Good evening, SF. As usual, I’ve got questions.

Last weekend I got to shoot an AR with SIG’s SRD762QD attached. At short range (15 yards or so), nothing spectacular happened (aside from it wasn’t as quiet as we thought it would be).

At about 40 yards, the holes on target were elongated, left-to-right, and skewed leftwad. At 85 yards, holes were small and round again, but way left of center.

Left-right issues might have been scope issues; that’s relatively easily correctable. But the elongated holes are worrisome.

Later, we removed the upper and slipped a new .290” diameter arrow shaft down the bore, from the muzzle end. Looking at the exposed end, it wasn’t perfectly centered in the end of the suppressor.

There’s no evidence of impact at the end of the suppressor, so that’s a good thing. But I’m curious now how the suppressor gets non-concentric with the barrel bore. Does that have to do with the quick-disconnect mount?

We can’t find the suppressor manual at the moment, and it’s not on SIG’s site for download, so we’re a little pinched for information.

Could the non-concentric condition be a contributor to the leftward skew and the tumbling? Or is there something else going on that I haven’t learned about yet?

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ETA specifics: 300 Blackout, 1:8 twist barrel, 200-gr subsonic (Winchester Super Suppressed) hollow points.

First off, I’m an idiot. Second off, I’m new to centerfire suppressors. That being said the first thing I would check is the muzzle device. Everything I’ve read says don’t use crush washers and nearly every rifle I own uses, wait for it, crush washers. So now I’m replacing them all with new MD’s sans crush washers. Other than that, no idea.

I don't get how a .290 shaft went down a .223 bore? I always check the alignment with a dedicated alignment rod and they never are exactly centered. And it would have to be a massive problem to have a .223 projectile affected by a bore that exceeds .35 or .36.
One issue not typically discussed is were you putting any pressure on the suppressor like resting it on a bag or barricade etc.

quote:

Originally posted by hrcjon:
I don't get how a .290 shaft went down a .223 bore?

I’m a newbie to this, too, so there’s a ton I don’t know.
But 300 Blackout is a .300 bore, not .223, right?

I did have the rifle on a bag, but it was the handguard resting on the bag. The barrel and suppressor were “hangin’ out there in the breeze.”

That would be me 'assuming' incorrectly. When someone says AR without any qualifications I think .223.

subs or supers? twist and bullet and barrel. Do it with different ammo? I don't fully follow oblong, but that means you think its tumbling?
Does it do anything without the suppressor?
What's the mount you are using?
Thanks for a bit more info.

Subs. 1:8 twist, 16” barrel.

The mount is SIG’s QD mount that came with the suppressor.

Oblong: the holes are ovals, where the long axis is left-right. I’ll try to get a pic posted in the morning.

I haven’t tried different ammo yet.

I don’t know what the rifle does without the suppressor (yet). Also from the “I’m a newbie” file: I can’t get the darned quick-disconnect to release. I’ll work on that some more tomorrow as well.

Thanks for trying to help!

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ETA: 40-yard Pic

This message has been edited. Last edited by: vthoky,

Interesting about the elongated holes at 40 yards. Two thoughts:

It’s not necessary for the misalignment to be great enough to cause an obvious baffle or end cap strike. The threading of one custom Thompson/Center Contender barrel I have was misaligned just enough with the suppressor that the bullet grazed the end cap hole. I could tell only by a small wipe mark in the carbon firing residues around the edge of the hole. That was obviously enough to cause significant stability and precision problems. I later eyeballed the hole in the suppressor with the barrel bore and it was obviously off slightly. (Did not have an alignment rod then.) I had the barrel threading redone by a different suppressor manufacturer.

But FWIW, I would expect quick detach mounts to be more susceptible to misalignments than direct threads (until the latter become loose as happened to me once ).

One thing I learned from reading Bryan Litz’s books is that often bullets are less stable in early flight and then settle down, or “go to sleep” farther along. I don’t know at what distance(s) we might see some instability (excessive yawing), but at 40 yards? Perhaps. The test would be to shoot at the same distance without the can. It’s also why when using a new heavy bullet load, I first test it at very close range to see if it shows any signs of instability before shooting it through my suppressor.

Proper alignment rods are available for most calibers, even from Amazon, and I recommend them as cheap insurance that we’re not going to have any expensive problems.

This is my mantra anytinme someone has a single setup that's odd. let's shoot some other conditions and gather some diagnostics. Is this factory ammo? Shoot it unsuppressed. Shoot some other factory ammo including both supressed and not. Confirm concentricty with a real alignment rod.

