What can cause a POI shift when using a suppressor?

[aslap]

I'm the same guy who asked for critiques on my K baffles. I realize there are many, many variables at play here but let me lay out my situation.

I had my first opportunity to take my suppressor to the range today and had it on my Ruger 10/22. The Ruger was sighted in and put on target. The suppressor was attached and all of the shots were being thrown way high and way left. By "way" I'm talking a foot or so at 25 yards. The POI shift was very consistent with Gemtech subsonic; Amercan Eagle subsonic; Remington Thunderbolt; and Winchester white box. The same ammo w/o the suppressor printed to POA.

I took the suppressor apart and there is no evidence of baffle strikes anywhere. The baffles were aligned using a length of -.261 drill rod. The bore of each baffle is 0.285. They fit snugly into the tube. The baffles were assembled into the tube in no particular order. There was no kind of excessive carbon build up since I only fired about 75 round through it.

I'd appreciate hearing any thoughts on this. I can't imagine with all of the oversize bore hole used in the baffles that this would be like having a bad crown on a barrel but I'm sure I could be wrong. And If you remember, I'm using a die spring thread holder as the final exit piece of the suppresser and it is not a hole - it's a hex cut out for a 1/4" Allen wrench. Maybe that's a problem.

Thanks

Tony S.

[a_canadian]

Well the 1/4" Allen hole gives you exactly 0.25" diameter bore... plus the little corners, but they don't count, it's essentially a 6-sided 1/4" hole. That means you have to be accurate in your threading concentric to the bore such that by the time the bullet gets to the 1/4" hole the 0.015" on every side of the bullet is sufficient clearance. Your exit hole is by far the closest tolerance of your suppressor bore, while being the furthest hole from the crown and therefore the most likely to be out of line. I'd suggest getting yourself a laser boresighter, one known to be dead accurate, and popping it in there. Put a piece of masking tape across the muzzle and press it into the Allen wrench hole. If the little laser dot isn't EXACTLY in the centre of the depression in the tape, you have your answer. That is... if your chamber is concentric to the last half inch of the bore... which is not guaranteed but is fairly likely. I'd guess that the laser will appear closer to the lower-right edge of the hole, bouncing the bullets slightly up and to the left, and that the hard metal of your plug just isn't showing any damage from the impacts of the soft lead grazing it on the way by.

If this proves to be the problem, you could drill out the 1/4" hex hole to match the rest of your baffle bore holes. Or go slightly smaller than that and test it, then drill larger if it's still messing up POI. Even aluminum K baffles will survive quite a few very slight baffle strikes from a .22". Larger calibres are likely to get destroyed, or a more serious misalignment will do the same in .22", but a very light touch on any baffle edge will be enough to throw off the POI while not showing much or any evidence of a strike for quite a lot of shots.

[aslap]

Thanks. I don't have access. To any kind of laser stuff. That is a great idea. however.

I have had every baffle under a 5x magnifier and I can not see any evidence at all of a baffle strike. The carbon build up on the face of each baffle is uniform with no kind of little disruptions in the surface.

Here's what I was thinking. The sharp corners would be a perfect place for any pressure to magnify itself. That is to say that if the bullet is coming out of the hex hole a bit off center - but not enough for a strike - and the corner was acting as a pressure builder, that could then be enough to push the bullets off course. The deflection was amazingly consistent.

I have a hunk of mild steel 7/8" x 14 threaded rod and I just made an end piece out of that with the same size bore as the baffles. It's supposed to be crummy weather tomorrow but I have to get to the range and try this out. I'll report back.

Tony S.

[a_canadian]

I've not tried the boresight laser trick myself, as I've only made suppressors for PCP air pistols and those pellet chambers don't allow for such a device to be inserted. Just seemed a good chance it'd work on a firearm.

