Went to the range today to do some plain 75yd. sighting in of an ACOG and something strange happened. ( 4 times )

When I let off of the trigger, sometimes I would fire another round. If I let off fast, it fired a second round fast sounding like a double tap. ( felt like a bump fire )If I let off slow, it still happened only later. It didn't happen every time, or when I was concentrating on my follow through, or release, but I've never had it happen at all in the past with my other ARs. It made me very uncomfortable considering it is a non rapid fire range. I don't think it's the rifle. It's a Colt 14.5in. M4. It has to be something else that I'm not used to.

Things different about this configuration than my other ARs =

RAS with K-grip. ( Second time shooting with one, maybe I'm pushing, or pulling when I shouldn't be )

G-27 Grip. ( larger in width than my others )

Lighter trigger. ( this lower has the smoothest trigger out of my AR lowers, and I was saving it for this rifle )

I was shooting benched, holding the K-grip, using sandbags under the RAS HG.

Any ideas ?

KZ

Sounds like your fire control group is out of whack.  Did someone do a trigger job on the lower?  

As I understand it, the mil spec FCG is surfaced hardened only, so if you take off metal when doing a trigger job, then it will not last and will eventually mess up (such as firing on release).

The FCG is out of time.

It's in the [url=http://ar15.com/forums/announcement.html?b=3&f=66&id=187]FAQ[/url]

I'm firing LC XM193
Bolt is lubed well with Mil-a-tek



You may be right about the fire control group.

I played around with it in the garage after I made the initial post.

The best way to describe it is that at the end of releasing the trigger it seems to titer on firing again. I was able to make it happen every time. ( fire at the end of the release )

The only time it would not happen is when I consciously released my finger from the trigger all of the way.

A buddy at the local gun shop did the sear match, and expected me to be surprised at the smoothness of the pull, and I was. Even more so now.

[rolleyes]

[thinking]

[banghead]

I'd simply throw out your FGC and just get all new parts.  It's OK to POLISH the engagement points, but DON'T REMOVE ANY METAL . . . and don't let your "buddy" mess with it again.

The easiest and best way to improve a stock trigger - IMO - is to just polish the engagement points and install some JP 4.0 tactical springs.  I also cut off the "tail" of the hammer to lighten it.  This lightening helps insure ignition of even very hard primers with the lightened JP springs.  

I've personally fired 10,000+ rounds with this set up and had 100% ignition with many types of ammo including Wolf.

The JP springs cost $10:  http://www.jprifles.com/

your semi auto disconnector or your hammer is worn out, or the sping is weak or missing from under the semi auto disconnector.

Either pay the $95 to get a RRA 2 stage match trigger or spend a bit more and get an adjustable single stage trigger like the JP.

Thanks for the help guys.

I have a couple extra FCG's, but I think I'll order a JP single stage . I know they're nice.

Just when you think you have an AR where you want it, something always comes up. [:(]

Pisses me off, but I guess it's all a learning process.[8D]

When you remove your finger from the trigger quickly enough you allow the trigger to run forward fast enough to catch the hammer. You need more disco engagement. In polishing the trigger your "buddy" shortened it. That allows the trigger to sit farther forward and it gets a head start on the trigger. Messing with any one part in the FCG requires modding others if they all started out correct.

Right now you can look at the problem in various ways[list]
[*]The hammer sits too far forward
[*]The trigger is coming up too slow
[*]The hammer is going forward too quickly
[*]The disco is releasing the hammer too early[/list]

To fix the first condition requires new parts,  effectively undoing the trigger job. Fixing the second requires a stiffer trigger spring which will increase the trigger pull. Slowing the hammer down by decreasing the stiffness of the hammer spring increases lock time and increases the chance of light primer strikes.

The easiest fix, given that other parts haven't been modded and the rest of the parts are in order, is to remove a small amount of material from underneath the tip of the disconnector. This will allow the disco to rotate farther forward so it will release the hammer later. "Later" means that the hammer will be pulled further to the rear than it would otherwise be. The delayed hammer release allows the trigger to catch up with it.

Thanks for the info Tweak.

The hammer sits about 1/16in. higher than the others.

You're obviously the guy to go to with FCG MF's. You know a lot more about what's going on in there than I do. I learned a lot tonight.

Thanks again,

KZ

I bought a "new RRA" lower kit from someone on the board and had the same malf on every shot.  Sure enough, the sear engagement on the hammer was worn down.  My local RO told me to keep the parts.  [lol]  (I didn't.)

Comes from having to "adjust' every one I put together back when I was full time. That, and explaing it a lot. The FAQ covers a lot of it tho. I'm patiently waiting for the Archive server to come up. There were some doozy Tshooting threads back in '98.


fizassist,
Even new (never fitted) parts can double, that's why most of them show some signs of fitting even when new. What "sear engagement on the hammer"? Do you mean the lower hammer hook?

I'm not sure what the proper name is:  The surface on the hammer that engages the trigger (not the disconnector) had been filed to some extent.  None of the other half-dozen or so RRA kits I have bought from LegalTransfers and others exhibited this filing, and all worked flawlessly.

The rifle would double on release of the trigger.  As soon as the trigger was far enough forward for the disconnector to release the hammer, the hammer would slip the trigger and the 2nd shot would fire.  If the trigger was not held tightly after a shot, the 2nd shot was immediate.

That's the lower hammer hook. Lower, middle, and top (on auto's) hammer hooks.

"Filing" on it creates the same condition as messing with the sear face of the trigger. The hammer will sit further forward in its arc, i.e "negative engagement".