Suppressor and mount are now removed; we'll get some of the same and some different ammo through it next week for comparison.

Alignment rods -- stainless? Carbon fiber? Heck, I've even seen titanium advertised. I'm leaning toward carbon fiber, unless you guys tell me that's a bad idea.

There are budget ways to go on the alignment rods, carbon fiber arrow shafts and metal rods from Home Depot or McMaster-Carr. I think there are some part numbers on the internet and you could likely get it down to $5 per caliber. I just hit the easy button on the Strike Industries carbon fiber set. I needed 3 calibers and the set comes with 6 different calibers for $75. It works well and gave me some piece of mind as I built my 5.56 and .300blk hosts up from stripped lowers.

quote:

Originally posted by ibanda:
I needed 3 calibers and the set comes with 6 different calibers for $75.

Can I ask where you found that bargain? I can only find that set for $99 or more.

I can say for sure the rods from McMaster are inadequate, too much run out.
I use rods designed for this application, mostly mine came from Geissle. And they have worked great. I have no experience with carbon ones which look quite economic, but I would note the runout is 3x that of the Geissle ones. Matters or not I have no idea.

I emailed SIG last night, asking for a copy of the manual for the suppressor (RTFM, right?); received the download link this evening.

Good grief. Please renew my “I’m a doo-mahs” card.

It appears we had the crush washer in the wrong place.

We’ve got it all apart now, ready to be cleaned, and I poked the “buy it button” on an alignment rod from Amazon.
In a couple of days, we’ll have it all back together and see how much it’s improved. Heck, maybe it’ll even be a bit quieter then, too, right?

Why is there a 'crush washer' involved in this discussion at all. Simply not applicable in suppressor mounting. you use shims, if needed.

According to SIG’s manual (P/N 8501925-01 REV00), It goes between the QD base and the muzzle brake.

When I took this all apart, there was a washer in between the QD base and the barrel.

quote:

Originally posted by vthoky: Can I ask where you found that bargain? I can only find that set for $99 or more.

Caught a sale at Midway USA, but I see it's at $99 now. Sounds like you are solving the alignment riddle and you caught everything before you had a baffle strike.

That's a bizarre QD system on the SIG. I couldn't understand what you were talking about in regards to the mount and crush washer. I downloaded the manual, you mount a QD device to the barrel, then a washer facing the correct way and then, the muzzle brake. It does help to read the manual I guess! That's certainly proprietary SIG QD system, and I don't know that I would have gotten that right the first time either.

I may have spent way too much time overnight thinking about this.

Admitting I don’t know enough yet, I stewed for a while over the idea that the muzzle brake stays on when the suppressor is in use. Is that weird?

I would have thought it was an either/or thing. But again, I don’t know enough yet.

Its a learning curve. I always try and think metric for bullet size , so I don't put a 7.62 down a 5.56 barrel.
I don't think there's a problem with the bore and suppressor alignment. I always tend to "boresight" a suppressor when attached. A rod helps but a peep down the barrel is quick and easy.
Your 300 BO appears to be tumbling or keyholing. You stated Winchester subs-200 gn--barrel length needed. I would re try with reg 300 BO to see groups and tumbling.

Seeing the actual photos of the bullet holes plus the fact that they are elongated like that at 40 yards but not at 15 or 85 yards makes me strongly believe that the bullets are just experiencing a large degree of yaw due to a too-slow barrel rifling twist rate, not because of anything due to the suppressor. The bullets may start fine, then start yawing more than normal, but then “settle down” at longer distances.*

The proof, though, will be when you shoot without the suppressor.

The short barrel 300 Blackout I have has a 1/6" twist rate and does fine with subsonic loads. Another has a 16 inch barrel with 1/8" twist and I haven’t noticed any problems, but its longer barrel might do a better job of stabilizing heavy subsonic bullets.

Another example of bullets with excessive yawing due to inadequate stabilization were these. They were fired from a 308 Winchester subsonic load by Beck ammunition, and it was probably dues to their low velocity and a 1/10" barrel.*





* FWIW, the bullet holes pictured above were not caused by bullets that were tumbling, just yawing excessively. Below are examples of impacts from bullets that were actually tumbling. Just two examples are circled, but I no longer recall why I did that. The other large rectangular holes were also from tumbling bullets.





Bullets that were actually tumbling would probably not even hit a desired target at 85 yards.

* I decided to check rather than relying on memory. The above groups were fired with a newer Tikka that has the 1/10" barrel rather than an older TAC A1 with 1/11" twist.

This message has been edited. Last edited by: sigfreund,