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Posted: 6/10/2006 12:17:45 PM EDT
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Rifle: LMT 14.5" w/perm flash hider, LMT bcg, and LCW lower by LMT w/J&T LPK and Colt CAR stock/tube. Less than 600 rounds total through this rifle. Mags are 2 CP mags w/green followers, 1 NHMTG w/Magpul gen2, & CP SS w/Magpul gen2 1st round of 100 was HSM 55 grain ammo. After approx. 70 rounds, I had a light primer strike, but recocked and it fired. 10 rounds later (4th mag) I had 3 light primer strikes in a row, all of which fired after recocking. I assumed it was a bolt carrier/group problem and swapped with my Colt M16 bcg. Today: Ammo: Federal XM193, Lot 6 -- 100 rounds w/25 in each mag Mags: 1. CP alum w/grn follower: 2d & 24th FTFire --recock & they fired 2. CP alum w/grn follower: 23d FTFire (did not fire after 2 strikes which appeared marginally adequate. 3 NHMTG mag w/magpul gen2: no problems (coincidence? I don't know) 4. CP SS w/Magpul gen2: 11th round FTFire -- recock & it fired Is this a lower or LPK problem? I have another lower which is an LCW as well, but has a CMT LPK in it. I had this happen 1 time previously, but again assumed it was a hard primer as the strike looked adequate at the time.  TIA. |
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Check a few of the other spent cases and see if the primer ignition marks are as light as the ones that didn't fire the first time out as well. Really need to know if the firing pin is just light striking (FP protrusion not between .032 to .037), or this fail to ignite problem is coming from the bolt just not locking up correctly (back of the carrier blocking the FP from achieving full protrusion since the bolt is not locked home). |
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I compared the primer of the round which failed to fire twice to those on numerous other rounds and didn't see any significant difference. The chamber seems to be excessively dirty after shooting the XM193, although it didn't seem to be dirty after shooting the HSM ammo. I'm not sure if that's significant or not. I have not changed anything to do with the FCG. |
That's my first thought too. The hammer spring on my M16 was weak (prior owner had kept it cocked while it was stored in his safe for 20 years) and caused that exact problem. Mine was having several light strikes per mag. |
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What is the round count for the barrel? I am thinking the headspace may be excessive and that the cartridge case may be seated in the chamber and driven forward by the hammer and firing pin. Also since you have not changed out the FCG parts have you checked to make sure the hammer spring is correctly installed? Charles. 
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Round count is <600. After checking, I'm thinking that the hammer spring may be the problem. I do not have a way to quantitatively check the hammer pull to cock it, but when I compare the finger pressure to manually cock the hammer, the resistance is noticeably lighter on my problem child, when compared to my other lower which is much stiffer. I'm going to switch lowers and see if I still have the problem. I do have a military no-go gage so I'll double check the headspace. Thanks for the help with this!  ETA: Wouldn't close on a military Field Gage, so headspace is good. |
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Check the hammer spring. The hammer spring should be tightening in the coils when the hammer is cocked back into the hammer/sear engagements. Sometimes people put the hammer spring onto the hammer backwards. When put on backwards you will function check okay, and it will fire most of the time okay, but will occasionally have light primer strikes. INCORRECT (Hammer Spring is on Backwards, notice the uncoiling)  CORRECT  CY6 Greg Sullivan "Sully" Chief Instructor SLR15 Rifles TheDefensiveEdge.com |
Ding!Ding!Ding! We have a winner! As much as I hate to admit it, it appears to be operator error. The hammer spring is in backwards. My other lower is correct. Dang it, I hate it when it's my fault!   Thanks Sully! (Also thanks to everyone else's excellent suggestions.) I very much appreciate the help. --Chuck ETA: I just explained to my wife what I did wrong and how the incorrect spring is not as stiff as the correctly installed spring and she informed me that "stiffer is always better!" 
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| Even though you diagnosed your problem. It is worth pointing (IMHO) that light primer strikes can also happen if you over lube the firing pin. When shooting ammo that is "dirty" or leaves behind a lot of carbon, the lube will attract the carbon, creating quite a mess in the firing pin channel in the bolt. As a result, it is always prudent to leave the firing pin and inside of the bolt dry or very lightly lubed. |
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Did a little shooting today-- not a single failure to fire. It's amazing what a properly installed hammer spring will do. I did identify one problem mag that would try and feed the next round before the spent cartridge was ejected, causing a type of 'double-feed' jam. Easy to fix, but I could repeat it and determined it was definitely the mag. |
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