Why do suppressors change POI?

[Rookie]

I shot my 308 today with my suppressor and my shots were way low and way right. Why?

 

[Bfish]

From what I understand, the can on the end and it's extra weight changes barrel harmonics and how it flexes. Most change is usually low I believe, but my 22 shoots high when suppressed. How low and how right is it shooting? As long as it's repeatable I wouldn't worry too much.

 

[Rookie]

About three inches low and 1 1/2 right. I zeroed it.

 

[Bfish]

About three inches low and 1 1/2 right. I zeroed it.

That's not terrible by any means I don't think. If you remove the can and then put it back on does it still impact the same? If so you're good to go. I'm assuming you zeroed without the can and got the low right results.

 

[LastShot123]

A few inches is normal, consistency is the key as bfish eluded to. I wouldn't be too worried

 

[indyjohn]

Fish is on it - harmonics change hanging that tube off the end of your rifle. And you're essentially adding length to the barrel and therefore adding more time for gases to push the projectile, even if the pressures are marginal in the suppressor.

 

[iChokePeople]

Two more data points for your reading pleasure, from folks who would SEEM to be pretty good sources:

What is POI Shift?
POI Shift | Liberty Suppressors - Firearm Sound Suppressor and World Class Silencer

 

[amafrank]

First and foremost the weight of the can changes the frequency of the barrel and how it reacts to the pressure. It is not unusual for a can to change the point of impact vs the point of aim and heavier cans will generally have greater changes. A heavy barrel will generally have less change than a lightweight thin barrel.

It is not always lower, it can be anywhere on the clock. Knights Armament made suppressors for the HK USP and SOCOM pistols that could be rotated about 10 deg at a time to adjust POI for the minimum change from unsuppressed. By rotating the can you can find the point of minimum POI change. You can do this with a thread on can as well but its a one time thing if you machine off the shoulder of the barrel or can to do the rotating. You can try using a shim of solid metal to change the rotation but be sure its flat. Using crush washers or shim material that isn't flat is a good way to ding a baffle. Don't try leaving the can loose as that allows it to droop and once again baffle strikes are in your future.

Another effect found with suppressors is the gas flow changing the bullets flight. The gasses in a High Velocity rifle travel substantially faster than the bullet. For a round doing about 3000 fps the gas behind it will do about 5000fps once its released. When the bullet clears the muzzle the gas can pass the bullet and in doing so can affect its flight. If the gas comes out non-symetrically it can deflect the bullet and as it passes through the can the baffles can redirect gas in a way that pushes the bullet off course too. Some baffle types are more likely to do this than others. Bullet weight also affects the change.

Most of us make a note of both unsuppressed zero and suppressed zero and use the dope for whichever we are shooting at the time. Once you're dialed in the changes to your scope are the same amount for either setup, only your initial zero should change.

Hope that helps

Frank

 

[oldpink]

Everyone else already gave you the answer for why your POI changes with the can.
It's worth mentioning that the world record benchrest group was shot with a bolt action rifle equipped with a can.
Apparently, the can helps shunt the gases away from the rear of the bullet as it exits the crown of the muzzle, which in turn helps stabilize its flight.
It's also possible that the can has a positive effect on barrel harmonics, but the ballisticians are the ones who would know more about that one.
I don't know the technical details of the current record holder's rifle, but it's probable that the current record holder was using a suppressed rifle, too.

 

[Rookie]

I sighted my 308 in. If I remove the suppressor to use it on my Ar15, will my POI be different when I put it back on my 308?

 

[roscott]

I sighted my 308 in. If I remove the suppressor to use it on my Ar15, will my POI be different when I put it back on my 308?

That is indeed the magic question. If you have quality gear and index the suppressor so that it ends up threaded back on in the same place, you should have repeatable POI shift each time. A small sharpie mark or scratch on both the barrel and can should help you make sure you're right back where you want to be.

 

[singlestacksig]

I sighted my 308 in. If I remove the suppressor to use it on my Ar15, will my POI be different when I put it back on my 308?

no, not if your threading is good... I have a can that I use on 2 rifles and it returns to zero for each rifle once set up.. I do make sure that the thread adapter & can are screwed on nice & tight..