I've spent a lot of time staring down the muzzle end of suppressors and shining various lights from the other ends to figure out if my alignment is perfect, in cases where suppressors have seemingly been clipping pellets and throwing off POI. A very fussy business, trying to spot alignment by comparing the rifling to the suppressor bore visually. I've found it easiest with a fibre optic snake attached to a flashlight so I can push that right into the bore and wiggle it around until it's centred. Still tricky, but with the pistol clamped in a vise and a lot of blinking and staring it's possible to get a good idea. 1/8" drill rod checked for straightness by rolling on thick glass in a few directions, then shimmed out with a few turns of tightly wound tape until it's an easy but close fit to the bore helps somewhat, but there's still too much flex in the drill to be sure and the tape isn't an absolute measure. Problem for me is threading; all I have is a small hobby lathe so threading is done on that with a tap in the drill chuck and the rear cap in the 4-jaw. A bit hit-or-miss as threading goes.

Perhaps another check you could do would be to fit a very thin, accurate leaf washer behind the suppressor, marking the suppressor first to show the direction the bullets are flying off incorrectly, then shooting again with this shim washer in place. If your POI is now realigned on the target, but consistently with the felt pen mark or whatever, then you know there's something about the suppressor itself which is consistently throwing your shots in that direction.

If the POI on the other hand is still high and left even with the suppressor rotated differently in relation to the barrel (say, 1/3 of a turn less far onto the threads courtesy of the washer) then it would seem more likely that the threading was suspect, and that the bullets are lightly scrubbing against the same edge of the bore at some point owing to misalignment between barrel bore and suppressor bore.

I'm a bit dubious about asymmetrical pressure at the end cap causing POI shift. Lots and lots of suppressor guys around here have tried lots of different K baffle cross-vent alignments, some doing the 180 degree thing with every second baffle, some keeping them all lined up in the same orientation. In my experiments with airguns I've found there is no difference in POI between these and various spiralling or 1-2-3-4 patterns with 90 degrees between them or just random. Alignment of bore to bore is critical. Contact between any baffle or cap and the bullet is fatal to accuracy, just as is a crown imperfection. Sometimes I've found that a too-small bore (0.250" with an 8" tube on a .22" PCP shooting around 600fps, with slightly less than ideal alignment but definitely no contact) can influence accuracy slightly, but nothing so wildly off to one side as you're reporting. Seems to me that 0.25" for a .22" is minimum if everything is perfect. Drill bigger if it doesn't check out as perfect. You've already drilled bigger, except for the end cap. Anyway, good luck with the range experiment tomorrow

[aslap]

I know it sound funny but it's a "last act of a desperate man" kind of thing". I agree that one would think that the K baffle itself would be enough to send the bullets tumbling but apparently it doesn't. It's just something about those sharp hex corners that has me all agitated. Any time you put that kind of a sharp situation in the path of a gas or liquid, trouble can start.

Well, we'll see tomorrow. If everything flies straight with a round hole then there's a good PhD dissertation here somewhere!

Thanks again for answered and the insights. I appreciate it.

Tony S.

[aslap]

I can kiss that PhD goodbye.

The bigger hole allowed the deflection to reduce in magnitude but it was still there. Once again, no evidence of strikes anywhere that I can see.

That led me to check the can vs. the barrel and - bingo - I'm not dead centered. The offset is small, really small but it's there. So I think KC is the winner since it appears the bullets are grazing the inside of the exit hole. Making the hole bigger made the grazing event smaller which is reducing the amount of deflection. I'll open the hole bigger on the end cap and I'd expect to see the bullet hit somewhere else maybe down and to the right. The corners of the hex hole did, in fact, have nothing to do with anything

Expensive lesson.

Tony S.

[Capt. Link.]

Don't give up on your Doctorate yet.
Working as a gunsmith and building testing and repairing suppressors over the years has lead me to a theory I call BOUNCE.
The signs are a repeatable POI shift that tracks with the rotational position of the suppressor but no indication of a strike is present.

A projectile is surrounded by a envelope of gas.When a projectile passes close to a surface other than a knife edge a localized compression of gases occur that will destabilize the projectile because of uneven pressure.

[Hatchetjoe]

Don't give up on your Doctorate yet.
Working as a gunsmith and building testing and repairing suppressors over the years has lead me to a theory I call BOUNCE.
The signs are a repeatable POI shift that tracks with the rotational position of the suppressor but no indication of a strike is present.

A projectile is surrounded by a envelope of gas.When a projectile passes close to a surface other than a knife edge a localized compression of gases occur that will destabilize the projectile because of uneven pressure.

