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Discussion Starter #1
Hello all:

Is there a known accuracy issue with Suppressed SBR H&K UMP's beyond 15 yards? I know two other people besides myself who are having significant problems with 7.8 inch HDPS M16 x 1 left hand threaded barrels and AAC Tirant 45 cans with the fixed barrel spacer.

We have tried six different commercial ammo types, using iron sights vs. Red dot/Eotech sights, supported shooting positions, cleaning and remounting the suppressor, cleaning the barrel, but the only thing that makes a difference is the presence of the suppressor.

The ammunition used was all factory new (no reloads) including Federal American Eagle, Federal Champion, Blazer Brass, American Eagle Subsonic, and Winchester White Box. All were 230 grains with clean paper hits and no keyholing. Typical 25 yard group sizes were 1.5-2 inch without the suppressor, 12-13 inches with the suppressor. Shooting at 15 yards or less produced tight groups with or without the can, but at 20 yards the groups opened up to 8 inches with the can. Without the can, 20 yard groups were less than 2 inches.

On my can the distal three baffles had light strikes. The other guys cans had light baffle hits as well. Mine was sent back to AAC, and they replaced the internals (including the "piston" which threads onto the barrel and a new fixed barrel spacer). This did NOT solve the problem and the inaccurate groups continued. Trying different cans on different guns did not help either.

All shooters have 25+ years experience, most current/former police.

Any insight or advice appreciated.
 

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Sounds like you have something Loose. you shouldn't see that type of grouping .
I used my Tirant 45 tends to get loose . I know ! I sent that can in 4 times in 3 years time .
You said you used different can and received the same results . I'd say check the barrel
 

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I've been thinking of one of those barrels for a USC to UMP conversion to use with an Octane 45 or Hybrid (still in NFA jail). This has me somewhat concerned about the choice. At least barrels are easy to swap.
 

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Discussion Starter #5
I thought the interface between the barrel and the can might be the source of the problem. The barrel itself is very accurate. I should have mentioned that the can is accurate when it is on a Glock 21. So each part is capable of accuracy, just not when used together. One of us knows a machinist who said he could take a look at the threads.
 

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I thought the interface between the barrel and the can might be the source of the problem. The barrel itself is very accurate. I should have mentioned that the can is accurate when it is on a Glock 21. So each part is capable of accuracy, just not when used together. One of us knows a machinist who said he could take a look at the threads.
I am no expert here, but I would definitely have the Threading checked. If you are even finding copper filings inside your suppressor, then most likely the bullet is barely making contact with the baffles. I am Not sure anyone currently makes a rod in 45, but a good smith will be able to diagnose the issue.

Please report back so we all have the knowledge of what the issue was in case we run into the same problem!! Hope is is something simple for you.
 

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Just to be clear - Is this built from a USC, or is it an all factory HK gun with a factory barrel?

If you are getting baffle contact, you have either an alignment issue with the can, the barrel, the spacer (or the tolerance stacking of all three) or a bullet stabilization problem.
 

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Discussion Starter #8
All three are USC to UMP conversion with HDPS 7.8" M16 x1 left handed threaded barrel and AAC Tirant 45 cans.
I think the bullets are stabilizing fine as 1) we have never seen key holing with any of them and 2) accuracy/group size without the can is very good.
We are definitely getting baffle strikes.
 

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All three are USC to UMP conversion with HDPS 7.8" M16 x1 left handed threaded barrel and AAC Tirant 45 cans.
I think the bullets are stabilizing fine as 1) we have never seen key holing with any of them and 2) accuracy/group size without the can is very good.
We are definitely getting baffle strikes.
I don't have a TiRant but rather a Griffin Revo .45 with the same barrels. I noticed some keyholes at 5 yards but luckily no noticeable baffle strikes. I was using the full length and not the K config. Glad you brought this up because I thought it was maybe my configuration. I'm subscribing to this thread as well.
 

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mihkmihk,

As others have suggested, it sounds like there's something amiss with the threading.

Does your AAC piston (with fixed barrel spacer) bottom out when threaded onto your HDPS M16x1LH barrel? Even if the threads are concentric, this interface could still pose a problem.

For metric threading, the suppressor indexes on the face of the muzzle, not the shoulder of the barrel -- so there should be a noticeable gap between piston and shoulder when the suppressor is screwed on all the way. I have three OEM HK USC45 barrels in UMP conversions that have been cut, threaded, and crowned by ADCO to M16x1LH threading (AAC specs) as diagrammed on page 8 here: AAC Thread Guide. All three barrels are accurate with or without suppressors attached (AAC Ti-RANT 45M and SilencerCo Octane 45K). Same tight groups, no keyholing, no baffle strikes.
 

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Does it have a relief cut on the final thread area? My guess is it probably does not so the suppressor never properly shoulders the barrel. You can try a machine type or supppressor washer to test if this works. But, if the area tapers up in diameter the washer will be stopped short as well. I have gotten very particular about the final thread area having a good relief cut and no taper going to the shoulder of the barrel.
 

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Discussion Starter #12 (Edited)
MudPhud: Yes, the AAC piston with Fixed barrel spacer is bottoming out, with no gap. And the appearance of the end of the barrel and threads is not the same as the AAC Thread Guide diagram you linked to.
The other thing I noticed is that the can screws together differently when using the fixed barrel spacer. The part of the can that holds the piston (which AAC calls the ASAP Housing) has a gap with the main tube of the can. If I take out the Fixed Barrel Spacer and put in the spring, there is no gap when screwing together the main tube and the ASAP Housing. Not sure if that is by design.
 

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Discussion Starter #13
4C Dragon: if the relief cut is that Detail B in the AAC Thread diagram link, then No it does not have that.
 

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4C Dragon: if the relief cut is that Detail B in the AAC Thread diagram link, then No it does not have that.
Been 6 days since your last post. Any resolution?
 

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Discussion Starter #15
I have bought a properly sized Viton O ring on the advice of the guys at ADCO, put it in, and put the can back on. This goes inside the piston of the can, not outside on the shoulder of the barrel. If it works, it is a 30 cent solution. Unfortunately, I've not had the time to test it. I will post an update when I test it.
 

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Tell Tom to stop taking his guns apart ;-)
 

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Discussion Starter #17
I placed one Viton O ring inside the piston shoulder (not the barrel shoulder). It still screwed down against the barrel shoulder. It produced a lousy 10 inch group at 25 yards (shooting seated and supported). I added a second O ring and tried again. This time the piston of the AAC can was not seating against the barrel shoulder (I am told that that is how it is supposed to be) but still shot a lousy 11 inch group.
 

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Discussion Starter #18
I placed one Viton O ring inside the piston shoulder (not the barrel shoulder). It still screwed down against the barrel shoulder. It produced a lousy 10 inch group at 25 yards (shooting seated and supported). I added a second O ring and tried again. This time the piston of the AAC can was not seating against the barrel shoulder (I am told that that is how it is supposed to be) but still shot a lousy 11 inch group.
I should note that then I removed the can and shot a 1.5 inch group at 25 yards.
 

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Discussion Starter #19
Update:my HK barrel came back from ADCO, and now with the can is is doing 2 inch 25 yard groups. Problem solved. So my conclusion is that HDPS barrels and AAC cans don't necessarily play well together.
 

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Awesome, so glad it all ultimately worked out for you. Did you acquire a 16" HK USC barrel and then have it cut with AAC spec'ed threading? I've been really impressed with ADCO's work for me. Perhaps your two friends with similar issues can duplicate what you did -- and then the fun really begins!
 
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