View Full Version : Technical: slam-fire, bolt-follow, etc.

January 29, 2001, 11:11
Hello everyone,

I got my first kit to build a roommate for my L1A1. Itís an Imbel South American kit.

This one came with a full auto selector. I read in a previous thread that having the selector in the FA position on a semi auto rifle isnít a problem. As I understand the operation, the trigger/sear has a hammer catch in semi that holds the hammer back after firing. The trigger assembly in FA position just lets the hammer go, so it is following the bolt carrier riding on the firing pin as it becomes exposed. Then if the primer is good and hard, the gun will not fire and you will have to rack the operating lever, spit the unfired round and load a fresh one.

So I look at the FA ejector block and see that it has a little pawl that catches the hammer and holds it back until the bolt drops down into the locking recess which also actuates the pawl releasing the hammer.

So I am trying to understand is having the selector in FA position without the FA ejector and pawl a recipe for the slam fire scenario or what. BTW, this board is exceptional for the amount of information shared. Thanks much to the owner, moderators and all the members for being so willing to share.



January 29, 2001, 12:06
I would avoid having the selector in the FA position. While you can assume that military primers are hard, it's not something I'd want to count on. I've had a round go off when I hit the bolt release to chamber a round (high primer, soft primer, both, who knows?). Fortunately, it was pointed in a safe direction.

I recall a post where someone's rifle went full auto at the range. The problem was attributed to faulty fire control parts. The sear wasn't catching enough of the hammer to hold it back after each shot. The result was some exciting slam fire. It's all fun until a round goes off out of battery.

Bruce Allen
January 29, 2001, 12:29
In a good condition there will not be slam fires in a FAL.

As far as the selector being set to auto the most that should happen is the hammer will follow the bolt carrier down after firing the first round and you will wind up with an uncocked hammer.

With a fully functioning select fire FAL there is an additional part in the upper receiver called safety sear. It is held in place by the hinge screw and is spring loaded. As you look down into the receiver while holding the rifle it would be located on the left side. Examine your bolt carrier and you will see two "tabs" on the bottom. One is noticably longer than the other. The long tab is there to trip the safety sear as the bolt carrier slams home and releases the hammer to strike the firing pin after the bolt carrier group is in battery. The saftey sear holds the hammer up a fraction of a second, like a built in hesitation, giving the bolt carrier group time to go into battery.

The weapon should not fire when the hammer just follows the bolt carrier down, without the safety sear, because the carrier group is not in battery when the hammer hits the firing pin.

Also that is why the upper receivers have to be handled by FFL holders as that is the part that the full auto parts would go into if they were milled in the correct spot, and had the correct ejector block.

January 29, 2001, 18:05
Sorry duplicate text

[This message has been edited by cycle_rcr (edited January 30, 2001).]

January 30, 2001, 14:15
I'm still trying to understand this. In a rifle with the selector in FA position, after the first round is fired, the bolt rebounds, cocks the hammer. The hammer is not caught by the sear and so follows the bolt forward until it comes to rest with the fresh round.

Is there a design feature in the weight of the hammer that prevents it from pushing the firing pin through the primer via inertia?

Sorry to beat this subject so much, but I have actually wittnessed a bolt rifle blow up because of a case rupture. I'm really glad the shooter is still here(Dad).

My concern is that maybe a combination of conditions could cause the slam fire scenario and maybe an unfortunate incident of case rupture (due to an incompletely chambered round being detonated) next to someones face.



January 30, 2001, 14:41
Yes you've got it right. Nothing catches the hammer as it rides back up under the bolt carrier. In a perfect world, the inertial of the hammer is killed by friction against the underside of the bolt carrier so that when it eventually comes to rest against the firing pin (as the bolt starts to come into battery and stick out the back of the carrier), it doesn't have enough energy to lite the primer. That's when you have a live round in the chamber but the hammer is uncocked.

The problem I tried to point out is that in the real world, where a company may produce millions of rounds of ammunition, there could be a few soft primers in the mix. Or maybe the primer pocket was a just a hair too tight so the primer is just a hair too high. Or perhaps that lot of primers has a bit more lead styphanate than a previous batch or the primer cup material is slightly thinner. The end result is that it only needs, say, 1/3 of the normal energy required to set it off.

So now you've got a sensitive primer with the firing pin scraping against it and it goes Boom! With some luck, the inertial of the bolt carrier has already closed and locked the bolt down before this happens and you just have a little excitement. But if that bolt isn't fully locked, look out.

On your next range trip, what setting are you going to use?

January 30, 2001, 14:48
Not wanting a repeat of the ruptured case scenario, I think the answer is clear.

Thanks much Wadman and Bruce Allen


January 30, 2001, 14:53
As a former CDN Forces Armourer, I will relate some information that we derived from testing rifles in the late '70's.

The FAL design incorperates the "safety sear" in both semi and select fire rifles. The purpose of the safety sear is to engage the notch on the hammer keeping the hammer cocked and off the trigger sear until the bolt has fully closed and the bolt carrier has tripped the safety sear. Once tripped the safety sear is released and allows the hammer to move slightly forward engaging the trigger sear without the momenteum it would have had had it not been stopped by the safety sear. This prevents the possibility of the hammer over riding the trigger sear and allowing the hammer to continue forward.

In testing the FN rifle with a full auto selector, in the full auto position, without a safety sear could result in a slam fire with standard military ammunition. This did not happen all the time, but was noted to happen about 30% of the time.

To avoid the problem, install a semi selector or weld up the full auto slot in the full auto selector. Another method is to install the longer semi auto trigger plunger in the trigger return system. This prevents the extra movement needed in the trigger for the trigger sear to be unable to engage the trigger.

Of course, the proper method, is to have the gun correctly outfitted with a "safety sear" and then use a semi auto selector and trigger plunger to prevent the possibility of the hammer over running the trigger sear. But unfortuantely the Government powers that be have mistakenly deemed the "safety sear" to be a full auto sear and have required it's removal.

What is truely ironic, is that here in Canada, part of the conversion requires the removal of the "safety sear" as well. You would have thought in a country that had the rifle in it's military for so long would have realized they were mandating the removal of a safety item and not done it. But no, that would have made too much sense.

So, anyway, the use of any FN without a safety sear must be done with caution and attention paid to the engagment of the hammer, trigger and trigger sear along with the correct selector slot dimensions and trigger return plunger to minimize the potential for accident.

I hope this helps. http://www.fnfal.com/forums/smile.gif