Bravo, Capt' Link, I've held that theory for 2 decades and no ones else has even admitted to the possibility.

[aslap]

OK, Capt.! Kind of like my theory with the sharp corners. I'll be leaving Pittsburgh tomorrow for a 2 day trip to Fort Lauderdale. When I get back I'll do this:

1. The can as it was today goes back on with a washer to change the rotational position. If it tracks and moves I'll add another washer. If it tracks and moves again, we've both earned our PhD's and Historian is going to be accompanying us to Stockholm when we pick up the Nobel prize. We'll need a worldly guy like him to guide us around!

If the issue is with the can/barrel tilt then rotating the can shouldn't change anything. I don't think.

2. If number 1 proves out, I'm going to open the end cap up to ridiculous size just to see if anything changes. If number 1 is real, I'm guessing it shouldn't. If the issue is the problem with the small (maybe 0.002") tilt in the can wrt the barrel, then some other thing should happen and I don't know what that will be right now but it should be different.

If it is #1 then that should lead to a lot of spirited discussion.

I'll post towards the end of next week with the results.

Thanks for chiming in!

Tony S.

[aslap]

Mystery not solved but problem fixed.

I went to the range today equipped with every tool I could think of end every possible thing I could need to fix the POI issue with the suppressor. I first shot the rifle and verified the POA and POI were aligned with both Remington T-Bolt and Gemtech subsonic ammo. I attached the problem child suppressor.

It shot exactly to the POA! What! There was no issue whatsoever. In fact, I was probably more accurate with the suppressor than without. And then I saw it.

The 10/22 I have is equipped with the standard AR style A2 flash hider. This piece uses a fairly thick (0.16"), conical washer behind it to help in centering or making up distance in the hider or whatever. When I remove the flash hider I normally remove this washer along with the hider. Not this time. I removed the suppressor and removed the washer and reinstalled the suppressor and kaboom - right back to everything being off high and left by quite a bit. Washer back on, all A-OK. By A-OK I mean I was hitting everything I was aiming at out to 50 yards with either type of ammo.

Back home I made some measurements. From the rear face of the suppressor to the lip of the cone shape of the first K baffle is 0.8265". The insertion length of the muzzle of the rifle w/o the washer is 0.7015". So in this case the distance from the lip of the cone to the muzzle is 0.125". The addition of the washer brings the baffle/muzzle spacing to 0.285".

I would have never guessed if you told me this that there would be this dramatic of a difference in performance by the addition of such a small amount of additional clearance, but there it is. I have no idea what's going on or just what the extra spacing has fixed. I could certainly see if the muzzle was actually touching something that could have caused an issue with barrel twist or some alignment issue in the can but that doesn't appear to be the case. I'd sure like to hear some opinions as to what might be happening.

Thanks

Tony S.

[Capt. Link.]

Mystery not solved but problem fixed.



It shot exactly to the POA! What! There was no issue whatsoever. In fact, I was probably more accurate with the suppressor than without. And then I saw it.

The 10/22 I have is equipped with the standard AR style A2 flash hider. This piece uses a fairly thick (0.16"), conical washer behind it to help in centering or making up distance in the hider or whatever. When I remove the flash hider I normally remove this washer along with the hider. Not this time. I removed the suppressor and removed the washer and reinstalled the suppressor and kaboom - right back to everything being off high and left by quite a bit. Washer back on, all A-OK. By A-OK I mean I was hitting everything I was aiming at out to 50 yards with either type of ammo.

Back home I made some measurements. From the rear face of the suppressor to the lip of the cone shape of the first K baffle is 0.8265". The insertion length of the muzzle of the rifle w/o the washer is 0.7015". So in this case the distance from the lip of the cone to the muzzle is 0.125". The addition of the washer brings the baffle/muzzle spacing to 0.285".

Tony S.

You found your problem but its not the spacing.It sounds like the threads are straight but the shoulder on the rifle is not!A true shoulder forces alinement of the suppressor the washer is forcing alinement on the threads instead.You proved it when the can was installed without washer.

