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Discussion Starter · #1 ·
SCAR 17 suppressor concentricity pics

I have been wanting to understand more about the POI shifts I have seen on my SCAR 17 while using a can. Basically my Specwar 762 caused about a 1.5-2.0 MOA POI shift on the 16” barrel, and a 3.5-4.0 MOA POI shift on the 13” barrel.

I would have expected the shorter, stiffer barrel to have less POI shift, not more. POI shift can be caused by several things: weight on the barrel causing static droop, the extra mass changing barrel harmonics as it flexes, and gas flow changes as the bullet exits the muzzle/suppressor. The last one can be greatly influenced if you have poor concentricity, imagine a damaged crown on a rifle barrel.

So I purchased a precision ground rod to use for checking bore concentricity. Below are some pictures that you guys may find interesting. Not the best quality photos but I think they get the message across. Please ignore the messy basement. :)

Specwar 762 on 13” barrel, Trifecta flash hider, bottomed out directly onto the barrel shoulder without shims.



Specwar 762 on 16” barrel, Specwar muzzle brake, bottomed out directly onto the barrel shoulder without shims.



I was expecting to see problems on the 13” barrel but it actually looks really good, even better than the 16” barrel in fact. I guess the very large POI shift I’m seeing on the 13” barrel is predominantly due to changes in the barrel harmonics after all.

So what happens when you don’t tighten things down all the way and your ASR mount backs off a couple teeth? Not good. This is why any mount with known wobble problems (e.g. 51T) is off my list, this was probably only 1 degree of rotation and would make for a bad day. Make sure you keep those mounts tight!




Here are some more:
Saker 762 on 13” barrel, Trifecta flash hider, bottomed out directly onto the barrel shoulder without shims. All good! I also noticed that the Saker mount was much more tolerant of not being perfectly tight since it has a more gradual taper that prevents it from wobbling.



Saker 762 on 13” barrel, Saker muzzle brake, bottomed out directly onto the barrel shoulder without shims. All good!



But the muzzle brake wasn’t timed right in that last picture. So what if I got lazy and decided to try and bottom the brake out against barrel nut after all, even though everybody knows that is a recipe for disaster?





Yep, there it is. You just shot your can halfway down the range. Just another friendly reminder to never ever use the barrel nut with a suppressor.

So how do you time a brake on the 17 then? Well as Jeremy from Silencershop pointed out the Silencerco shims will actually work. Here are some additional photos showing that this works.

Saker 762 on 13” barrel, Saker muzzle brake, timed using 1 thick shim and 1 thin shim. All good!





Well that’s it. Hope you found this useful if you are as OCD about measuring things as I am! I will be testing the POI shift with the Saker 762 soon and will report back.
 

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Looks good and nice photos too one question though, where did you obtain the precision ground rod? I suffer OCD as well and will need one for my soon to be suppressed 16's w/14" bbl. Thanks... :?
 

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For those reading this and interested in yet, wondering where to obtain a precision ground rod as Brett did, look no further I found 'em for both 7.62 AND 5.56.

Our friends at Geissele Automatics actually make them specifically for checking bore concentricity w/suppressors and they call them oddly enough...."Suppressor Alignment Rods". At $75 a pop they ain't cheap however the misfortune of a barrel bore-to-suppressor bore misalignment and its insuent potentially destructive bullet-in'ya-baffle strike is even more expensive. Git'ya one, maybe even two! :?
 

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Discussion Starter · #6 · (Edited)
Yes Geissele makes alignment rods that are ready to go. I thought that $75 was pretty steep so here is what I did instead. I ordered several different diameters of 3 foot length precision ground rod from McMaster:

McMaster-Carr

Of the ones I tried, the 0.2969” diameter, (part number 8893k219) seemed to fit the best:

McMaster-Carr

It’s only $5.45 + shipping however I did have to modify it a little bit to work. Basically you have to carefully cut off about .25” from each end without deforming the rod. I just did it gently with a hack saw. McMaster had sheer cut the rod to length so the ends were deformed. Don’t clamp the center of the rod in a vice as it will deform it, only clamp the ends that you will be cutting off and discarding.

