I have had this M4 for about a year, bur unfortunately I haven't fired it much until today.  The same problem occured as was posted a while ago on this board.  I have only 6 mags; I only took three today (yes I know this is sad, I should have 12-15).I have 3-30 rd Bushmaster and 3-20 rd Colts from Eagle Firearms.

This is the test I performed when I noticed it short stroke.

I took each mag with 3 rounds in it and fired while holding the rifle tightly.  I repeated the same 3 round test while holding the rifle loosely and somtimes even like a handgun.  I did this 3 times for each of the mags; I also tried just one round a mag.  Strangely the short stroking occured more often when holding it tightly than when holding it loosely.  My rifle will short stroke about 40 percent of the time, but it has never failed to feed or fire.  

I have tried cleaning the gas tube with carburetor cleaner, to see if the gas was being blocked.  I can't tell if there is a gas leak.  I haven't noticed any signs of the carrier snagging and causing excessive wear on any parts of the upper reciever.

I have been using American Eagle 55gr. FMJBT, a fair quality ammo.  I have also previously tried Winchester, Ultramax, but it short strokes all the same.  

Specs...

16" LMT M4
DD 7.0
RRA Bolt and Carrier
4 KAC rail panels
DPMS lower and lower parts kit
Collapsible stock with LMT SOPMOD clone stock.
H Buffer

I have 2 buffers, one normal and one heavy, and 2 CAR springs one longer and one shorter.  I have tried both with different combinations of weight, ect.

I have tried everything, I think, but I am sure that AR15.com will never fail me.

Thanks,

Mike

ETA:  (I just realized that I posted this in the wrong forum, sorry, please move)

even though its an LMT barrel which is pretty high qualilty from what i've heard, i would suspect the gas port is a little on the small size. before you get to carried away i would get some full pressure ammo like xm193 just to confirm this.  if it runs fine that the gas port might be the issue

a drill bit might be in order to fix the problem.  just be sure to get machinists bits since they come in very small increments like .002".  your stand drill bit sets will not work. i took a micrometer and messured the port diameter. i bough a few different bit sizes in that range just to have them around.

either way, any barrel should run with down loaded ammo like what you mentioned. i had a barrel that was cut down that would run on full pressure ammo but not the cheap stuff.

a broken gas ring sometimes causes these problems or so i hear.

i'm sure someone will come along and tell me i'm all wet but its the first thing that comes to mind.

Quoted: My rifle will short stroke about 40 percent of the time, but it has never failed to feed or fire.

Are you saying that 40% of the time the bolt fails to lock back? If the rifle has never failed to feed or fire I'd say you don't have a short stroking (gas related) problem but you might a problem with your bolt catch or some related issue that is preventing your bolt from locking back.

Dont mess with the gas port.Sounds like gas loss.Check the carrier key and check the tube thats potruding in the upper that makes contact with the key.Some times the tube can wear on one side and cause gas loss from the key not aligning to the tube properly.Change out the gas tube and see,if it still happens chnage the key.In any event before I touched anything I send it back where I bought it and let them check it at the source.Dont put anything in the tube.There was guy here a few years ago who used carb cleaner then imediately fired his rifle and got flame out his ejection port...what ever goes into the tube can get into the barrel.I had the same problem and it was the tube it was worn on an angle and wasnt locking up flush to the key.It would feed and fire fine and not loick up sometimes on the last round even with new mags of known quality.Tube replaced and it works fine.If its feeding firing and then stopping on its rearward motion as it ejects and only takes the empty half way or more out with out ejecting then it could be the key.

Quoted: Quoted: My rifle will short stroke about 40 percent of the time, but it has never failed to feed or fire.   Are you saying that 40% of the time the bolt fails to lock back? If the rifle has never failed to feed or fire I'd say you don't have a short stroking (gas related) problem but you might a problem with your bolt catch or some related issue that is preventing your bolt from locking back.

along that thought line,,, can you manually cycle the action on an empty mag and get the bolt to lock open? maybe its a weak spring or dirty magazine preventing the follower from coming all the way up?

the short stoking i experienced that was port related, it caused a failure to lock back on the last round AND a failure to clear an empty case, intermittently.

Sources of short stroking are:

1. Not enough energy delivered to the carrier.

2. Too much energy sucked off by drag in the mechanism.

3. Too much energy taken up by extraction.

4. Too much energy taken up by the springs.

Not enough energy delivered to the carrier - Obstructed barrel port or front sight base, poor fit of the carrier key to the carrier, carrier key screws not tight, gas tube end or carrier key opening worn and leaky, carrier gas path obstructed, gas tube obstructed, gas rings gone, large clearance between tail of bolt and carrier, too small a gas port.

Too much energy sucked off by drag in the mechanism - Binding or rubbing carrier, binding recoil spring, carrier and hammer binding on each other or inadequate lube on reciprocating parts. You can feel this during manual cycling, both with the trigger released and with the trigger pulled.

