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Posted: 3/13/2011 6:17:52 PM EDT
| I recently purchased a CMMG conversion kit and tested it out yesterday, after the first 150 rds I loaded another mag of 25 rds, pulled the trigger and 6-8 rds shot out, then when I emptied that mag and loaded another mag, it triple tap 2 times. I have an Aero lower receiver with a RRA LPK, has anyone with this setup experience this? Can anyone explain why this happened, I was told by a friend that I might need a weaker trigger spring. |
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Quoted:
I would say the opposite. Stronger hammer spring. Also, check the installation. There is nothing in the bolt to cause a burst. Is the RRA a notched or rounded hammer? Dave N Sounds like you nailed it. I had a similar issue with a notched RRA hammer(2stg trigger) and JP yellow springs on a Spike's .22 upper, but it was intermittent and easily fixed with orange springs from PK. I just tested my CMMG stainless conversion the other day and it worked fine with a DD notched hammer, but I forget which springs are in that lower, stock or JP yellow. |
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Quite a doozy for your first post. I call B/S. These things just dont start shooting 8 rnds automatic unless they have been tampered with. Yes they do when you shoot ammo with the perfect blowback power and the hammer and recoil spring all work in balance enough to not let the trigger group recock. Stronger ammo or some spring tuning will fix this problem. |
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Quite a doozy for your first post. I call B/S. These things just dont start shooting 8 rnds automatic unless they have been tampered with. I had a bad trigger job (or good trigger job and an out of spec lower. WTFE) and it would double on my TacSol upper. If I didn't care and not even shoot that FCG after it malfunctioned I could have easilly strung it along for 8 shot bursts. That FCG has been destroyed (along with my interest in getting a one stage trigger modified ever again). Who knows if the OP is full of it, but I wouldn't call B.S. so quickly. Posted Via AR15.Com Mobile |
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I've seen several rifles do this. It can be from using a lighter than standard ammo or it can be
from a light hammer spring as was mentioned. There are combinations that will cause bursts. Hopefully, "The Man" sees that the poster is trying to eliminate the situation. |
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Quoted:
Quite a doozy for your first post. I call B/S. These things just dont start shooting 8 rnds automatic unless they have been tampered with. I had a bad trigger job (or good trigger job and an out of spec lower. WTFE) and it would double on my TacSol upper. If I didn't care and not even shoot that FCG after it malfunctioned I could have easilly strung it along for 8 shot bursts. That FCG has been destroyed (along with my interest in getting a one stage trigger modified ever again). Who knows if the OP is full of it, but I wouldn't call B.S. so quickly. Posted Via AR15.Com Mobile Fair enough. Maybe I jumped the gun............but not many post the "ive got an accidental machine gun" for their first post. Besides, who has a built AR that isnt already on ARFCOM? 
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Assemble your CMMG kit without the recoil spring, and place the kit back into your upper. Mount the upper to the lower, and move the bolt fore and aft with your fingers. You want to feel if anything is dragging on the bolt. Sometimes the bolt catch is too tall. If you have any excess drag on the bolt, weak ammo, or too strong of a recoil spring, the hammer does not get reset, and you get to experience the problem you just described. I have also had to use a JP Enterprises reduced power hammer spring along with cutting the tail off of the hammer.
With a little effort, you can figure this out |
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Hello all,
Thanks for the replys, both positives and negatives. I appreciate the help and information that can assist me with my current rifle situation. Like I stated before, new stainless steel CMMG kit, Aero Lower and RRA LPK. The ammo used was Federal Lighting 40 grain from Walmart. The Hammer is rounded not notched. I assembled the lower following the direction on this site, along with some youtube video. I doubt I assembly it wrong as it is pretty straight forward. I picked up the Aero lower receiver and LPK at a local gun shop in Vegas. As you all know, decent parts for a cheap build. This lower will be for a dedicated 22 build currently in progress. Because of the BS replys, I started to second guess myself, so I went out to the desert and took some videos of me shooting the rifle and sure enough it happened again, a couple of double and triple taps and two instances where it shot out a 5 or 6. I got home and seperated the upper and lower, inspected the BCG, the firing pin moves freely, I doubt it has anything to do with the BCG. I would like to take the BCG apart, as suggested by davestoll, but I don't want to void the warranty, unless CMMGinc says it's ok. I guess I will start with a stronger trigger spring and see how it goes from there. Also how do I verify that the hammer was cut correctly? Thanks again for your help. |
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You can take the bolt/carrier apart. Be careful doing so.
