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Suppressors LWRC M6A3 Short Barrel 10.5 inch Rifle with AAC M4-2000 Suppressor in 5.56 NATO

NevadaZielmeister

Sergeant
Full Member
Minuteman
Mar 1, 2013
693
12
Northern Nevada
Gentlemen,

Maybe you can help me out, not sure. I have a problem with my rounds "cooking" off in the chamber prior to the primer being impacted by the firing pin. So basically, I am getting the barrel so hot that the rounds are "pre-detonating" when the powder reaches flashpoint. Pretty much, the rifle goes full auto for 3-4 rounds then stops.

When I run things without the suppressor, everything is fine. The problem is not reproduced. Only when I run the can, and only after about 7-8 shots in a semi heavy string, maybe 2-3 shots per second pace. To detail, I have an adjustable gas block and I am sure to have the "S" selected for suppressor. It has an "H2" buffer.

Any ideas other than shooting smaller strings with my shots more spread out to let the barrel cool down?
 
NZ,

This is an odd one. Cook off in an AR platform is rare, rarer still with semi auto firing schedules, rarest still without having the round sit for seconds before igniting. Your rig would have to be red, glowing red hot, I would say your problem is "other than" cook off.

Supression "cook offs" are also rare and most instances this effect occurs when running a High pressure can. High pressure cans can maximized suppression values but are designed for low cycle firearms (bolt action). In a high pressure can we find greatly accelerated pressure/heat retention in the entry of the can. But, the M4-2000 is traditionally not a high pressure can and I have seen them used successfully in FA down to 10.5" I would even go as far to say that I dislike this model because its suppression values are poor as a result of its pass through and amount of internal real estate dedicated to the QD. But....If it is generating higher than nominal internal pressures, all that retained pressure could theoretically translate to additional heat and, in a high pressure can in a heavy firing schedule, high heat in the diverter/primary/blast baffle set as well as a high propensity for unburnt powder to actual ignite in the can, increasing heat again. All this heat could then saturates a short barrel, quickly moving back to the upper, etc.

Note, your using a switch block ("S") limits the returning gas system flow to avoid over powering the bolt, that means more gas left in the barrel to enter the can. As such your gas block is not a contributor.

So, to begin, with we must look elsewhere.

1. Contact the maker.
2. Swap out your auto bolt/carrier and see if that has an effect.
3. Check your firing pin/hole dimensions. Check for burr. The reason I say this is that if the firing pin is actually sticking then you are experiencing a whole different effect whereby the can is actually providing low pressure, the gas block is actually providing low pressure and your carrier is being effected by less momentum, just enough to cycle and providing a lazy pin the opportunity to apply just enough kinetic energy to slam fire the rounds.

Your problem is going to get fixed fast. Please do let us know what you find out.
 
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Thank you RollingThunder51. I appreciate the time with your assistance. Here is a photo of the rounds that are (maybe) being cooked on the right and the regular rounds on the left:

IMG_3244.jpg

The primers on the right are slightly bulged and you can see firing pin cupping.

I will contact the manufacturer on Monday to discuss. I hope it is not round cook off.
 
Doesn't look or sound like cookoffs to me. I bet the increased gas pressure is causing extraordinary BCG closing speeds that cause a slamfire. I would suspect the upper is grossly overgassed, or there is a defect with the bolt carrier or firing pin.
 
Possibly needs a heavier buffer when suppressed to slow down the bolt and avoid imparting the additional inertia to the firing pin to cause slamfires. Also try a titanium or other lightweight firing pin if you haven't already.
 
I have an LWRC m3 in 6.8 with a 10.5 inch barrel that I run with a can. I note even on S it is over gassed. I am planning on taking corrective action. Installing an H2 and stronger recoil spring didn't help much.
 
I have an LWRC m3 in 6.8 with a 10.5 inch barrel that I run with a can. I note even on S it is over gassed. I am planning on taking corrective action. Installing an H2 and stronger recoil spring didn't help much.

Yes, I think the system is over gassed. Mine already came with an H2 buffer. The cyclic rate is much lower when I run the system with a suppressor. I can feel the return bolt carrier group taking its time getting back into battery.

We will see what LWRC has to say. With your comments, it would appear I am not the only one. Maybe that is why they stopped producing this model.
 
Doesn't look or sound like cookoffs to me. I bet the increased gas pressure is causing extraordinary BCG closing speeds that cause a slamfire. I would suspect the upper is grossly overgassed, or there is a defect with the bolt carrier or firing pin.

So Zak, you are the winner, kinda. Nonetheless, the PROBLEM HAS BEEN FOUND!

The problem = AMMUNITION. Get this. I was using reloads to test the rifle. I was using Federal #205 Small Rifle Primers. It is my belief that these primers are too soft for my application. So when the bolt carrier group slammed forward, the free floating firing pin (the LWRC design does not have a firing pin return spring) would slid forward and strike the primer. Thus the term, "slam fire".

Solution = I switched to PMC .223 ammunition and the problem stopped. I went back and forth between magazines, first with the reloads, then the PMC, then back to the reloads, then back to the PMC. Every time I used the PMC, the problem ceased.

So, in the end, it was the ammunition. Thank you Zak and everyone else for the assistance. Yes, I agree, round cook offs would have been very rare.