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Heavy hammer spring causing short strokes?


xtian999

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Had a cmc trigger installed on an Aero lower and took it to a rifle class where it short stroked repeatedly. Instructor put the upper on his gun and it ran fine. His diagnosis was that the hammer spring was too heavy. Looking at his spring, it was much lighter. Bought a stock hammer spring and tried to install it and realized that it will not fit into the cmc trigger group. Before I put it all back together, I would like to ask here if the heavy spring is the problem and if so, where would I find a lighter cmc spring? They are not listed on the website.

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By "short stroked" you mean the bolt would not cycle "fully to the rear" and strip the next round from the magazine and load it into the chamber? 

 

I doubt a "heavy spring" is the problem.

 

I've used three different CMC trigger cassettes and never had a cycling issuing.  I did have one CMC where the housing didn't fit the lower that well and would bind a bit, but i rectified that right away with some pliers and a metal file and have had 0 issues with it.  

 

Did the rifle cycle fine before installing the CMC trigger?  Did the person installing it know what they were doing or possibly booger something up?  Keep in mind there are two other variables in the AR cycling mechanics that are in the lower:  recoil spring and buffer.  

 

If the rifle cycles fine with the same upper & lower but with different trigger group (like the fire control group the rifle came with), then there might be something wrong with the CMC trigger you have.

 

Also, it's not recommended to disassemble the CMC cassette and replace parts.    

Edited by uewpew
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Yes, it did not cycle fully to the rear. Bolt did not strip a round off mag or jammed it at an angle. Bolt sometimes  not locking back after last round. Happened dozens  of times in class. Used brand new Magpul Pmag 30rds. 

No, it did not work well on that particular upper before replacing the trigger. That is why we put a new trigger in it. But it did work well with an Aero 300 blk upper before that.

I think the installer had a good idea of what he was doing. He sold me the upper and he is a custom gunsmith. He has tried several things to make it work: New bolt, new gas block, new trigger. 

I did disassemble the cmc cassette. Did not see any warning labels. New spring does not fit. Did not try to reassemble.

 

Bottom line is this: I need a rifle that works. I don’t really care whose name is on the parts. What do I need to do to get this thing running?

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Personally, I’d grab a $20-30 DPMS (or similar, just pick a brand) lower parts kit and drop a mil spec trigger into the gun. As others have mentioned, there are other things in the lower tha could contribute.

 

Change nothing but the trigger and test it, until you’re certain that it’s definitively the trigger.

 

Then look at another comany with a stellar record like Gieselle or Hiperfire or whatever the magic brands are these days... and try one of those.

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From American Rifleman: “Short-stroking occurs when the bolt and carrier do not travel fully rearward upon firing. When this insufficient travel prevents the bolt from picking up the rim of the top cartridge in the magazine on its forward stroke, feeding issues result. Short-stroking can also prevent the fired case from being thrown fully out of the gun. Short-stroking may be associated with a failure to stagger the slots in the gas rings on the bolt, a broken, bent or plugged gas tube, or carbon or dirt in the carrier or upper receiver. Even going from a lighter to a heavier hammer spring may cause a marginally-functioning system to short-stroke.”

 

So, maybe it is the spring. If cmc does not have a light spring, then a stock trigger group with a stock spring will go in and we will see... 

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On 4/16/2018 at 11:43 PM, xtian999 said:

 

So, maybe it is the spring. If cmc does not have a light spring, then a stock trigger group with a stock spring will go in and we will see... 

 

Please do report back with your test results. =)

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It could be a heavy hammer spring, though in a CMC trigger I would doubt that.  It could be the buffer ( too heavy; is it an H,or H2 or H3 buffer?)  It could be a heavy Buffer ( Recoil) Spring. Absolutely agree with plan to put standard trigger group in the rifle, and try again. If it does the same, your now trashed CMC trigger was not the cause. If buffer and Buffer spring are standard stuff, ( sort of easy to check the buffer, even if it is heavy but not MARKED as such. Mistakes happen ).  A gun that is under-gassed will short stroke.( I not an expert, have had several over-gassed carbines that performed better with a H or H@ buffer installed.) As your upper was tried on a different lower, that seems to be ruled out.

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