[knall]

I asked this question before looks like the thread was lost in the website update.
My gun feeds FLAWLESSLY, and shoots pretty darn well BUT every so often (100 rounds or so) the firing pin fails to fall after pulling the trigger.
Doesnt appear to be a headspace issue..... gun "locks" up tight (and the next 100 rounds fire) I have the trigger adjsustment screw ALL the way backed off.... I polished the moving pieces......
With the gun partaily disasembled...... locked and cocked......pull the trigger slowly ..... ya can see the rear of the bolt drop downward....... before it begins to move the sear... is this normal? If ya JERK the trigger it seems to fire more reliably...... but I dont wanna jerk the trigger.... and it is happening so infrequently that I am having trouble diagnosing the trouble.
Firing pin is definately "cocked" action is definately "locked" I am leaning toward a "break in" issue or a trigger travcel issue..........
MY FIRST 1919a4 I am baffled. I wanna start grinding but i am hesitant.
The FRONT BOTTOM of the kmp trigger should it come into contact with barrel extension AT ALL when the trigger is fully pulled?? My barrel extension is ground "45* or whatever at that area as it should be..... but it still rubs a bit. Seems as though if I could pull the trigger FARTHER it might fire....... but trigger pull is absurd allready..
Any ideas?
Oh its a semi auto built on a kmp trig / sear and and a ORF plate

 

[kaotic01]

Well first of off what is the actual deminsions for the barrel extension back. (to be ground.) I am assuming your meaning the part where the front of the KMP trigger hits when fully depressed. I believe its 45 degrees but someone else should chim in on that one as I am not sure , perhaps someone else can get exact deminsions and even a picture or 2.

 

[30calmachinegunner]

A couple of things to keep in mind, if you are doing the internals mock up outside the gun is that the bolt and barrel extension are not still "locked up" with the lockframe when the gun is fired, the accelerator has moved them further forward. This may be giving you some "false" information if you are not taking it into account. I assume you have also checked to make sure the trigger is not being stoppped in the slot on the backplate also. Are you positive that when the gun does not fire that the trigger is still engaged in the sear and is pulling it down(ie. the disconector is engaged on the sear ears), it may be that the disconnector did not reset. It has to be something minor like this to be only happening every 100 rounds or so. Good luck and let me know if I can be of any help, Brian.

 

[knall]

I can not be %100 sure that the trig is reset and pulling the sear, no SAFE way to check if it is. Hell of a thing to try and diagnose.........

 

[knall]

Allthough it makes NO SENCE to me, the failure has become more common. Now happens every 15 rounds or so. I have NO explanation for why this has happened BUT it has made the diagnosis MUCH easier. Trigger seems to be hitting the cuttout on the backplate. Plan to make the cut deeper when time permits.

I will be modifying this before the 4th of July and letting you all know if this fixed it shortly thereafter.

 

[loboslanding]

Hey Knall, take a look at this barrel extension. You mention that the tang is cut at 45 degrees, mine are more steep in order to clear the trigger when it's pulled. You can take a grinder to that part and relieve some more with no danger of messing up...just go slow and don't get it too hot.

Also, lock your driving rod into the bolt and remove the backplate and cycle the bolt and pull the trigger without the backplate...just hold forward pressure on your charging handle so the bolt stays locked up real good. You don't need the driving rod tension or backplate in place to check these guns and you can easily see what's going on with the BP removed. On my new builds I cycle it this way without the backplate so I can check all the engagement surfaces and view how the parts slide in the sideplates for any binding or areas that need dressing...plus you can view the trigger/sear engagement and function thru the rear.

If it works fine with the backplate removed then your slot theory is dead on.

Also, we're focusing on the trigger pull, but this problem can still be because it's not resetting properly. If you can get the gun to misbehave, try pushing down real hard on the trigger while simultaneously pulling rearward and then pull it up normally to see if the fp drops. I had to file a groove in my bottom plate to get that tad more trigger travel to properly reset. Or heat and bend the trigger.

I've noticed on some lockframes that the trigger retaining pin head is much smaller than the hole in the rsp that it locks into which causes the lockframe and barrel extension to move and if your trigger disconnector is already a snug fit with the sear, that little bit of slop can cause them not to re-engage. The lockframe needs to be locked solidly into the rsp with very little, if any, fore or aft play.

Hope you get it tweaked soon...that can be mind boggling and frustrating.

 

[tomt]

trig

the pull shuold not be hard at all. make sure the trig is not topping out on the back plate. if your bolt is moving downward when pulling the trig, you could have some wear problems with the bolt/ barrel extinsion. that will cause you problems also. lmk. thanks. tomt

 

[John Powell]

Ensure that you have adequate bend in the correct direction of the sear spring to hold the sear up against thefiring pin tohold it in battery.

 

[knall]

I shaved some material off the backplate to allow further travel of the trigger, all is well now.

QUESTION When I pull the trigger the bolt DEFINATELY drops maybe a 1/16 or an inch.........before the firing pin falls, is this abnormal??
Willsomeone please open up there top cover, dryfire while watching there bolt 4 me. Make me feel beter.

 

[knall]

I am now back to having the problem every 100 rounds or so. ANOYING AS HELL.

But I sapose putting 100 rounds of 8mm downrange without failure will have to be good enough.

 

[belt fed frog]

sounds like you have a worn bolt /barrel extention and it is getting worse

 

[HottAK47]

I believe you just might be right, Frog. Wonder what would happen if you peened the threads on the barrel a little bit? Should take some of the slop out.

 

[belt fed frog]

HottAK47
i am thinking it is the bolt rail /extention rail interface that is worn not the barrel threads though that is one area i hadn`t thought of as a wear area

 

[knall]

UMMMMMMM the threads on the barrel, and barrel extension seem fine, no noticeable slop there.

Whats this about
"HottAK47
i am thinking it is the bolt rail /extention rail interface that is worn"

Any more info about that would be helpfull....... again the falure ocours every hundred shots or so....... and it is the trigger failing to trip the sear.

Could it be as silly as a lubrication issue?

 

[tomt]

bolt

your bolt should not move that much. something is worn. about .020 or so is fine. but if the bolt is tipping 1/16 of an inch you've got other problems. jmo. tomt

 

[belt fed frog]

knall take you barrel extention and bolt out of the gun then slide the bolt into the barrel extention and proceed to wiggle it from side to side you should be able to get no more than 2 pieces of 20Lb paper between the two with the bolt all the way to one side
then with the bolt all the way forward in the extention try lifting and pushing down on the back of the bolt once again at the back of the extention you should be able to get no more than 2-3 pieces of paper between them lifted noe less than one piece pushed down this is where i bel;ive your problem is either the rails on the bolt are worn or the groves in the barrel extention are worn or both Tom T is right the bolt /extention should only have .020 play in it
my bolt rails mic .143-.146, the extention grooves mic .151-.152

 

[knall]

AH HA , yep the bolt has more travel up and down than I think one would expect. Seems to be the groove in the barrel extension more than the bolt itself.
Lots of meat left........ doesnt SEEM dangerous........ I'll just keep an eye on it.