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Posted: 2/16/2006 7:25:56 PM EDT
[Last Edit: 2/16/2006 7:51:32 PM EDT by 1911user]
I installed a DPMS LPK into a lower and had an unusual problem. With a known good upper in place, the bolt carrier would catch and be held about mid-stroke after retracting it with the charging handle. After swapping hammers, the problem went away. Turns out the DPMS hammer was tall enough to catch the back part of the firing pin and that was what was holding the bolt open.

EDIT: after looking around some, I think the top of the hammer may not have all of the cuts needed. Here's a pic of my hammer: DPMS hammer
Compare that with the hammer in this picture from the Build section: www.ar15.com/content/guides/assembly/lower/lowerKit.gif

My other hammer that works fine has the same notch cut at the top front of the hammer. Should I take a file to the DPMS hammer or would that cause other problems?
Link Posted: 2/16/2006 8:00:55 PM EDT
Link Posted: 2/16/2006 8:15:35 PM EDT
[Last Edit: 2/18/2006 6:30:56 PM EDT by 1911user]
I don't think the tail is too long. I did an experiment where I held the hammer lower by using the disconnector and inserted the bolt group. It slid in freely.

I think the width is fine due to the mark on the hammer face from the back of the firing pin from dry firing it a few times.

I will remove part of the L notch material (have grinder :)) and try again. If that fixes the hangup, I may ultimately cut the tail off and make it a speed hammer.

thanks for the help and grinder tip.

EDIT: I decided to avoid the speed hammer since I don't have a light hammer spring.
Link Posted: 2/16/2006 8:27:45 PM EDT
Link Posted: 2/19/2006 1:07:15 PM EDT
I had the exact same problem. I changed out the hammer and have had no problems since. Stay away from DPMS hammers! I won't even use one that has had the face milled.
Link Posted: 2/19/2006 3:46:28 PM EDT
[Last Edit: 2/19/2006 3:47:03 PM EDT by Dano523]
Link Posted: 2/19/2006 4:00:56 PM EDT
I just find it curious that all of the other LPKs I've seen use the notched hammer where the DPMS is un-notched. Is the DMPS hammer only made for certain types of bolt carriers?
Link Posted: 2/19/2006 4:40:26 PM EDT
[Last Edit: 2/19/2006 4:41:53 PM EDT by Dano523]
Link Posted: 2/19/2006 4:48:17 PM EDT
DPMS hammers are used with 9mm Colt uppers with non ramped bolts I think as well per the 9mm thread.

Link Posted: 2/19/2006 5:02:57 PM EDT

Originally Posted By Dano523:

To add as a tidbit: To see if the modified hammer started it’s life an a-1 or a-2 unit, look at the disconnector sear on the hammer. If the disco sear is wide (overhangs) on the ejection side, then it started its life as a M-16 A-2 hammer.

It does have the wide disconnect sear. I noticed that earlier, but left it in place since it wasn't causing a problem. A buddy who bought several of the DPMS LPKs on sale is also having the same problem in his build; I bought an LPK from him.

I am glad that I had another set of parts to compare with. When I was grinding on the hammer, I may have went too far on top and will end up replacing it anyway (slight dragging now). It is an excellent gunsmithing lesson though. thanks.
Link Posted: 2/19/2006 6:08:34 PM EDT
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