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Discussion Starter #1 (Edited)
Okay guys I need some help diagnosing a bizarre problem that occurred at the range today. This is an all German MP5K-N converted using a SP89 with factory MP5K-N barrel installed by the HK service center in GA, factory fullauto bolt group, 28 locking piece, HK marked pdw stock and a DLO frame with all German mp5 internals including a roller sear. Cans used were a SWR QD3K purchased in '03 with many many rounds through it (first baffle is completely filled with carbon along the periphery of the "cone" and the bore of the first baffle does have some stalactites but the diameter appears sufficient and I see no copper deposition here) and a SiCo Octane9 with very few rounds through it. Ammo was 115gr blaser and Speer brass case. I predominantly shoot 115gr brass case, usually Fiocchi, Speer or Blaser. Three different HK mags, two thirties and one fifteen. All appear to be in spec based on my eyeball.

I ran two mags, shooting a steel plate at about 50 yds on FA in controlled 2-3 round bursts using the qd3k. POA and POI were congruent. Next mag, I hand it to my friend. POI was about 3 feet left and 2 feet low. I shoot it and get the same results. Pull the qd3k, inspect it and discover the carbon fouling. No signs of baffle or end cap strikes, but some very slight copper deposition on the ID of the endcap' bore on my qd3k. So we swap to the Octane and get the same results. Pull the can and notice that the knurled thread protector is slightly loose. Shoot it unsuppressed and voila back on target, but now getting double feeds. Strip the gun and notice tons of carbon in the trunion (especially in the roller recesses....like a few thousands of an inch thick) on breech face and with the annular recess of where the barrel mates with the trunion. Trunion and barrel are intact, no movement. So I clean the trunion and remove all carbon. Shoot again without a can an we are on target, no malfs. Re-test with both cans and we are on target.

I don't have feeler gauges so I have yet to check bolt gap. That's my next step. We also did not isolate cases that were ejected when the change in POI first occurred.

What gives? I believe the malfs were caused by the crud in the trunion's bolt head and roller recesses. But could this cause a change in POI? Both cans have a slight amount of up/down play; very very slight. My qd3k 3-lug mount appears slightly worn where it locks up on the lugs. The 3-lug looks fine. Could the loose thread protector cause the can's bore to no longer be properly aligned with the barrel's bore? No obvious baffle or endcap strikes but there was some faint copper deposited at the bore of the endcap. So maybe the loose thread protector was along just enough misalignment to let the bullet "kiss" the bore of the baffles or endcap? I saw this exact problem with a change in POI occur with a friend's can that had a loose 3-lug mount.

Sorry for the long post and lack of photos. I appreciate any suggestions.
 

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You might want to strip the firearm and look down the barrel with the suppressors attached. Looking for concentricity, of course.
Check the target for keyholing, too.
You're probably right about the carbon causing the double feeds. Maybe it just didn't manifest until the backpressure from the suppressors went away.

Edit: I think the loose thread protector could be part of the problem (I don't have a three-lug myself though). The melting point of pipe tape is around 500 or 600 F; that could tighten it up but I don't know how hot your barrel gets.
 

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You might want to strip the firearm and look down the barrel with the suppressors attached. Looking for concentricity, of course.
Check the target for keyholing, too.
You're probably right about the carbon causing the double feeds. Maybe it just didn't manifest until the backpressure from the suppressors went away.

Edit: I think the loose thread protector could be part of the problem (I don't have a three-lug myself though). The melting point of pipe tape is around 500 or 600 F; that could tighten it up but I don't know how hot your barrel gets.
I wouldn't use pipe tape, it tends to make little shreds that get in the wrong place. I use high temp anti-seize grease on my end cap and really tighten the sucker down, it seems to stay just fine - I too have the factory MP5K-PDW barrel.
 

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Okay guys I need some help diagnosing a bizarre problem that occurred at the range today. This is an all German MP5K-N converted using a SP89 with factory MP5K-N barrel installed by the HK service center in GA, factory fullauto bolt group, 28 locking piece, HK marked pdw stock and a DLO frame with all German mp5 internals including a roller sear. Cans used were a SWR QD3K purchased in '03 with many many rounds through it (first baffle is completely filled with carbon along the periphery of the "cone" and the bore of the first baffle does have some stalactites but the diameter appears sufficient and I see no copper deposition here) and a SiCo Octane9 with very few rounds through it. Ammo was 115gr blaser and Speer brass case. I predominantly shoot 115gr brass case, usually Fiocchi, Speer or Blaser. Three different HK mags, two thirties and one fifteen. All appear to be in spec based on my eyeball.

I ran two mags, shooting a steel plate at about 50 yds on FA in controlled 2-3 round bursts using the qd3k. POA and POI were congruent. Next mag, I hand it to my friend. POI was about 3 feet left and 2 feet low. I shoot it and get the same results. Pull the qd3k, inspect it and discover the carbon fouling. No signs of baffle or end cap strikes, but some very slight copper deposition on the ID of the endcap' bore on my qd3k. So we swap to the Octane and get the same results. Pull the can and notice that the knurled thread protector is slightly loose. Shoot it unsuppressed and voila back on target, but now getting double feeds. Strip the gun and notice tons of carbon in the trunion (especially in the roller recesses....like a few thousands of an inch thick) on breech face and with the annular recess of where the barrel mates with the trunion. Trunion and barrel are intact, no movement. So I clean the trunion and remove all carbon. Shoot again without a can an we are on target, no malfs. Re-test with both cans and we are on target.

I don't have feeler gauges so I have yet to check bolt gap. That's my next step. We also did not isolate cases that were ejected when the change in POI first occurred.

What gives? I believe the malfs were caused by the crud in the trunion's bolt head and roller recesses. But could this cause a change in POI? Both cans have a slight amount of up/down play; very very slight. My qd3k 3-lug mount appears slightly worn where it locks up on the lugs. The 3-lug looks fine. Could the loose thread protector cause the can's bore to no longer be properly aligned with the barrel's bore? No obvious baffle or endcap strikes but there was some faint copper deposited at the bore of the endcap. So maybe the loose thread protector was along just enough misalignment to let the bullet "kiss" the bore of the baffles or endcap? I saw this exact problem with a change in POI occur with a friend's can that had a loose 3-lug mount.

Sorry for the long post and lack of photos. I appreciate any suggestions.
OP....I had an identical problem. What I found was the thread protector was the problem. If you remove it completely, shoot rounds....I think you'll see a substantial difference. OR...Locktite the thread protector.
 

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Discussion Starter #5 (Edited)
Thanks guys. I shot it again this past weekend using a SWR trident9 3-lug can and made certain the thread protector was tight. Issues with POI resolved. No feed/ejection problems when using blaser brass case 115gr. My friend shot some of his Speer lawmen 115gr and it had some of the same issues with FTE and double feeds. Swapped out for blaser brass case 115gr and no more issues. I believe the thread protector was the issue. My trusty QD3K can was also a likely culprit. Tons on carbon buildup internally and I suspect projectiles were kissing the bore of the baffles. That can must have a conservative 10k rounds on it. I shoot it on my K-N and fullsize UZI with 3-lugged barrel. So off to SiCo it will go...

This has me not really liking the Navy threaded/3-lugged muzzle. So much so that I've decided that the 94 I recently purchased (off this forum) will get a 3-lug only. It's already marked MP5-N, but functionally I don't want to deal with the thread protector coming loose.

The K-N will get some locktite on the thread protector.
 
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