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Posted: 2/16/2006 7:25:56 PM EDT
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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? |
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Ways to attack the problem, You can lower the top edge of the firing pin contact pad. You don't need to make the entire L cut, just draw a line from point A to point B of the outer edges of the L and take down the material. It may be just as simple as the tail of the hammer being too long, and removing material off of the tail will allow the hammer to be lowered down without binding between the carrier bottom and the back of the disconnector. Or it could be that the bottom of the carrier is not ramped wide enough and the hammer is catching on the milled sides of the carrier ramp. When all else fails, use a maker and smoke the bottom of the carrier (ramp section), and the tail/top edge of the hammer to see just where it is binding to figure which of the above is the problem, and needs to be resolved. P.S. The hammer is heat treated so you will need to use a grinder to remove metal on it. |
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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. |
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If you are going to speed hammer it, you just need to take off the tail, then make a small 45* cut on the top of the firing pin collar instead of the full L edge to edge cut. Really, you are just making a top edge beveled that is allow the FP collar to glide over/make contact lower on the firing pin contact pad, instead of the FP collar contacting the top edges as the carrier moves back forward over it. P.S. With the new speed hammer, do confirm that you have .032 to .037 FP protrusion, or the mod may lead to light primer strikes with a lightened hammer spring. |
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The problem with some hammers having problems is not always directly related to the hammer it's self/alone, but also to the geometry of the hammer as well (distance from the center line of the pin hole to the front edge of the sear). To keep is short, the FCG must spec as a unit to work in any given rifle, and when installing either a new part or a full FCG, there are times that you will need to re-time or mod a part of the FCG to get the complete working FCG into working specifications. Since this disclaimer is always present with every single part that you buy, "Should be installed by a competent gunsmith", Some fitting/tuning may be expected from even the best of suppliers, especially when the complete rifle may be made up of sourced parts by many different manufactures. On the bright side, this is more than a handful of members here that can walk anyone threw the simplest of needed modifications if problems present themselves when a part/combo of parts need to be fitted to bring the parts as a whole into working order. Note: It’s either fit the parts, or you better have bins of them to start swapping until you find the combo’s that will work in the rifle. |
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The non-L cut hammer is just a M-16 hammer with the auto sear tab ground off. The L cut hammer is a Colt type designed ar-15 (read SP-1) hammer that worked as a safety device with an un-shrouded carrier to prevent hammer follow threw if the disconnector ever failed. Since a Modified m-16 hammer leaves the hammers firing pin pad full height, it all boils down to the receivers FCG pin placement, and FCG total geometry if the hammer will work as a drop in unit, or if the hammer is going to catch the FP collar or the hammer tail to bind on the disconnector when lowered by the carrier. 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. |
www.biggerhammer.net/ar15/hammers/ |
I"ve bene studying up because I'd like a pistol caliber AR soon as well.
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