Please help me diagnose my slam fire

Status
Not open for further replies.
D

Danco411

Member
Joined
Dec 4, 2009
Messages
30
Today I had my first slam fire. HOLY CRAP that was scary! It happened on my LR308 while doing a ladder test. The rifle was fortunately in a Caldwell Led Sled DTF rest. The first five rounds shot fine. Upon reload I released the bolt and had an immediate discharge. Upon inspection the primer had a perfect hole all the way through the size of the firing pin. I disassembled the rifle and found nothing wrong that I could see. Other than the hole the primer looked fine. No flattening or a high seated primer. This was a low range load of 38g RL15 over a 168 hornady BTHP. Upon reloading the 5 round mag the first fired fine. The next 3 went full auto! It did not fire the fifth round. I packed up and went home. Upon inspection the 3 fired rounds had rather shallow primer strikes but otherwise fine.

I am thinking either a sticking firing pin or something out of adjustment on my Geiselle SD-E trigger. Could the sear be disengaging? I'll disassemble the trigger tomorrow. Any thing else to check?

Buying new jocky's tomorrow!

Thanks.
 
I'm not familiar with the LR308, but sounds like TurtlePhish has it covered...check and make sure the firing pin is not sticking, or broken, and then check to see if sear is disconnjcting or you are having 'hammer follow"..i.e. the hammer is immedaitely following the bolt, without the trigger being pressed.

Is it possible you were 'bump firing'? Had just enough pressure on the trigger that when the gun returned to battery, the trigger was actuated?
 
is the charge too low? might it be possible that the bolt is picking up rounds but the BCG is not going back far enough for the hammer to lock back? (I'm no expert, just trying to picture the functioning of an AR and imagine anything that might cause that problem)
 
is the charge too low? might it be possible that the bolt is picking up rounds but the BCG is not going back far enough for the hammer to lock back? (I'm no expert, just trying to picture the functioning of an AR and imagine anything that might cause that problem)
Click to expand...

If the bolt locks back on an empty mag, no. I had a short-stroking AR that cycled enough to re-cock the action and chamber a new round but had difficulty locking the bolt open when empty.
 
Primers

Just curious, since you reload....

Do you use mil-spec primers, i.e. , CCI No. 34? These stopped the slam fire for my Garand. I had loaded hundreds if not thousands of rounds for my Garand and not experineced a slam fire since using these primers. (had a few before!)

CCI No.34 primers (large rifle type) are supposed to have a less sensitive surface.
http://www.cci-ammunition.com/products/primers/primers.aspx?id=30
 
What primers? Read to what Uncle Grinch posted.

The DPMS LR-308 does not have a firing pin spring to slow the forward movement of the firing pin when the bolt closes. The ArmaLite firing pin has a single flange, and a cylindrical section before it that mounts a firing pin spring. The firing pin spring greatly reduces the light firing pin strike produced in M-16 type rifles when the bolt locks, and thus reduces the possibility of slam fire.

I recall reading something about modifying the LR-308 bolt to accept the ArmaLite pin and spring.
 
Thanks for the suggestions. I use both the #37's and standard #200's in my loads. I have hundreds of rounds through this rifle with both. I think these may have been #200's but I'll have to check.

After reading a bunch of advice this is what I think happened. I'll be able to confirm tonight when I dissasemble the BCG and trigger. I think the firing pin caused the first discharge upon releasing the bolt catch release. That heavy bolt slamming forward caused the firing pin to puncture the primer. This may have been a high primer or a stuck pin. The primer blowout caused all the gasses, carbon and unburnt powder blow back through the firing pin gunking it all up which caused the 3 round slam fire. The reason it did not empty the magazine was it worked itself loose again. Since the first discharge was upon bolt catch release and not a follow up round I think the trigger is not the culprit. I will dissasemble anyway to check it. I'll completely dissaemble the bolt to see what it looks like. My bet is a dirty mess. I'll report back.

Thanks!
 
You sure you are not firing an SKS?, :)your problems sound more like SKS slamfires than AR slamfires.

The SKS has a totally free floating firing pin that is not mechanically retracted. If the firing pin is stuck forward, the rifle will slamfire multiple times, like yours: You can see the videos at Murray’s Guns: http://www.murraysguns.com/sksown.htm I think what they did is suicidal as that SKS could have slamfired out of battery.

I can think of several firearms with free floating firing pins that use springs to reduce the inertia impact of the firing pin on the primer. A FAL is a good example. Another is the Armalite AR10. Armalite had inbattery slamfires with LC Long Range ammunition when they took the firing pin spring out. So they put the firing pin spring back in. Firing pin springs reduce the risk of a firing pin initiated slamfire. These are very rare, going from 1:9000 events to 1:64 million events (based on William Davis’s analysis of slamfires in AR15’s) depending on primer sensitivity. My Chinese SKS’s don’t have a firing pin spring but my Russian SKS’s do. Obviously the Chinese used very insensitive primers in their ammunition.

What boggles my mind is the multiple slamfires you were having. Really comes across as a mechanical problem but, and this is a big butt, on the AR designs the firing pin is mechanically retracted on extraction and does not extend until cam down. Maybe it is stuck forward in there somehow.

Check to make sure that all primers are below the case head. It is more difficult to set off a high primer than is generally understood. The anvil has to be firmly seated on something and it has to be pushed into the primer cake. If the anvil is just dangling over air space the usual effect is a misfire. However, lets say your primer is cocked, part of the anvil is firmly set, and some of the primer is above the primer pocket. I believe, though without data, that a high primer could go off if impacted by the bolt face.