Too much energy taken up by extraction - Rough, sticky, or out-of-shape chamber, as evidenced by marks on cases and bent extraction rims. Ammo/chamber that make excess pressure, as evidenced by pierced, cratered, flattened, or lost primers, buttons of case metal extruded into the ejector plunger hole, case head growth in firing. Dirty chamber - clean with gun solvent, then lacquer thinner.

Too much energy taken up by the springs - Extra power recoil springs and hammer springs can be too much to allow full cycling. Start with GI, and if they work OK, then play with stronger ones.

Notice how many sources there are. Eliminate the gas path problems by checking them all, put in GI springs, clean and lube everything, and try it. Look over the case carefully in a good light for bent rims, trumpet or barrel shaped bodies, signs of rough chamber, excess pressure.

What I ment by short stroking 40% of the time was that 40% of the time the last round does not cause the bolt to lock open.

Should I try my lower on someone elses upper and vice versa?

hrmm

try having the gas tube opened up some. that will increase the blow back of the bolt causing it to cycle?

Were you shooting wolf????? Everytime I shoot wolf, my rifle short cycles, but cycles fine with black hills, q3131, and q3131a.

Quoted: try having the gas tube opened up some. that will increase the blow back of the bolt causing it to cycle?

Opening up the gas port should be the last resort.

Does the bolt lock back on an empty mag if you manually pull the charging handle to the rear?

Quoted: Quoted: try having the gas tube opened up some. that will increase the blow back of the bolt causing it to cycle? Opening up the gas port should be the last resort. Does the bolt lock back on an empty mag if you manually pull the charging handle to the rear?

Yes, it does without a problem.

Quoted: Does it feed live all rounds?

Yes, without a single problem.

Quoted: Quoted: What I ment by short stroking 40% of the time was that 40% of the time the last round does not cause the bolt to lock open. Should I try my lower on someone elses upper and vice versa? Could be a problem with your bolt catch mechanism on your lower. Does it feed like it should?

That's what I'm thinking. Especially since he says it feeds, extracts and ejects fine. I'm certainly no expert but that would lead me to believe he does not have a short stroking problem. I wonder if his carbine simply has some issue holding the bolt open.

M11293 - Try your upper on a different lower.

From the troubleshooting checklist in the troubleshooting forum:


---TEST FIRING PROCEDURE---
-Unload the rifle and verify that it is clear.
-Check the bore for obstructions.
-Load the rifle with one round from the magazine. Use quality ammunition and a proven magazine for this.
-Aim downrange and fire, the bolt should catch on the bolt stop. Hold the rifle as loose as possible.
-Reload and repeat the process if the rifle does not lock open Depress the bottom of the bolt stop before firing this time. This allows for weak magazine springs, sticky followers, or an overly strong bolt catch spring.
-If the bolt does not lock open then there is a gas system problem. Investigate and repair the problem.
-If it only locks open with the bolt stop depressed, repeat the process with a different magazine. If the rifle still fails to function then look at the bolt stop as a probable culprit.

Quoted: Put one round in a magazine, chamber it and press in on the bottom of the bolt catch.  Then go bang. If it engages with the little assist from you, it is proabally either an incorrect spring in the catch or the catch is binding.  This will definately eliminate short stroking. This is why we do not drill the gas port first.  It is a very reluctant last resort.

I don't quite understand your procedure.  How does little assit from me mean my bolt lock is working?

Sorry for double tapping. Just wanted to bump up my post count. LOL!

Quoted: Put one round in a magazine, chamber it and press in on the bottom of the bolt catch.  Then go bang. If it engages with the little assist from you, it is proabally either an incorrect spring in the catch or the catch is binding.  This will definately eliminate short stroking.

Quoted: I don't quite understand your procedure.  How does little assit from me mean my bolt lock is working?

No, his procedure eliminates the possibility of short stroking/gas system related issues. It is checking to see if the bolt latch assembly/mag is faulty.

1. Check your mags.  Are they new?  Do they need new springs?
2. Lock your bolt to the rear and leave it that way until next time you go shooting.

Hope this helps
Hardwarz

Spend $10 and replace the gas tube and see.

That is impossible, clearly this guy messed something up here since it is an LMT. It is probably a combination of your ammo, oil, cleaning, gas tube, magazines, break in, or probably the owner as last result. Had a few malfs with my LMT as well. Good luck.

Quoted: Put one round in a magazine, chamber it and press in on the bottom of the bolt catch.  Then go bang. If it engages with the little assist from you, it is proabally either an incorrect spring in the catch or the catch is binding.  This will definately eliminate short stroking. This is why we do not drill the gas port first.  It is a very reluctant last resort.

I tested my rifle on Monday.  It definitely catched with almost no help from me.

Now what?

Thanks

Quoted: I tested my rifle on Monday.  It definitely catched with almost no help from me. Now what? Thanks

If it locked back when you held the bolt catch in manually while firing, then it isn't a short stroke but a defective bolt catch instead.

+1 for what M4Madness said.

So, I need to purchase a bolt catch and the bolt catch spring?  Should I replace the extractor spring also?

Where can you purchase these Extra-Strength springs?

Quoted: Where can you purchase these Extra-Strength springs?

Wolff