We m4 CB doesn't cock with rear body pin in?!?!

[serioussniper]

Quick question all!

I've been given an m4 to look at as the user hasn't been able to get it to work.

Cock the rifle
Pull the trigger
Click
Recock and the trigger doesn't set and the charging handle doesn't fully return.

Take the pin out and you can cock and click until your heart is content!!

Exactly the same with the mag in, works lovely without the pin in!!


Obviously the pin doesn't get in the way of anything so its got to be something being compressed and not moving freely


Any starters for 10 ready for when I can look at it properly?

[tisane]

Will the forward assist push the bolt forward?
If you cock it for the 1st time with the mag in does it fire and return normally and then fire the 2nd shot normally?

If the forward assist won't push the bolt forward then the bolt could possibly be moving too far rearward allowing the hammer to ride up into the void at the front of the bolt, then when you release the rear pin it lifts the bolt by a mm or so and it clears and returns fully.

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

Thanks once again for replying tisane! I'll run through it all later when I have time but the forward assist doesn't seem to help in the slightest!

That does sound like an extremely plausible explanation

[tisane]

Look how close the distance is between the buffer tube and the buffer retaining nipple, if there is any gap at all try screwing the buffer tube in so it touches the retaining nipple.
If it still has the issue whip the buffer out and check that, if that's ok then use a few coins or something similar in the end of the buffer tube to reduce the bolt travel.

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

Ok so here are my notes:

Buffer and nipple are fine, tested with coins.

Forward assist doesn't do anything, in fact the button is totally loose? (And falls off)

I think it's a selector thing, but might be wrong as I'm not used to m4s
Unable to select safe unless rifle is cocked
Auto doesn't appear to work, selecting it just works the same as semi

There is definitely something catching inside with the pin in, as it jams enough that the pin has to be tapped out. On doing this there is an audible click as the body parts system frees up

[crasstoe]

Safe is only selectable when cocked iirc. Could be the pin isn't from a WE M4 and is pushing the upper receiver, and the bolt too far into the lower receiver?
Sent from my Windows Phone using Board Express Pro

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

Ok thanks!

No I thought that also, but you can see when cocked the two halves don't want to sit together and line up properly.

Just tested with the pin in and a mag... Can cock it fine but it doesn't fire, the trigger pulls but just clicks. Then the same issue without the mag, it's impossible to re-cock

[tisane]

Right then the semi only part is easy you will find that's a broken sear, and that leads to another possibility.


http://youtu.be/ARx7Bq30zYQ

The video is an Australian gent showing how to make imported WE rifles semi only, (as is there law) either WE or customs simply break the rear 10mm off the sear preventing it from firing on full auto.
Left is the broken standard part, giving semi only and right is what the part should look like to give full auto capabilities.

If its broken with wear then its quite feasible that its totally worn on the "hook" part meaning that the trigger isn't being held back at all, quite possibly then instead of locking the hammer back and out of the way as soon as the bolt strikes the hammer its continually pressing against the bolt.
But there is the possibility too that the hammer could also be worn along with the sear

Oh and Crasstoe is correct, most WE rifles can only be safetied once they have been cocked.

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

I believe sir, you might have hit the nail on the head ...

When I ran the bolt just along the top of the trigger (with the bolt out of the upper) it didn't 'feel' quite right to me.

Tomorrow I'll strip the trigger group out and see what is what.

I swear my SCAR safeties un cocked..but that's as close to an M4 as I'll ever own in all honesty!

- - - Updated - - -

Ahhhhh evidently he realised and wanted it to be semi only but does have a spare sear so that resolves that little mystery!!

[tisane]

With the upper and lower apart can you manually lock the hammer back?
Does its owner realise that dropping it into safe can damage the sear?

Some WE rifles can be put into safe uncocked, and some can't, I had a OB converted HK416 that would go into safe uncocked, but both my current HK416 and M4 won't.
Its just luck of the draw I think.

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

Yes, I can manually lock the hammer back, and as I said with the body just held together by hand the damn thing cycles just fine!
So the issue is (thinking out loud)

The rifle can be cocked fine once..
The trigger can then be pulled and the click of the hammer heard but then the rifle can't be cocked again, AND with the body pin in the hammer obviously isn't coming forward far enough to strike the mag

Therefore I think its not something which isn't holding the hammer back correctly, its something restricting it going forward all the way....which is then jamming the bolt up..... I think that makes sense.... actually that does make sense as that would also explain the click that can be heard when the body is separated, its the hammer falling fully back into position


I am sure he does, I just didn't realize they wouldn't safety without being cocked, thankfully I didn't try and force it either! I *think* both the SCARs I have safety fine when they're un-cocked!

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

Grrr so I am completely convinced that for some reason the hammer isn't being allowed to fall fully forwards.

The trigger group is fully working as it should be.

What that 'something' is, I guess can only be the bolt...

I assume, it is the bolt not going fully forward, or twisting or something... But I can't find what it is...

image.jpg

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

Got it now I know the problem!! Going to try this now

http://www.gasguns.info/forum/viewtopic.php?f=54&t=5853

- - - Updated - - -

Sorted. So simple once you know what to google!!

[tisane]

Couldn't have been any more simple.
I have to admit I'd never have got it without it in front of me, if its open bolt I'm in my element, however being closed bolt things are very different in that area

[serioussniper]

Re: We m4 CB doesn't cock with rear body pin in?!?!

I know :-/
Ah well, nothing like a good old process of elimination.

Thank you for the help though, much appreciated as always!

Its the first CB I have ever seen or used and I was surprised how nice it is to use.