. In order of preference these are the primers I have used: CCI #41, CCI-450, Remington 7 1/2 and CCI-400.
When loading .223 ammo meant for a bolt action rifle I use either CCI-400 primers or CCI BR-4 primers.
CCI replied to my inquiry the next day!
.......................................................
"the CCI 400 primer has a standard cup and the CCI-450 primer has a thick cup, this is the difference and if slam fires are a concern then you should be using the CCI #41 primer. It has a thick cup magnum priming charge and an anvil angle change to prevent slam fires. "
Make Every Shot Count!
Justin M.
CCI/Speer
.......................................................
"the CCI 400 primer has a standard cup and the CCI-450 primer has a thick cup, this is the difference and if slam fires are a concern then you should be using the CCI #41 primer. It has a thick cup magnum priming charge and an anvil angle change to prevent slam fires. "
Make Every Shot Count!
Justin M.
CCI/Speer
ArchAngelCD,
With no * or other flag, I missed the note at the bottom of the page. I consulted that page but completely missed it. Not a good way to put that somewhat important information out there.
Lesson learned. Thanks
Humbly,
Art
ArchAngelCD
Member.
Art,
No reason to be contrite. This isn't a competition, we are all in this together. It doesn't matter who comes up the the answer as long as in the end we all get the correct information.
Welcome to the forum...
No reason to be contrite. This isn't a competition, we are all in this together. It doesn't matter who comes up the the answer as long as in the end we all get the correct information.
Welcome to the forum...
243winxb
Member
Head Clearance
I wonder if head clearance is a factor when using sensitive primers ?? When comparing head to datum of both lots of ammo, would a difference of .010" have an effect? If the slam fired rounds only had .002" head clearance, would the primer be struck by the inertia firing pin with more force? The rounds with .010" head clearance would have more slop in the chamber. A light tap from the pin may move the round forward in the chamber, not causing a slam fire? But maybe the extractor would keep the round from moving forward? This is with all primers seated correctly, about .004" below the case head. Just thinking out loud.
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Good ideas, but I would think the rather powerful ejector spring would keep the round seated as far forward as possible, either against the extractor, or against the shoulder whichever comes first.
Fwiw, I fired another 50 of the "suspect" rounds, with zero slamfires. I'm on target when I close the slide in any event. Probably a good habbit in general.
edfardos
(OP)
Fwiw, I fired another 50 of the "suspect" rounds, with zero slamfires. I'm on target when I close the slide in any event. Probably a good habbit in general.
edfardos
(OP)
I had this happening with my disconnect. If I pulled the trigger and held it bottomed out then the disconnect would work fine. If I pulled the trigger really really slow and careful like I was shooting for accuracy I could hold the trigger half way and the disconnect wouldn't catch. I had it pump 3-4 off at a time while figuring it all out.
sinbad339
Member
This isn't the first mention of a failure of the disconnector to cause a slam fire, but I understood that the AR15 bolt will not allow the hammer to follow the bolt home all the way; instead it is "captured" with the bolt not completely closed. The intent is to prevent simple conversion to full auto (although not select fire) of an AR15. Can someone verify this?
rcmodel
Member in memoriam
The way all AR-15's/M-16's work is, the bolt slides in and out of the bolt carrier.
It is only locked shut at full foreword travel of the bolt carrier.
As the bolt carrier telescopes over the bolt, the cam pin rotates the bolt into full lock-up in the barrel extension.
At all other times?
The firing pin is too short to reach the primer until the bolt carrier has already locked the bolt shut.
Even IF the hammer could hit the firing pin before the bolt is locked?
It is too short to reach the primer until the bolt is rotated into full lock-up by the bolt carrier & cam pin.
In a full-auto M-16, the hammer is tripped (by an unnamed part) of the trigger group AFTER the bolt is fully locked by the carrier.
If the hammer simply followed the bolt carrier down, it would not have enough energy left to fire the primer.
rc
It is only locked shut at full foreword travel of the bolt carrier.
As the bolt carrier telescopes over the bolt, the cam pin rotates the bolt into full lock-up in the barrel extension.
At all other times?
The firing pin is too short to reach the primer until the bolt carrier has already locked the bolt shut.
Even IF the hammer could hit the firing pin before the bolt is locked?
It is too short to reach the primer until the bolt is rotated into full lock-up by the bolt carrier & cam pin.
In a full-auto M-16, the hammer is tripped (by an unnamed part) of the trigger group AFTER the bolt is fully locked by the carrier.
If the hammer simply followed the bolt carrier down, it would not have enough energy left to fire the primer.
rc
HisStigness
Member
I was told that ARs shooting 300blk are susceptible to slam fires so I have been using cheap tula primers with a hard cup. Does anybody know why 300blk would be more likely to slam fire?
sinbad339
Member
rc,
I understand about an AR15/M16 not firing out of battery, but I also read where the AR15 bolt is designed to "catch" the hammer if it is riding the carrier forward.
"Another difference [between the M16 and AR15] in the bolt carriers is the additional metal removed on the underside to expose the collar of the firing pin. This is intended to catch the hammer and prevent firing if the carrier is not fully forward when the hammer releases. This occurs if the hammer is "riding" the carrier back (i.e. no auto sear is holding back the hammer). On the M16 bolt carriers, the firing pin collar is not exposed."
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