New Build that is Double Tapping!!! HELP!!???

[LHClarke]

O.K. Grendel Guru's! I have acknowledged my shortcomings, and now I need to be forgiven and helped. I LOVE THE GRENDEL, more than any human that has played a part in it's inception and creation.

I have just built a 6.5 Grendel. It is made from a 19.5" Alexander Arms barrel and bolt, a Del-ton M-16 Bolt Carrier and bolt carrier parts. It is an Alexander Arms 6.5 Grendel Upper Receiver, a Alexander Arms 6.5 Grendel Lower Receiver. The LPK however, is a CMMG. I bought it in March of 2009 during the Obama Crunch and didn't realize it. I don't think that there is anything inherently wrong with the LPK. At least as far as I have been able to read on the web. It seems that their LPK's are indeed Mil-Spec and that should be the end of it..........HOWEVER:

I fired it for the first time today. I got my scope mounted and bore-sighted and place 3 Wolf 120gn MPT's in the 15 round C-Products magazine and dropped the bolt. All is well.
Then......I pull the trigger.........POP-POP-jam...stovepiped the 3rd round. HHMMMM????

Tried it again......POP-POP-jam.....stovepipe the third. O.K. something is amiss. I decide to load one at a time to zero the scope because I drove a long ways to show this rifle off to my buddies???!!!!

I zero the rifle this way and she is a shooter!!!! .750 groups, outside to outside with a Caldwell JR. front rest and a sand-sock for the rear. I like!!!

I let it cool down while I shoot the 6.5 Swede.

I then try it again with three rounds.......POP-chamber.........GREAT!!!! I pull the trigger..............nothing. The trigger didn't reset, but the primer on the chambered round is indented BIG time. Must have been CLOSE, no more than .0005" to detonate.

What gives? It has a Wolff Extra Power Buffer Spring and a D.P.M.S. 9mm Buffer in it. I think the buffer weighs around 6.5g's. I am wondering if the spring and buffer is not letting the carrier cycle all the way and the hammer is following the carrier down and causing the double tap? Any insight would be great. I am out of witticisms because I've not run into build problems like this before.

So to sum-up: It is a new build and has a Wolff Extra Power Spring and a 9mm Buffer in it and it is double tapping. When it doesn't double tap the trigger is not resetting where I can pull the trigger, but the primer on the chambered round is massively indented but not detonating. ANY IDEAS. I am thinking "replace the buffer spring first." ???Yes????

Then go from there until I get feedback.

Thank You for all Your help!!

LHClarke

[bwaites]

Was any work done on the trigger? Any polishing of the sear, etc.

If not, then there IS an issue with the LPK, most likely.

[LRRPF52]

LHClarke,

Try it without the 9mm SMG buffer-those are way heavier than even an H2 buffer. You shouldn't need that heavy of a buffer. Also, it's very difficult for the hammer to follow the bolt carrier, but your guess at the hammer not resetting due to the extra resistance from the insane buffer mass makes sense, as well as the stove pipes. AR15's rarely stove pipe...

Be advised that most/all of the major manufacturers (assemblers) of AR15's source their MIM'd small parts from the same places, so CMMG, RRA, DPMS, etc. might not be all that different.

Put a Spike's ST-T2 or an H2 buffer in there with a standard recoil spring and see what happens. When putting a new AR together, I highly recommend having 3 different buffer weights to try out with your gas system length, barrel length, and loads. Those 9mm SMG buffers are that heavy because there is no locking mechanism on them, so they are completely blow-back operated, and recoil like a son of a gun.

Is this on a carbine extension tube?

LRRPF52

[LRRPF52]

By the way,

Start proofing your system with one round fed from the magazine, and ensure that the magazine follower locks the bolt back. Also take note of what direction your spent brass flies, and how far. Look at the fired case before proceeding, checking for any pressure signs...cratered/flattened/pierced primers, eschar on the brass, split case necks, swollen bases, and ejector recess brass flow (where the molten brass has filled into the ejector channel against the force and mass of the ejector and ejector spring).

Do NOT use that SMG buffer in a gas-operated system.

LRRPF52

[Darkop]

I suspect the disconnector is not holding the hammer. Check the disconnector spring. Is it installed? If so, the large end should be down in the hole in the trigger groove. If this checks out I would replace the disconnector and see if that corrects the problem. CMMG LPK's are not the best, could be a problem there. Let us know what you find out.

Until that day,
Darkop

[rkflorey]

+1 on the disconnector. Check to see that the spring is installed correctly and that the disconnector is machined correctly. I found one out of the box that had not been inspected and acted just like the problem you describe.

[tx4guns]

I would suspect the disconnector, too. They can be sticky and stay in the down position, which will essentially make the rifle full auto. Also, but by chance is your magazine touching the bench? That can induce a bump fire. My old 10mm Auto AR used to do that when I'd shoot prone and it would bounce causing a bump fire double tap. Sold it.

[warped]

Take that damned buffer out of the rifle please, it is way too heavy.

the H3G is only 155grams H3 is 152!

The 9mm buffer is for a blowback operated rifle, that is not what you want.

[warped]

I suspect the disconnector is not holding the hammer. Check the disconnector spring. Is it installed? If so, the large end should be down in the hole in the trigger groove. If this checks out I would replace the disconnector and see if that corrects the problem. CMMG LPK's are not the best, could be a problem there. Let us know what you find out.

Until that day,
Darkop

This, is it missing or misformed?

DO NOT USE THAT 9mm BUFFER

[LHClarke]

O.K. just got in from "work". Anyway, Warped, no work was done to the LPK. I did not file or sand or anything on any of it because I have a Geissele on the way.