[aslap]

Cap't,

Bingo!!! The backside of the washer is cut with a concave conical shape that would force the washer to "true up" the face of the shoulder. So if I were to take flat washers and set them to the same thickness I'd be right back where I was before.

Cap't - brilliant long range analysis!!!!!

Tony S.

[Fulmen]

First off; you can get changes in POI even without physical contact. The bullet will generate a pressure wave when it moves through air, this will interact with nearby objects and can cause deflection. In addition you have the propellant gases flowing alongside the bullet that can cause similar effects.

Rule number one when fitting a can is to sight through the bore to check for misalignment, with some training you can see fairly small discrepancies.

As for the barrel in question I concur with Capt's assessment, sounds like a bad shoulder.

[aslap]

I believe that to be the case also. However, I have a few more tests planned just to be sure that it isn't something else. If I find anything out that isn't the bad shoulder, I'll post it. If the thread goes into oblivion, then that's what it was.

Thanks for all the help and ideas!! Made for what I'd consider a pretty successful F1 can.

Tony S.

[Shift1]

I believe you should not use a crush washer when mounting a suppressor.....some warn against it

[aslap]

This will be the final post in the subject here. It is, definitely, the shoulder of the barrel not being square to the threads. I just got back from the range and I tried a number of different things and they all led to the same place. If the washer that came with the gun is not installed like this:



and the suppressor is mounted to anything but this washer mounting orientation, I had know way of knowing where the bullets would strike no matter where the muzzle was positioned wrt the first baffle; or the internal configuration of the baffles; or the use of the hex hole in the front v. a round hole; or a 0.250 hole in the exit cap v. a 0.280 hole in the exit cap. Whatever the other suppressor configuration I used, setting the washer to this position and attaching the suppressor always and immediately returned the rifle right back to the same POI that I had when the suppressor was removed.

The gun is a Ruger 10/22 Tactical with a 16.5" barrel and it comes threaded from the factory and has an A2 flash hider on it. I have no idea if mine is the only one like this but this may be something to pay attention to for those that either have one or or thinking about one as a host gun.

Thanks again for all the input and great discussion!

Tony S.

[MCKNBRD]

The gun is a Ruger 10/22 Tactical with a 16.5" barrel and it comes threaded from the factory and has an A2 flash hider on it. I have no idea if mine is the only one like this but this may be something to pay attention to for those that either have one or or thinking about one as a host gun.

Thanks again for all the input and great discussion!

Tony S.

Sorry, but your post isn't the last one.

IIRC, Ruger threads the barrel to centerfire .22 specs, that is, .600-.620 overall length to index on the shoulder behind the threads (They did this to use the same flash hider as the Mini-14 & SR556 lineup). The 'industry standard' for rimfire is closer to .400-.438 for indexing on the shoulder behind the threads. The threads are too long for your application, and the crush washer is 'taking up the slack'.

I'd chuck up the barrel, index it properly in the lathe, and square up the shoulder and reduce the threads OAL to .400-ish. Just be careful and don't take off too much...you're close to SBR range if you part off the threads & try to start over...

Byrdman

[aslap]

Byrd,

You're right! My opening statement there was far to final.

I don't quite understand what you're saying about the length of the whole thing. If the shoulder is square, then it shouldn't matter how long the threads are as long as I can screw the can all the way to the shoulder (which I can), right? But now I'm wondering about the washer itself. Just what is it doing? I don't think it can be what you're suggesting because it has to be used in a certain orientation. If it was just taking up some space, I'd think it wouldn't matter a whit whether the cone was facing the front or the rear but it really does. If I turn that washer around the bullets start to spray everywhere. Get the cone facing back towards the shooter and the POI is right on with the POI without it.

But why the orientation issue? There is no corresponding cut out in the barrel that the cone would seem to be being forced into. One thing I didn't do was try and just use a couple of flat washers and see what happens. In the meantime, I better order a couple of those washers from Ruger. If I lose this one it's all over daddy-O!!!

Thanks

Tony S.

