Broke a hammer pin in 9mm AR 15

[crimsoncomet]

Hey all. I was shooting my CMMG uppered 9mm AR and broke a hammer pin. I didnt even notice! Found it when I broke the rifle down. I wasnt even shooting hot loads! I think 9mm AR's are famous for this. BTW it is a DPMS LPK. Heard they are junk.

Here is the setup.
Standard lower with DPMS un-notched hammer
CMMG ramped bolt
Heavy wolf buffer spring
STX 9 Spikes long heavy buffer

Just curious if there is anything certain i should look for that would cause this? I already have KNS pins on order.

I have also heard that that hammer tail being to long could cause this. I definatley feel more friction when when the hammer is installed. Would it hurt to mill 4 or 5 mm off the hammer tail to be sure it is not causing it?

Thanks in advance.

 

[ApacheCoTodd]

Show us a photo of the hammer and the underside of the tail of the bolt - the portion interfacing with the hammer.

Do you see signs on the top of the tail of the disconnector indicating heavy strikes from the hammer?

 

[crimsoncomet]

Do you see signs on the top of the tail of the disconnector indicating heavy strikes from the hammer?

I don't see anything indicating heavy strikes to the hammer. I do see a shiny spot on the tail of the disconnector (pretty sure i am looking at the right spot) right at the tip. On the tail of the hammer, i also see a shiny spot where it looks like the hammer is hitting the back of the trigger? The face of the hammer also has a slight groove where the back of the bolt is hitting it.

Ramped bolt:


The sharp corner under the firing pin is what is causing the groove in the hammer:


Shiny spot on the back of the trigger where tail is hitting:


That shiny line is where the sharp corner of the bolt is hitting:


Trigger with shiny spot on disconnector i think?


Back of trigger tail. Some anodizing missing:


Thanks for any advice. I really appreciate it. If you guys need anymore pics please let me know.

 

[ApacheCoTodd]

It's a Colt pattern bolt without the cross cut some select fire systems have so that's good in that it won't hang on a step front hammer.

The lower will benefit from switching to a RRA/Colt(small pin) 9mm hammer. See that line on the hammer face about half way from the step on the front and the striking point of the firing pin? When that portion of the hammer hits the corresponding portion of the bolt, it becomes a fulcrum to transfer force back to the hammer pin (and receiver) stressing things sometimes to the point of failure - you've seen it. The good 9mm hammers for that bolt have a relief notch at that point.

I would get a set of hammer/trigger pins from someone other than DPMS as well.

Of course, a fella could slow file that relief cut as well, I suppose.

 

[crimsoncomet]

Thanks so much for the info! I have kns pins coming. Figure I don't won't to screw up a registered lower. Are the RRA/colt hammers dedicated for 9mm? Can I just buy a hammer or do I need the whole trigger assembly? Thanks again for the awesome info! BTW, tht would be a lot of filing!

 

[ApacheCoTodd]

The 9mm hammer should work just fine with your trigger and disconnector and in turn perform with a .223 bolt carrier if all the existing components are proper spec. I run two separate multi caliber lowers with 9mm hammers, one a Colt pattern stepless hammer and the other is one of the "light weight" hammers which were all the fashion a while back.

 

[crimsoncomet]

Parts should be at my house. I got kns pins and rra hammer.

 

[crimsoncomet]

The kns pins worked great. The RRA hammer did not. I could not even charge the gun with the hammer installed. Seems like the ramped heel of the bolt was catching in it. So, I trimmed some of the spur off my DPMS hammer and and rounded the spur. I also polished the face of the hammer. The seems to be much less tension there now. I think it will work great. Only time will tell.

 

[ApacheCoTodd]

That sucks about not being able to charge it with the new hammer. It is specifically copied from the Colt as is your carrier though many other manufacturers put a lot of relief cutting into the underside of the carrier to allow it to work with .223 hammers.

I wouldn't be surprised if there was some displaced metal/burring at the point of impact between the .223 hammer and the carrier below the firing pins contact face. This would correspond with the horizontal impact line on the .223 hammer, this line being the indicator of what's going wrong in the first place.

It sounds like the action is hanging up on the very cut in the hammer that is machined/cast to keep the stress off the hammer pin. Ironic, that. If the .223 hammer continues to strike the carrier at the bottom of its pin shroud the stress if not breaking the new pin - would be beating on the receiver holes.

At a minimum, I would relieve the hammer along that impact line on its face.