Short Stroke

Sorry about repeating the try new mags thing. I missed that in your OP.

Pictures. Try showing the bolt feeding (or not feeding) rounds from the mag. Also, how far back the bolt travels when pulled open all the way by the charging handle. Like Clobb says, it could be your bolt is not going all the way back so it can't properly engage the round in the mag.

Another check, with an empty mag in the gun, does the BCG lock when you pull the charging handle all the way back? Does the bolt catch engage the front of the bolt lugs?
 
If it has a regular buffer, that should HELP feed issues. The standard CAR buffer has the least damping and throws the bolt back faster, making feeding more positive. In some shorties it will be too positive and give bolt bounce, but bolt bounce won't explain your problems.
 
Claven2 said:
If it has a regular buffer, that should HELP feed issues. The standard CAR buffer has the least damping and throws the bolt back faster, making feeding more positive. In some shorties it will be too positive and give bolt bounce, but bolt bounce won't explain your problems.
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Does anyone know if the weight of the buffer spring is different between the LMT MRP and the regular Defender carbine? Could a stiff spring be causing the short stroking?
 
If the top round is not being picked up when the action is cycled manually, that suggests that the round is not being presented properly. Does this happen when you insert the magazine with the action closed, and then cycle it, or when the magazine is inserted with the action locked open, and the action released? Or both? Given that this happens with different magazines, it sounds as if there is a problem with the relationship between the seated magazine and the carrier group.
Have you tried the upper on a different lower, and the lower with a different upper?
 
Why don,t you talk to Questar they probably can help you with the issue and you never know it might be something warranty related or LMT might assit you. they have been really good with others.
 
tiriaq said:
If the top round is not being picked up when the action is cycled manually, that suggests that the round is not being presented properly. Does this happen when you insert the magazine with the action closed, and then cycle it, or when the magazine is inserted with the action locked open, and the action released? Or both? Given that this happens with different magazines, it sounds as if there is a problem with the relationship between the seated magazine and the carrier group.
Have you tried the upper on a different lower, and the lower with a different upper?
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I am pretty sure that it is short stroking. filled a mag last night and manually cycled it without problems. Additionally, last time I was at the range I don't recall the bolt staying open after last round was fired.
 
martiy said:
I am pretty sure that it is short stroking. filled a mag last night and manually cycled it without problems. Additionally, last time I was at the range I don't recall the bolt staying open after last round was fired.
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The buffer spring is the same so that shouldn't be the problem.

Ammo can cause what you're describing... have you tried the gun with full power 5.56NATO rounds? Weak or underpower ammo will cause the gun to short stroke, especially on 10.5" LMT's which are often more sensitive to this issue. The 10.5" LMT is specifically designed to function with 5.56NATO ammo.

The LMT 10.5" guns come from the factory with standard "standard" buffers. The Piston rifles use an H2 Buffer. I would suggest you change the buffer to a "standard" weight buffer if you're using that lower with the 10.5" DI upper.

It's hard to diagnose problems without seeing the gun and knowing the full history, but you need to confirm that the upper is within spec... if it was bought "used" then it's quite possible that the previous owner changed or damaged something.
  • Has the sight post been removed at any time?
  • Is the original barrel installed and has it ever been removed?
  • Is there any issue with the gas tube?
  • Is the gas tube properly aligning with the gas key?
  • Has the gas key been modified or removed at any time?

Any problems in those areas could explain/cause the short stroking issue.

Mark
 
Questar said:
The buffer spring is the same so that shouldn't be the problem.

Ammo can cause what you're describing... have you tried the gun with full power 5.56NATO rounds? Weak or underpower ammo will cause the gun to short stroke, especially on 10.5" LMT's which are often more sensitive to this issue. The 10.5" LMT is specifically designed to function with 5.56NATO ammo.

The LMT 10.5" guns come from the factory with standard "standard" buffers. The Piston rifles use an H2 Buffer. I would suggest you change the buffer to a "standard" weight buffer if you're using that lower with the 10.5" DI upper.