carefully pull one side of the rail out of the collar. Twist the collar down and pop it out. From there disassemble. Same putting it back together. CMMG knows there will be times the BCG may need to be taken apart. If the rails get bent out of spec, problems arise. I've taken them apart many times. Just pay attention. Dave N |
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Quoted:
Quite a doozy for your first post. I call B/S. These things just dont start shooting 8 rnds automatic unless they have been tampered with. Don't be too quick to call bullshit the same thing happened to me with a ciener kit, except it was 3 -5 round bursts. i never missed around with the kit just installed it in the rifle, which worked fine with .223 ammo. I switched from the winchester ammo to federal and the problem went away, but if I go back to winchester ammo it happens again. I threw out the winchester ammo and have used fedral ever since. |
| Ok, I took the recoil spring off and and reassembled the BCG, installed into the into the upper with the lower. The bolt slides back and forth drag free. I will go out and purchase some 36 grain federal and winchester and give those a try to see if I get the same results. |
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I have never had this happen but would the hammer follwing the bolt into battery cause this? Maybe the contact surfaces with the sear are out of spec. Yes the hammer following the bolt will cause your problem with some weaker bowback power ammo, hammer spring or recoil spring needs some tuning so the bolt will come back just a little more so the hammer will recock and lock back. I tune all of my springs to function with the weakest ammo then most any ammo will cycle good. function check your trigger group too as this can cause the same problem. |
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Quoted:
Quoted:
Quite a doozy for your first post. I call B/S. These things just dont start shooting 8 rnds automatic unless they have been tampered with. Yes they do when you shoot ammo with the perfect blowback power and the hammer and recoil spring all work in balance enough to not let the trigger group recock. Stronger ammo or some spring tuning will fix this problem. Yup. |
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Pan -
I had problems with my .22 conversion, too. 2 or 3 rounds bursts were a blast, but unpredictable. All CMMG (lower, parts kit, and upper). I was using the recommended Federal bulk pack. It shot fine with 5.56. I emailed CMMG tech support. After checking a few things and a few trips to the range, they sent me a call tag. I had a replacement in 4 days. 1000+ rounds later and it's been trouble free. |
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Quoted:
Quoted:
I have never had this happen but would the hammer follwing the bolt into battery cause this? Maybe the contact surfaces with the sear are out of spec. Yes the hammer following the bolt will cause your problem with some weaker bowback power ammo, hammer spring or recoil spring needs some tuning so the bolt will come back just a little more so the hammer will recock and lock back. I tune all of my springs to function with the weakest ammo then most any ammo will cycle good. function check your trigger group too as this can cause the same problem. While I see that for whatever reason, there are some that seem to have had this problem, I have to believe something else is adding to this problem. I've tested multiple kits in multiple set ups and lowers. I have never had to change springs or anything else on these kits. The same kit with 0 changes has run in 5.5" through 16" with ammo ranging from Remington subs(until it gets gummed up)(also, well broken in kit) to CCI Stingers. Six kits I have right now have no changes and run. Never doubles or hammer follower with G.I. hammers and stock springs. I wish a kit would do that so I have some experience as to what is causing it. Dave N |
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Quoted:
Quoted:
I have never had this happen but would the hammer follwing the bolt into battery cause this? Maybe the contact surfaces with the sear are out of spec. Yes the hammer following the bolt will cause your problem with some weaker bowback power ammo, hammer spring or recoil spring needs some tuning so the bolt will come back just a little more so the hammer will recock and lock back. I tune all of my springs to function with the weakest ammo then most any ammo will cycle good. function check your trigger group too as this can cause the same problem. While I see that for whatever reason, there are some that seem to have had this problem, I have to believe something else is adding to this problem. I've tested multiple kits in multiple set ups and lowers. I have never had to change springs or anything else on these kits. The same kit with 0 changes has run in 5.5" through 16" with ammo ranging from Remington subs(until it gets gummed up)(also, well broken in kit) to CCI Stingers. Six kits I have right now have no changes and run. Never doubles or hammer follower with G.I. hammers and stock springs. I wish a kit would do that so I have some experience as to what is causing it. Dave N With the longer barrels most of the time no tuning is needed but if you play with 5.5" or shorter barrels and like to shoot sub ammo then some tuning is sometimes needed for good function without a can. Not all of the recoil springs are exactly the same strength and you get one that has just a little more strength it can keep the bolt from coming back that extra 1/4 inch that may be needed for perfect function. CMMG has no control of the ammo or hammer spring that the end user has in the host and sometimes a hammer spring with just a little extra strength can keep the bolt from coming back far enough too. |
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