You are very fortunate that you were not firing a Garand/M1a/M1 Carbine/Mini 14 as those rifles will slamfire out of battery as the firing pin is only retracted on cam down.

I still don’t understand why the Murray’s SKS did not explode in their faces.
 
It happened to me today, it scared the heck out of me. I just cleaned the firing pin/channel so I don't understand how that happened. Any advice?
Click to expand...

What type of mechanism?

Reloads or factory?

If reloads, what type of primers?

How did it happen: Did you put a round in the chamber and drop the bolt? was it from the magazine? How many rounds had you fired prior to the slamfire?
 
You might consider this first...

As we well know, there is a world of difference in the recoil of 5.56/.223 and .308/7.62. However, when we have fired a lot of AR pattern rifles in .223, we tend to hold in the same manner, with anything that looks the same. In short, our rifle is not as firmly back into the shoulder as it should be.

Congratulations! You've set yourself up for inadvertent bump fire.

Would this be the case in a lead-sled? With a trigger originally designed for the AR platform, maybe not--but with a match trigger installed, the sear engagement surfaces are drastically smaller.

We, as shooters, like to pull our triggers all the way back--that's called follow through, and it is one of the basics of good shooting. A remote trigger does not allow follow through.

I accidentally bump fired an M1A. Rifle was not tight enough into my shoulder. Result? It bounced forward, right onto the trigger finger. Fastest double tap ever with a military rifle without the go-faster switch.

Suggestion?

If possible, replace the innards with the "normal" AR-looking fire control parts, and fire off a mag. Your pull weight will be much harder--but I'll bet the problem will go away.
 
Not to sound like a nervous nelly, but for me the first slam-fire would be last shot I fired with that gun and ammo until I had a handle on the cause. I had an SKS that slam-fired because of a sloppy fitting third-party stock that buggered the trigger mechanism; that was the last I fired the gun until I got it squared away.
If you are on the battlefield a quick fix is vital; at the range it's foolhardy.
IMHO
 
I have never had one of my AR`s slam fire, I have had my SKS & MAS slam fire, but they have floating firing pins, has to be in the trigger group.............
 
SlamFire1 said:
What boggles my mind is the multiple slamfires you were having. Really comes across as a mechanical problem but, and this is a big butt, on the AR designs the firing pin is mechanically retracted on extraction and does not extend until cam down. Maybe it is stuck forward in there somehow.
Click to expand...
The firing pin on a AR-15 (and AR-10s as well unless specifically modified) is completely free floating with no spring involved either. The hammer cannot fall on the firing pin unless the bolt is in battery. Likewise the firing pin protruded very little from the bolt face and the case head is pushed under the extractor and against the bolt face when the bolt goes into battery. The two scenarios that cause slamfires in AR style rifles are excessively sensitive primers being ignited when the bolt slams into battery and the free-floating firing pin hits the primer under inertia hard enough to ignite it or the firing pin breaks and the front end jams in place in the bolt protruding from the bolt face.

The first condition is a problem with the primer, not the rifle. All ARs put a small dimple in the primer when a round is chambered. This will not set off 99.9% of primers. The OPs description of his problem points to this being his issue. If it was a "double", i.e. a failure of the disconnector to work, then it would not have pierced the primer. The pierced primer here is the key and points to the primer cup being too thin/soft for the application. CCI#41 primers are the standard mil-spec hard cup primers. Federal 205 (not 205M), Rem 6 1/2 and Wolf Small Rifle are somewhat notorious for being soft/thin and in the case of the Rem and Wolf they note not to use them in high pressure loads or loads for semi-autos.
 
look at the hole in the primer VERY carefully.

is it punched in, or blown out- is it the size of the firing pin?

or the slightly larger size of the firing pin hole in the bolt face?

I have seen a primer that stood proud get slam fired by the boltface, but blow a hole back through the primer at the un-supported firing pin hole.
 
did you use large pistol primers instead of rifle primers? For a firing pin to go through a primer would indicate a very thin primer material or over pressure load or both. Just a thought.

Like others have said, no spring on AR or LR bolt carrier assembly. So, the firing pin is free to move forward after last round is chambered. It really needs a hard surface to hit against (and not fire).
 
Dang, I have never even heard of an AR type rifle slam firing. Sure, I've heard of lots of them them NOT firing at all.. ;)

I'm curious as to what the root cause is.

Add FS2000 to the list with a spring-loaded retracted firing pin. They recalled the early batches of FS2000's to add springs to the firing pins, to forcefully retract them, due to slamfire problems. Later models (such as mine) came with the spring on the firing pin from the factory.
 
Status
Not open for further replies.

Similar threads

G
  • Locked
AR15 Double Fire/ Slam Firing
2
Replies
32
Views
5K
madcratebuilder
M
O
  • Locked
Avoiding slam-fire on SKS?
Replies
15
Views
4K
Dave Markowitz
Dave Markowitz
dh1633pm
  • Locked
Savage 110FP 223 Slam Fire
Replies
18
Views
2K
Milt1
Milt1
outdoorman63
  • Locked
cetme's and firing pins?
Replies
3
Views
1K
ceetee
C
W
  • Locked
Slam Fire and Safety
Replies
7
Views
786
762 X 54r
7
Back
Top