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Posted: 9/3/2005 12:39:23 PM EDT
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I built my first AR after researching here and around the web. A 14.5+phantom CMMG upper with chrome standard BC, RRA LPK, RRA M4 stock and an Oly lower. This combination is not working for me. It is short stroking badly, cycling normally only about 5% of the time. It is also damaging the firing pin. CMMG told me the notch in the RRA hammer is causing the problems. ADCO (my supplier) told me all RRA hammers have the notch. I read here and elsewhere to never use a standard bolt carrier but to instead use a 'shrouded' or 'enhanced' style. This was all news to me. I thought I was buying the best LPK with the RRA. Had I know about the notch in the hammer and problems it could cause I would have chosen another brand. The 'unshrouded' standard BC came with my upper from the EE. Had I known the problems with these interfering with the hammer, I would have not bought it or at least asked for a different BC. CMMG is sending me a used rounded hammer ($10) which it says will fix everything. We'll see. Is this old news to everyone? Why do some people recommend RRA LPK's? Will the 'brand X' rounded hammer I get from CMMG work with the rest of my RRA FCG? Does anyone here use an unshrouded BC? Can I round off the RRA hammer notches with a grinder? Will a rounded hammer fix the problem or will I need a new BC? http://tinypic.com/dc4hlf.jpg http://tinypic.com/dc4hsy.jpg http://tinypic.com/dc4hw8.jpg |
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I use only the ramped type carriers that expose the firing pin and the notched hammers. I have never had any problem that looked as bad as yours looks. The parts are called SP1 style parts and are designed so the hammer will catch the collar of the firing pin if the disconnector fails to function properly. It keeps the hammer from following the firing pin on the return cycle if the hammer is not retained by the disconnector or the trigger. The problem many have is the firing pin collar is incorrectly sized on some of the pins being sold and this causes problems with the hammer hanging the firing pin and also it tends to bend the firing pin retaining pin. Also some of the notched hammers are not notched correctly and may hang a properly sized fiirng pin. But your firng pin seams to shows wear marks that I have not seen associated with this problem on my guns. Usually you only see the collar being beat up but there are other wear marks at other locations on the pin that I have never seen caused by too large of a collar and a notched hammer. You will find that the rounded hammer is not lower as you have it drawn but is the same height with the round being because no notch was cut. It will still drag across the firing pin as the carrier cycles rearward if you use a ramped SP1 style carrier. If a hammer is rounded to far down or if the notch and top of the hammer removed to much the carrier will not push the hammer down far enough to catch the disconnector and possibly cause a dangerous condition where the rifle fires more than one shot when the trigger is pulled. Most SP1 style firing pins I have measure around .330 across the large collar area allowing them to clear the hammer notch. Have you checked to see if your hammer spring is installed correctly or tried a new firng pin? Does the lower group pass the function test? A notched hammer should function properly with either type carrier if all other parts are correctly made and installed. When I buy DPMS kits with a rounded hammer I dremel the notch in them because I prefer to use the SP1 style parts. |
| I just checked my AR (built with an M&A kit), and my bolt carrier is unshrouded (well, it looks about 1/2 way between the two you have in your pics) and my hammer notched, but I am not experiencing any of the problems or unusual wear you are seeing. It shouldn't be an issue if all the parts are in spec, but sometimes there is an advantage buying all the parts from one source in that you can usually be sure they will all work together. |
I must be one of the kool-aid drinkers, because I've always regarded the Colt LPK to be the best.  You might try a M16 firing pin (perfectly legal), as the collar is smaller in diameter and may not drag on the hammer. |
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M4Madness, You might try a M16 firing pin (perfectly legal), as the collar is smaller in diameter and may not drag on the hammer. -------------------------------------------------------------------------------------------------- I thought the M16 firing pin had the larger collar ? |
This is incorrect, The m16 firing pin collar is larger not smaller than an AR15 sp1 style pin.
