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Discussion Starter #1
Gentleman,
Has anyone had accuracy issues with their Gemtech UMP suppressors?
I zeroed my EOTECH red dot sight with a bore laser at 25' and got a good grouping on target with the UMP/USC unsuppressed. I installed the suppressor and the shot pattern became wildly erractic with as much as 5-8" between where I was aiming and where the bullet hit the target. I removed the suppressor and got a very tight grouping again. Any Thoughts?
 

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First off, we need to define precision (hits in close proximity to each other) vs. accuracy (hitting what you're aiming at).

If the precision is fine with the suppressor mounted, but accuracy is off, that's considered normal. The suppressor changes the point of impact of the rounds due to a different recoil impluse plus from the added weight on the muzzle. This usually results in rounds hitting LOW compared to where they would unsuppressed. I've noticed this with all of my suppressed firearms including larger rifle-caliber guns.

If precision is off AND accuracy is poor, then you may have an alignment issue with the can which is allowing for baffle strikes, or movement of the can between shots. This allows either high pressure gases behind the bullet to reflect off of the baffles and impact accuracy, or the bullet is actually striking the baffle causing it to fly in an unrepeatable patter, resulting in poor accuracy. If this is the case, the can needs to go back to Gemtech, with the rifle, most likely, to get everything aligned. You would notice baffle strikes as dings in the exit opening of the suppressor and possibly also 'ringing' of the suppressor tube where a baffle has been struck, displaced from the stack, and forced outward against the suppressor tube.

Good luck, and let us know what you find.
 

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Merchant of Death (Admin)
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Excellent advice! You might also try a low tech trick to see if the can moves between shots. Use some masking tape to make an index line on the can that matches a similar line on the barrel end. If the lines separate between shots, you'll at least know part of your problem.
 

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Factory barrel?

Do you have a factory barrel? I have an AAC UMP Defender suppressor. On the factory UMP barrel that I have on my USC conversion it does not fit well. It acts like it snaps on, but does not stay on if you give it a good pull. On the other hand I have a Shockwave UMP flange on a .45 UZI barrel that snaps on rock solid. (I did have to send the Shockwave back for a little rework. I think he had it at factory specs, so it did not fit. After the rework it was perfect.) So, my thought is if you have a factory barrel it may not be locking up tight and that effects your accuracy. I did think it might be the AAC can, but someone else had posted that their can that was not an AAC was not snapping on the factory barrel. Hope this helps.

Brad
 

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Discussion Starter #5
Thanks

Gentlemen,
All your posts have great advice, Thanks!

Long Duck, right now I would say I am having BOTH a precision and accuracy problem. So I will check if the can is moving between shots using straightgrain's tape trick. I'll report back on the whether I see any marking or dings on the can itself.
Thanks again for your help!
 

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Merchant of Death (Admin)
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Please let us know what you find out. Suppressors are necessarily magic things, no matter who makes them. There are all kinds of things that can affect performance in a suppressed system, no matter who makes it and no matter how stellar the reputation. A changed point of impact is almost to be expected.

Other issues come up because the manufacturer of the suppressor does not have the ability to completely control the rest of the system and must make best guess accommodations. This is not a bad thing! It just means that there is likely to always be some fiddling needed. Get that thing running like a top and have fun!
 

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Personally, I think your mount is loose and allowing the can to move from shot to shot.

Suppressors tend to increase accuracy potential due to better control of expansion gases at the muzzle at the instant of firing, so any loss of accuracy is a sign that the mounting interface has some problem.

I doubt that the 'tape trick' will show you much with this mount due to the snap on design with the UMP barrel, but you can try it.

Also, are you using an HK Factory UMP barrel, or a Shockwave attachment? Good luck,
 

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Discussion Starter #8
LongDuck,
I am using a 7.87" HDPS barrel om my UMP/USC conversion. After your post yesterday, I went and checked the weapon and did notice that I can physically move the mounted suppressor slightly. This "play" in the mounted suppressor leads to believe that at the point of discharge the recoil can cause the can to wobble slightly but enough to affect accuracy AND precision. Any suggestions on a next step? Thanks again for your insights on this matter!
 

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I am using a factory barrel and Gemtech UMP can and have found little to no change in point of impact at close range 7-15 feet. I would look at your barrel mount. I had a similar issue with a three lug adaptor. the can did not have a good fit to the barrel because the three lug was out of spec. I recently bought a TROS 9mm AR three lug barrel hoping to fix the problem.
 

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I think your only option is to contact HDPS about their barrel. Would be convenient if you had access to a factory HK UMP barrel to test fit the suppressor, which would rule out Gemtech's mount as being part of the problem.

No mounted suppressor should have any hand-felt play in it, since the pressure on firing will play havoc with the interface. In fact, I'd go so far as to recommend that you stop shooting it until you get this figured out. With repeated firings, any play now will only increase as the can hammers itself against the mount. The more motion you develop, the greater the hammering effect, and the worse and worse your problem will get.

Short term, you could also try some shim washer material between the flat shoulder of the barrel that the can sits against. What you want is a snap on installation that leaves no movement, but is firmly locked in position. Only metal shim material will work, since rubber washers will still allow too much flex.

Again, good luck, and it sounds like you're getting to the bottom of it,
 

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Hey Gemini did you ever figure out what is up with your UMP and the suppressor? It has been a while so I figured I would ask.
 
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