It's hard to diagnose problems without seeing the gun and knowing the full history, but you need to confirm that the upper is within spec... if it was bought "used" then it's quite possible that the previous owner changed or damaged something.
  • Has the sight post been removed at any time?
  • Is the original barrel installed and has it ever been removed?
  • Is there any issue with the gas tube?
  • Is the gas tube properly aligning with the gas key?
  • Has the gas key been modified or removed at any time?

Any problems in those areas could explain/cause the short stroking issue.

Mark
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I bought the upper off a guy on CGN and it was advertised as a LMT MK18 CQBR (CQBR - Upper Receiver Group, NSN 1005-LL-L99-5996) 10.5" but I have no way of verifying that claim. It did not come with a BCG.

Appears to be original barrel and site post; however I have not checked the gas tube alignment yet nor am I sure I know what to look for within the gas system.
 
martiy said:
I bought the upper off a guy on CGN and it was advertised as a LMT MK18 CQBR (CQBR - Upper Receiver Group, NSN 1005-LL-L99-5996) 10.5" but I have no way of verifying that claim. It did not come with a BCG.

Appears to be original barrel and site post; however I have not checked the gas tube alignment yet nor am I sure I know what to look for within the gas system.
Click to expand...

Then start by ruling out what is NOT wrong.

  • Try some full power 5.56NATO ammo and see if that makes any difference (rules out ammo as cause).
  • Try your ammo and mags in someone else's gun to confirm they work without issues (helps rule out ammo and mags as cause).
  • Try your complete upper half on someone else's lower half (that will rule out the lower as the source of the issue).

If you rule out everything but the upper as the source of the problem then you may want to have a qualified AR Armourer find where the problem is with the upper. There are a variety of possible causes.

If you don't have someone local then you can contact our office to arrange for the upper to be sent to us for repair work.

Mark
 
Questar said:
Then start by ruling out what is NOT wrong.

  • Try some full power 5.56NATO ammo and see if that makes any difference (rules out ammo as cause).
  • Try your ammo and mags in someone else's gun to confirm they work without issues (helps rule out ammo and mags as cause).
  • Try your complete upper half on someone else's lower half (that will rule out the lower as the source of the issue).

If you rule out everything but the upper as the source of the problem then you may want to have a qualified AR Armourer find where the problem is with the upper. There are a variety of possible causes.

If you don't have someone local then you can contact our office to arrange for the upper to be sent to us for repair work.

Mark
Click to expand...

Thanks Mark. I will try each step and contact you if I cannot resolve the problem. Thanks so much for your help
 
Questar said:
Then start by ruling out what is NOT wrong.

  • Try some full power 5.56NATO ammo and see if that makes any difference (rules out ammo as cause).
  • Try your ammo and mags in someone else's gun to confirm they work without issues (helps rule out ammo and mags as cause).
  • Try your complete upper half on someone else's lower half (that will rule out the lower as the source of the issue).

If you rule out everything but the upper as the source of the problem then you may want to have a qualified AR Armourer find where the problem is with the upper. There are a variety of possible causes.

If you don't have someone local then you can contact our office to arrange for the upper to be sent to us for repair work.

Mark
Click to expand...

Well I found and tried some 75gr 5.56 match ammo and the gun ran flawlessly; however the action still would not stay open after the last shot.

When I went back to the 55gr 223 Winchester after the initial round, I would get FTF every round so obviously it is short stroking with the light loads.

Any suggestions or do I chalk it up that this rifle likes full power 5.56 rounds.

Oh and BTW I also changed the buffer and spring just to rule them out as causes.
 
Either you have too much buffer/spring, or not enough gas. I'm guessing gas (or rather the lack thereof). Time to take the handguards off and look for the more obvious stuff - plugged gas tube, kinked gas tube, misaligned front sight block.
 
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