Yes, it does. |
Oops! And to think that I actually own an M16 myself.  I figured that the AR-15 firing pin would have been the larger one so that it would be guaranteed to catch the hammer's notch in the event of disconnector failure. EDIT: I have a couple of spare AR-15 firing pins lying around and just pulled my M16 apart to compare them to the firing pin in it. The collar on the M16 firing pin is definitely larger than the one on the AR-15. The title of this thread (Learning from my mistakes...) fits me perfectly. 
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I've bought 3 RRA LPK's from RB Precision in the last year. NONE of them came with a notched hammer. I prefer the un-notched hammer and shrouded Enhanced carrier. I can get bump-firing fast enough that I hang up on the firing pin after 2 or 3 rounds once I get it percolating. WIZZO |
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I believe that most all AR15 semi-autos have the unshrouded BC's. It almost looks like something is wrong with your trigger group by the amount of wear across the ramp of the BC. It appears that your hammer is hitting some type of hard stop when the BC slides over it. It may be just the photo, but I've never seen the amount of gaulling that appears to be on the underside of your BC. |
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I thnk what looks like bent metal is actually just black CLP grease. I may have put a round fingerprint in the middle of the ramp when removing it. The ramp is smooth. I think the hammer or something is hanging up on the FP. We'll know soon when I get the measurement of the old FP and the new hammer. Here's a closeup of the ramp. It sure does look bent...but it's not. It's good to hear that most ar's use this type of carrier. I'd like to keep it. 
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Can you wipe off all the grease and gunk and stuff and take a pic? Might be clearer that way. |
Ah, crap. I gotta eat a little crow. Of the 3 RRA LPK's I bought off Robert, 2 had the un-notched hammer. One didn't.  The un-notched were bought in March/April and the notched was bought back in October of '04. They are RRA because Robert wouldn't misrepresent anything. Maybe they have changed suppliers in the last year. Also notice the difference in where the diconnector grabs. It's a lot bigger than the one I bought about a year ago. I have the one hammer laying around as a spare because I bought a RRA NM trigger and didn't need it for one build. WIZZO |
OK, that's better. The picture is a little deceiving as it looked like the hammer was biting into the BC and thus slowing its travel. Did the rifle lock back when a single round was loaded? I had a similar problem a while back and changing the FCG cured it. The FCG on this rifle had been in service for a long time and saw limited or no cleaning or maintence. The holes in the FCG were all well worn and I think the hammer was sitting higher because of this. I'd be interested in hearing how it does with another hammer. The old hammer may be just a little out of tolerance. |
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I read this post over the weekend, and checked with RRA when I was there this morning. ALL of their hammers have the notch, and always have had it according to the guys inthe shop there. I looked at several hundred in their bins and they were all notched, and when I looked at the several I have at home (bought "factory direct"), they're all notched as well. My two cents. Sluggo |
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If CMMG said your problem is the RRA LPK, they're FOS! I have a CMMG upper with a chromed carrier on a Ameetec lower with an RRA LPK and it works fine. I'm not knocking CMMG, but I'll say this, the upper I have is the third one. The first one would lock shut on empty cases, and when I say locked, I mean completely locked, the hand guard fit was awful and it had a canted front sight, I returned it and they gave me another one, this one had bad fitting M4 hand guards too and the delta ring was purple, I returned that one and now have one with M33 hand guards that fit nice and tight and no purple parts on it. I haven't shot it though so we'll see if all is well or not. I don't see any reason for CMMG to make shit up, but I can't see the RRA LPK being the problem. |
I got my upper back from CMMG. They said it test fired fine for them and it was returned to me along with a rounded hammer to replace the notched one. I took the rifle to the range along with 200 HSM reloads and some other ammo. The rifle worked much better than with the notched hammer. The first 50 or so rounds went perfect, then the short stroking started to happen again. The remaining 100 or so rounds would eject about 8 feet away but about 5 per 30 round mag would fail to go back far enough to completely strip a new round. The jam would leave the round trapped diagonally against the feed ramp. So even with the rounded hammer, it's still short stroking, just not as bad. I examined the brand new firing pin and it is again all dented and beat up.   Upon looking closely at how the bolt carrier moved, I realized that the firing pin was actually pushing the hammer back most all the way before the ramp even touches the hammer.   Do you guys with unshrouded BCs ever have this kind of damage to your firing pin after 200 rounds? Could something just be out of spec with my rifle? Could this rubbing of the FP against the hammer be causing the short stroke problem? My hammer sticks up 12.5mm. Is this right? 
