Concentricity Check


July 23, 2013, 06:13 PM
Finally getting around to swapping out all my crush washers on my A2 flash hiders so I can use my Halo on any of my 5.56 or 5.45 ARs.

Ran across one out of spec flash hider so I thought I show a couple of photos of a failed concentricity check, and what an acceptable one is after replacing the flash hider with one that is "in spec".

First the failure:

Note the contact between the opening and the drill rod. The rifle is propped straight up.

Here's a different angle:

Now after using a different A2 flash hider, this one while not "perfect" is acceptable. I wouldn't be surprised if this shows more POA/POI shift suppressed vs. unsuppressed than I'd like, but I actually have a 20" RRA that is a bit worse and any shift is hard to tell with a couple of 10 shot strings.

Again from a different angle:

The "drill rod" concentricity check is recommend in my HVT-QT manual. McMaster-Carr is a good on-line source for small quantity lengths of standard drill rod sizes.

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July 26, 2013, 09:39 AM
question: did you visually check your out-of-spec flashhider, with the can installed, by looking through the barrel from the chamber end?

July 26, 2013, 09:57 AM
Doesn't really show anything as the hole in the flash hider is a lot larger than the hole in the end cap. However with the flash hider off, from the rear its pretty easy to see the threads are not centered in the body -- just measured asymmetric wall thickness 180 degrees apart on the rear of the flash hider as 0.196" and 0.215". A good one measured in the same way is less than 0.002" difference

Since the Halo mounts on the flash hider, if its not plumb and centered, baffle strikes are possible.

The drill rod test is a good thing to do as its easy, and pretty much a final system check that will detect misalignment no matter the source.

Setting up peel washers is a PITA, I can see why everyone uses crush washers to index the flash hider these days.

July 26, 2013, 10:10 AM
If the end cap opening was that far off alignment with the bore, it would be easily visible from the chamber end. Size of the fh flare has no bearing when your looking through the barrel with the suppressor attached. You won't be able to see the FH at all but the misalignment of the end cap with the bore will be obvious, if its as far off as you've indicated.

July 26, 2013, 10:23 AM
If you can see ~0.020" difference looking over 24" away you obviously have a lot better vision than most!

If you want to trust your eyeball instead of making a simple test, its your can, if looks good shoot it and find out!

July 26, 2013, 11:32 AM
And yes, you can see that distinction. You should always only see the barrel and a circle of light at the end of it. If there is a football shaped light or anything other than a circle, then there is a problem. Certainly this check can be done on rifles like ARs and bolt actions where a visual check is possible from the chamber end by removing the bolt completely and the BCG. I am saying I would do this check first, prior to dragging a steel rod over my crown and down my barrel. Wood or plastic would be better, IMO.

But this is why I asked if you visually checked before sliding a metal rod down the muzzle. I was curious if it was noticeable but from your response, I assume you did not look.

July 26, 2013, 11:49 AM
Wood or plastic would be better,

Never gonna be straight and stiff enough. You are pushing things out the barrel at ~3000 fps and you are worried about a steel rod touching anything? It's not like drill rod has a rough surface, and its easy enough to "break the edges" if your rod end is not chamfered.

I looked and that is why I got the drill rod, I was confident enough for pistols but not for rifles -- can always see a circle or an ellipse depending on the exact eye alignment, hard to tell which to believe. As I said, its your can, trust your eyes if that satisfies you.

I prefer giving my old eyes all the help they can get!

July 26, 2013, 12:14 PM
I looked and that is why I got the drill rod

this is the answer i was looking for with my first post. Thank you for answering it.

and as to the drill rod, it is a much different material than a brass jacketed bullet flying down the barrel. and yes, i would be more worried about putting a piece of steel in the bore of my rifle, than shooting bullets. I am sure it CAN be done a limited amount of times fairly harmlessly, if you take the appropriate precautions, but I am personally not partial to it.

HDPE can be had is short pieces that are just as straight as steel.

July 26, 2013, 01:05 PM
It looks like McMaster-Carr sells annealed drill rods in very small size increments. Does anyone have a table of sizes by caliber (or a table of land diameters by caliber and some guidance on the amount of clearance needed)?

Edit: I ordered a .4375 rod for .45ACP (typical land dia.=.4421 --> 46 mil clearance)
and a .3860 rod for .40/10mm (typical land dia.=.3902 --> 42 mil clearance)
but a general guide would still be useful.


July 26, 2013, 10:33 PM
I only have .45ACP pistols and with only a 5" barrel and since both pistols came threaded from the factory (FN and SIG) I was comfortable with a visual only check, but this got me looking for something a bit more "objective" for after-market barrels or intermediate adapters like the HVT-QD or the HALO-A2 flash hider system. As I said, the drill rod idea was suggested in my HVT-QD owner's manual.

I was most leery of putting the HVT-QD on my Draco-C SBR as I'd heard lots of stories about bad concentricity in AKs, mine ended up as close to "perfect" as I've seen, but it still has the most POA/POI shift of anything I've tried. Barrel harmonics are a complicated beast.

I'm pretty sure my drill rod checks have prevented at least one potential baffle strike shown in the nogo photos.

Standard rod sizes worked well for 5.56 and .308, but I had to turn down a slightly over-sized rod to fit 9mm, as the next smaller standard size seemed too sloppy.

I used my shop drill set to test the shanks in the bore to find what size rod to order for a good fit.

HDPE can be had is short pieces that are just as straight as steel.

Got a source that sells small quantities? with tolerance specs? McMaster-Carr lists straightness and diameter tolerances for their drill rods.

I actually want piece a bit longer than suppressor length + barrel length to minimize the potential error from the diameter not being tight in the lands. I believe in instrumentation/measurement it is referred to as "sine error" or "Abbe error" where the angular misalignment in fitting the bore (two contact points) is magnified when extrapolated out the end of the suppressor.

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