After I cut off the ends I spun the rod in a drill and used a file to fillet the ends. I then use 800 grit sand paper while still spinning the rod in my drill to give it a final polish. Since this rod is hardened steel you need to be very careful that it has no burs or deformity before feeding it down your barrel.

Overall it was pretty easy to do though and works perfectly. There is a tiny amount of free play with the rod in the barrel but one size larger was too big and would not fit without me spending some time turning it down some on a lathe so I stayed with the .2969” diameter and it worked great.

So if you are handy you can do this yourself and save quite a bit of money. I haven’t tried one for .223 yet bit I would imagine there is a size for that caliber that works just as well.
 

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Nice post!!!
 

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Awesome post!

I had been thinking of picking up the Geissele rods, but the price was killing me. I haven't shot my SCAR / Saker combination yet and had just been eyeballing concentricity. I saw another site offering alignment rods for about half price, but this seems to be the best idea. Plus, McMaster-Carr is "right down the street" and I can pick it up today.

I do have a need for a .223 rod. Any idea on diameter or a method to determine the best fitting rod?

Thanks,
-Drew

Edit: Brownell's shows "SPECS: 5.56mm 0.2175" O.D." for the Geissele rod
 

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Discussion Starter · #10 ·
I haven't tried a .223 Rem rod yet but McMaster has 3 sizes that look like they could be in the right range: .2120", .2188", and .2190". Since they are only ~$5 each you could buy all 3 and see what fits best. Based on the clearances that were needed for the .308 rod to fit well, and seeing those Brownells specs for the Geissele .223 rod (and since a .223 Rem bullet is really .224 diameter), I would bet the .2188" McMaster rod would be a good fit.
 

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I haven't tried a .223 Rem rod yet but McMaster has 3 sizes that look like they could be in the right range: .2120", .2188", and .2190". Since they are only ~$5 each you could buy all 3 and see what fits best. Based on the clearances that were needed for the .308 rod to fit well, and seeing those Brownells specs for the Geissele .223 rod (and since a .223 Rem bullet is really .224 diameter), I would bet the .2188" McMaster rod would be a good fit.
Yeah. I just got back from McMaster-Carr and picked up the .308 rod you recommended and the .2120" and .2188" rods (along with some Sparrow o-rings - I love that place).

Unfortunately, it's 'honey-do' time and I have to go mount some drapes so rods and cans will have to wait....

Thanks again for your info.

-Drew
 

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Discussion Starter · #12 ·
Anytime. Make sure you chop off and clean up the ends of the rods first. If you look at them closely I'm sure you will be able to see where they are slightly deformed from the McMaster cuts.
 

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This has been a whole afternoon of fail.

Curtains the wife wanted were out of stock and had to be ordered, so back to guns.

.2120 is the correct rod for .223. .2188 did not fit (and of course was the first one I cut and cleaned up....). Then I did the .2120 and it fit fine. I figured I would check it on an AR first as the barrels are readily available and relatively cheap if I was completely stupid and left a burr.

Rod was fine. It indicated that my AAC M4-2000 / Brakeout 2 are mounted properly:

Optical instrument Photography Monocular Metal


Then I checked the Saker 7.62 (w/ 5.56 end cap)/ Trifecta that's mounted on a Tavor. Again, adequate results:

Rim Automotive wheel system Wheel Auto part Metal


Then I cut and cleaned the .308 rod (what was the size? .2969?).
...and the Saker got stuck on the damn Tavor! :icon_eh::th_anim_43::th_cry:

I haven't had it on there in a while and I guess I twisted it on too hard. A heat gun only resulted in me burning the hell out of my hands when the rag slipped. At least I didn't melt the Tavor. I'll have to shoot it off tomorrow. The SCAR / Saker (which was the main priority of this exercise) will have to wait a bit.

I guess it wasn't a completely failed afternoon. At least I now have piece of mind that my 5.56 muzzle devices are mounted properly.

-Drew
 

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A quick check of the specs for the Geissele Suppressor Alignment Rods sold at Brownells indicates; 5.56 = 0.2175" and 7.62 = .2969". The McMaster-Carr rod for 7.62 is spot on, even though we're splitting hairs -literally almost- for the 5.56, it doesn't take much for a costly baffle strike. FYI
 
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