[Shift1]

A rifle barrel, if less than 16 inches puts the gun in SBR (short barrel rifle) status meaning you need another $200 to be legal. The shoulder behind the washer may not be perpendicular to the bore causing a potential alignment issue (may be what you are experiencing). The crush washer is used to align and clock the flash hider. it has been said and advised not to use crush washers with suppressors because of the potential deformation and causing of miss-alignment. If the barrel shoulder is not square and you are using the already "formed" washer it can throw everything off. The cup portion backwards is more than likely fitting over the shoulder of the barrel and therefore does not fit the same as it would cup forward. Setting the barrel in a lathe between centers and making absolutely certain that the shoulder is square to the bore is not all to difficult or expensive to have done if your equipment is not long enough to do it. Would more than likely be only a few thou to clean up. Once that is done try again without the washer. Also could be the back of the flash hider may not be square and did not crush evenly on the washer. Pretty cheap insurance to your investment to make absolutely certain that everything is good and correct on the business end of your rifle.

[RJT]

If you thread it all the way down onto .600", you'll erode the blast baffle faster than at .400"-.430". You could machine a spacer similar to this, if you are worried about getting into SBR length.

http://www.silencershop.com/shop/rifle- ... er-1-2x28/

ETA - You'll also have one hell of a time removing it after use, as carbon, and lead will foul the exposed threads.

[aslap]

So this washer is actually an AR15 crush washer, huh? Regardless of why such a thing is needed in the first place if the shoulder of the barrel is square to the threads and they are concentric to the bore, more moving things around on my end still comes back to the washer on the rifle. On my M&P 22 pistol with thread adapter with shorter thread length and a shoulder to butt up against, the POI with and w/o the suppressor is the same.

Tony S.

[DanielWilson]

Don't give up on your Doctorate yet.
Working as a gunsmith and building testing and repairing suppressors over the years has lead me to a theory I call BOUNCE.
The signs are a repeatable POI shift that tracks with the rotational position of the suppressor but no indication of a strike is present.

A projectile is surrounded by a envelope of gas.When a projectile passes close to a surface other than a knife edge a localized compression of gases occur that will destabilize the projectile because of uneven pressure.

Bravo, Capt' Link, I've held that theory for 2 decades and no ones else has even admitted to the possibility.

That envelope of gas ... is that what we experience when parked on the side of the highway and a semi-trailer truck goes by, shaking the car? Or a different phenomenon?

In my example, the truck should be pushed with similar force that the car is, but being so massive, the force would be far less noticeable. However, with a bullet, that force could be quite significant.

So what is the solution to reduce (preferably eliminate) POI shift? Absolutely symmetrical baffles so that the push to the left is perfectly matched by a push to the right?

[whiterussian1974]

A projectile is surrounded by a envelope of gas.When a projectile passes close to a surface other than a knife edge a localized compression of gases occur that will destabilize the projectile because of uneven pressure.

That envelope of gas ... is that what we experience when parked on the side of the highway and a semi-trailer truck goes by, shaking the car? Or a different phenomenon?

In my example, the truck should be pushed with similar force that the car is, but being so massive, the force would be far less noticeable. However, with a bullet, that force could be quite significant.

So what is the solution to reduce (preferably eliminate) POI shift? Absolutely symmetrical baffles so that the push to the left is perfectly matched by a push to the right?

Similar. They are called Compression Waves or Compression Zones.
The Truck and Car don't generate the same values because they displace different volumes of air. The mass you mentioned only affects their resistance to disturbance, not how much they generate. Only volume affects generation.

And in a parked car, you aren't generating. The Truck would only feel the rebound of its own pressure waves. Thus, the bullet only feels its own pressure waves. The baffles can't generate, only reflect.

To mitigate effects, have the 1st 2 (or more) baffles use sharp-angled, thin, symmetrical aperatures. After 5/6 of the gas has been stripped away, move the baffle mouths to slightly larger ID. This prevents the compression wave from rebounding into the projectile until after passage. This way you can add assymmetrical cross-jetting structures to create turbulence.

This Principle is even used to accelerate projectiles using a series of explosive rings to "squeeze" its boattail like a watermellon seed b/t fingers, or penny on Railroad tracks.

[aslap]

WR,

How would you calculate the 5/6 stripped gas volume to know when to open the baffle mouths?

You seem to really know a lot about this stuff. How did you get the knowledge?

Thanks for sharing!

Tony S.