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Posted: 9/13/2009 11:30:40 AM EDT
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Posted this in Precision Rifles, but this is a specific AR problem. So:
Here's the Rifle: Krieger Barrel, DMR trigger 
The following are all 4 round groups: 69gr BH, 100 yds, WITH Smith Vortex FH: 
Same ammo, FH removed: 
77gr BH 100yds, WITH Smith Vortex FH: 
Same ammo, FH removed: 
According to the factory, the Vortex should be no more than finger tight. I have been using it exactly that way; also tried it with 10lbs torque. I have used Vortexes in the past without anything like these results. Is mine damaged? What FH would you try next? |
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We ran some tests a while back using a Vortex, Blackout, and QCB on a quality barrel with quality ammo. The QCB performed better, but not as dramatic a differential as you are presenting.
The gist of it is that muzzle devices can most certainly influence precision. Your target type changed for one of the strings. I am not suggesting that made all the difference, but please remember how important variable isolation is for further testing. Thanks for the data. 
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There was nothing obvious in terms of strikes. I will check again.
since I and many others have had good results with Vortex, I am thinking one of the tynes got knocked out of line, or something? Not sure if that would cause what I was seeing. The difference in the targets is noted, and in the future I will try to compare apples to apples; of course, in this case we went from 2.4in to 0.45in, far more than the target itself could account for. |
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Interesting results - everything I've ever read on the Smith Vortex Flash Hiders is that they "increase" accuracy? Can you post a close up pick of the Vortex from the front and the back? I'm wondering - as you suggested, if one of the prongs is off alignment compared to the others? Or, is there some excessive fowling/carbon build up on one side that's affecting the gas flow and causing irregular turbulence when the bullet exits the muzzle?
Sakic #19 
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Barrel harmonics can change when different torques values are used when installing a FH. I had a 16" carbine barrel with a standard GI FH that I had replaced with a XM177 Slipover FH. I torqued it on (no measurement, just torqued until it felt solid).
Prior to the Slipover the barrel groups were so so. After I put on the slipover, it shot amazingly tight groups. I never took that slipover off because I was afraid I would never find that "sweet spot" of torque again. Try torquing it at different amounts, if that still does not fix it, ditch it and try another FH. |
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1. No flash hider can improve accuracy by design.
2. Adding any weight to your barel can casue a random alteration of accuracy based on barrel harmonics. You can just as easily get better groups as worse when adding a Vortex. Would like to know how much your torqued it on if at all. Excessive torque on the threads is bad. |
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What is your thread spec? Is it .630" long?
That's a strange problem to have. I've had good results with them. Maybe you should check eccentricity of your thread. If it is eccentric .005" or .008" there is a tight free bore that pressure moving through could be causing negative impact on your shot group. It might have a tendency to want to throw or destabilize rounds if very eccentric at all. My LMT 10.5 was just measured by ADCO at .0095" out of concentric. It was trash for silencer use. |
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Quoted:
I have never had trouble with other vortex setups, including a different Krieger and a Noveske. I took this particular Vortex in to my dealer and we measured the gaps between tynes; there was significant variance. Can't recall the numbers. Oddly tho, this particular Vortex worked fine on another barrel. I will try a PWS556 today to see what effect that has. For the difference with/without please see my other thread. Look for bullet rub marks on that .260" bore area. Lets see if these longer bullets might be slightly unstable as they exit the muzzle and stabilize later downrange. That would be an easy fix. ream to some larger dimension .273 7mm or .281 9/32 Whatever is happening most likely can be keyed directly to that little freebore .260ID by .160 OAL. Could even be gas and not even a rub, but the bullet flight is only able to be altered by this one feature. By the time significant gas has entered the flash hider the bullet is in air. as a silencer manufacturer I have a lot of experience with bullets flying through bores. The .350 tine bore just isn't going to have any measured effect whatsoever. Certainly not like you've posted here in your pictures. Only other thing I could think of is something to do with harmonics of the barrel experiencing wave addition with the occilation of the tines. That's a little out there too, that's pretty unlikely. |
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I think you're going to find the longer bullets exit slightly unstable and touch that .260 bore.
The A2 shouldn't need to be reamed. A company called micor ? on brownells sells an M4QD clone mount and that's a CNC'd FH with phantom/A2ized slots it should be a little better than A2 probably. It will have a concentric bore and that will be around .310" so it should be bigger than Smith. |
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