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That chromed bolt carrier is out of spec. The ramp looks like it is cut too deep to me. Someone else also mentioned that their carrier's ramp was in between the two in your picture. Borrow another "Known Good" one and put another NEW firing pin in and give it a whirl. I bet that the pin comes out looking like new still. Randall |
All the Rock River kits I bought had a Notched hammer. The ones I have that look like those are out of DPMS kits. The ramp does not look too deep to me. If it was I doubt it would matter because the hammer bumps across firing pin on the SP1 style on the back stroke untill the carrier starts pushing it down. On the forward stroke the trigger or disconnector holds the hammer down out of the way. |
 Well I'm the guy that cugir bought the CMMG upper from. Unfortunatley I never fired it.  It is my assumption that there is something out of spec with the BCG since the upper functioned fine with a different BCG. Possibly a tolerance issue with the chrome lining. I have offered a refund on either the BCG or the entire upper. I compared the pics to that of a unshrouded carrier I have and it looks the same to me. I don't think it has to do with the ramp. The 11mm hammer height looks good too so I doubt the lower is out of spec. |
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Comp1911 (the seller above) and CMMG have both been great in helping me remedy my problem here. I will be getting a new shrouded style BC to try soon. I'm confident this will fix the short stroke type failures I've been having. I'll report the results here. It is interesting that so many people use the unshrouded BC with and without notched hammers without any problems. Maybe something is still out of spec with my setup.. or maybe it's just my black cloud, gremlin havin' self. |
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Cugir- In looking at your carrier pics and comparing it to my test fire carrier, I don't see a ramp problem after all. Something did jump out though. You mentioned 12.5mm from the top of the hammer to the top surface on the receiver. I grabbed my test fire lower to measure that dimension. What I found were 2 things... First, I must have had a similar problem to you when I built this lower because I clearanced and polished the end of the hammer.  Second, it measures 11.4mm where yours measures 12.5mm.  I think that's a key player in this... Oddly enough, we both have olympic lowers. Mine is a 1999 vintage. Possibly, there is a dimensional problem with the hammer and/or trigger pin holes in the receiver. Either they are both too high, causing the hammer to stick up, or they are not the right distance apart, also allowing the hammer to stick up too far when held by the trigger. Tweak would probably know if this was a common problem with OAI receivers. Randall |
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Update.. I purchased a new shrouded BCG from Bravo Co. This BCG fixed my short stroke problem %100 and the bolt holds open after the last round now. My new FP is perfect and undamaged after 300 rds. Here is a pic of the new BCG in black and the old one in silver  on the left is the new FP which is a larger diameter than the old. I don't have a good way to measure them.  Thanks for all the info everyone. |
DUDE!!! THeres a tick defiling your lower!!! RAPE RAPE!! get it off!! 
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He must be out of Heinekens |
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Just an small update on the BCG that cugir was having problems with. I refunded him for the BCG. I had a chance to try it last night in my CMMG MOD4 on DPMS lower, Vltor stock and RRA LPK with notched hammer. I ran a mag through it with no issues. I will put alot more rounds on it during Thanksgiving. I did add an o-ring and was running XM193. |
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Wish I knew what the problem was and offer a solution, but I don't. I will say that every AR I own has a shrouded carrier and rounded hammer and all run smooothly, assuming I'm not doing something that would cause them not to, like custom builds, known bad mags, crap ammo, or a combination of any - any or all of which I may be doing at any given time. |
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