I do believe that I should get shed of the 9mm buffer. I have ordered a standard spring and buffer so I plan on changing the spring out first and firing it to see what happens. If problems persist, I will change the buffer. Warped: I can see what you are saying about the "blow-back" operation. Makes sense. Thank You.


If problems persist beyond changing out the Wolff Spring, then I will change the buffer and I will have a "stock" carbine buffer and spring. I almost hope that this doesn't fix the problem.

I am hoping that I can swap the buffer and keep the Wolff spring. Why???? I don't know?!! I like the spring? Really it makes me no difference, I just want to get this lady acting right.

Thank You all for the insight. I read every one of your opinions and don't mean to leave anybody out. All suggestions make sense and I will try all hypotheticals. When I get her figured and running I will deffinately post the solution.

This forum is awesome. Thank You all again!!!

LHClarke

[LHClarke]

DARKOP, I WILL check the spring under the disconnector. I have built a few AR's but hey,,,, you never know. I will post results. It does seem like a LPK problem.
I loaded some brass with primers and "dropped" the bolt on the brass. I know that this is bad and can break the bolt but I needed to see if the firing pin had that much play...... I even cut a primer pocket .010" deep and set a primer up at .016" to see the indention on the primer. The firing pin is not the culprit.....the indention was the same with the .006" as with the .016" primer.

LRRPF52: The brass flew very nicely. It all ejected very uniformely, going to the same place. I feel that this is not an issue, but thank you for the variable.....

LHClarke

[LRRPF52]

LHClarke,

Another thing: Don't test it again at a public range. If someone want's to give you trouble over it, I have heard of similar cases where guys with AR15's that doubled were prosecuted to the fullest by zealous bureaucrats from the Elliott Ness crowd. That could really be your biggest worry. There is one case in particular where a guy borrowed an AR15 from another guy, and it doubled at a range. They went after the owner with the full force, and charged him with illegally transferring a machinegun. Some people are hungry for cases like this so they can line their career with convictions, and the facts are meaningless to them.

http://www.thehighroad.org/archive/index.php/t-375570.html

Dude went to prison for 2 years, and lost all rights to own firearms forever.

LRRPF52

[warped]

LHClarke,

Another thing: Don't test it again at a public range. If someone want's to give you trouble over it, I have heard of similar cases where guys with AR15's that doubled were prosecuted to the fullest by zealous bureaucrats from the Elliott Ness crowd. That could really be your biggest worry. There is one case in particular where a guy borrowed an AR15 from another guy, and it doubled at a range. They went after the owner with the full force, and charged him with illegally transferring a machinegun. Some people are hungry for cases like this so they can line their career with convictions, and the facts are meaningless to them.

http://www.thehighroad.org/archive/index.php/t-375570.html

Dude went to prison for 2 years, and lost all rights to own firearms forever.

LRRPF52

They do not care one bit why it doubled, only that it did and they can get an arrest and conviction.

[LRRPF52]

I also heard of another case years and years back where that agency had a track record of prosecuting people with AR15's as machineguns, where the helpful agents actually replaced the firing pins of the defendants/victims' rifles with heavy, weighted firing pins to induce a constant slam-fire, and lubed their BCG's with some super-slick compound, resulting in a striker-fired machinegun. The particular agent(s) even bragged about how well it bolstered their careers, getting them quick promotions and accelerated trajectories towards the upper stratosphere of management. Juries had no clue what evidence they were being shown, and trusted that these were honorable, trustworthy professionals providing expert testimony on deviously-errant citizens who knowingly violated the law with full intent to manufacture a device for killing scores of innocents.

This is no area to play around with, and in a best-case scenario, will involve temporary incarceration, massive bail fees if eligible, and tens of thousands in legal fees for a competent defense team that specializes in firearms/Constitutional Law. If, by some chance you are found innocent, you will be on said agency's target list for illuminating their incompetence, malice, and disregard for the law...and they will want vengeance. Those who treat others in this manner HATE it when the light is shone upon their true deeds. The Gunrunner Scandal is a perfect example of this, but in that ball of wax, a Border Patrol Agent lost his life with the offending agency's role as an accessory to murder. DOJ is squirming to avoid any more publicity with that case.

LRRPF52

[LHClarke]

Hey guys....been reading some of the replys to the thread. I still haven't fixed the Grendel yet. I got the buffer spring and buffer in a couple days ago but haven't had the opportunity to go back to my shooting hole.

FWIW LRRPF52, I was never at a public range. It was the first time to fire a new build and I NEVER fire a rifle for the first time anywhere near other human beings. I just cannot imagine maiming or killing a friend or anybody for that matter because of my incompetence, even if my incompetence was just a temporary lack of attention... We all make mistakes but I usually check and recheck and then recheck my rechecks and I use a check list.......so anyway, no public ranges when you are firing a rifle you've worked on for the first time.
ALSO, I don't just jam a full magazine in a new build and go for it either........I had fired 10 single rounds, first to look at my brass for signs of pressure and ejection marks, etc.... and I like to see how she is ejecting, all looked good. HOWEVER, I missed something, I missed the hammer or sear not resetting if this turns out to be the squib. I can justify this oversight simply because I had an unloaded magazine seated and every time the round went off the bolt would be held open, so it was simply a matter of placing a round on top of the magazine and releasing the hold open. In other words, I wouldn't have really had the opportunity to check for sear engagement using this method. I will be putting this on my list though.
It was after I went through all these procedures that I decided to load a magazine with 3 rounds in it to check for function, this is when the double tap and jam occurred. It was uncomfortable to say the least! So, after I calmed down, I loaded another 3 round magazine.......same thing. So the rest is history. I will (hopefully) get to my spot by this weekend, I'll report soon.....

Again, thank you for all your thoughts and insights......

LHClarke