Significant point of aim shift

[2manygunz]

So, I had my first substantial (to me) point of aim shift suppressing a 308 yesterday. I was sighting in a new 308 and, after setting the optic, put a can on. It resulted in a 5" drop in target impact at 200yds. Without the can, the zero returned and I was able to reproduce the effect in groups switching between conditions.

I'm glad I observed and measured as I now know to take the turret up a few MOA with the can on, however this is the first time I've seem that significant of a shift between suppressed and unsuppressed.

I'd be interested in thoughts on this phenomenon, both general causes and specific thoughts on this particular drop. My gut assumption is that the can is somehow taking velocity off the round, particularly seeing as how the results are a dead drop (no left or right). I'm wondering, however, if there's something more subtle at work, particularly as I use the can across rifles and this is the first I've noticed this issue. (Barrel flex from the weight??)

Some technical notes:

barrel length: 18"
Round: M118LR-type
Can: YHM Phantom QD
Twist: 1/10

[whiterussian1974]

I'd be interested in thoughts on this phenomenon, both general causes and specific thoughts on this particular drop. My gut assumption is that the can is somehow taking velocity off the round, particularly seeing as how the results are a dead drop (no left or right). I'm wondering, however, if there's something more subtle at work, particularly as I use the can across rifles and this is the first I've noticed this issue. (Barrel flex from the weight??)

Usually the POI will shift UP for suppressed shots becasue the barrel gas doesn't dissipate or depart from boreline as quickly.
For DROP I imagine that barrel flex is the cause. Your tech notes mentioned many variables, but not barrel profile. A bull or fluted barrel will suffer less than standard barrels.

[Emilio]

How heavy is your can? How skinny the barrel?

POI drop is not uncommon suppressed, 5" is a bit much though so I guess you have a heavy YHM or something.

[whiterussian1974]

It's not that a heavy can "bends" the barrel. The weight changes the vibration frequency and the bullet exits during a different point during the cycle.

You're fortunate that the drop is straight down. A 1" up, 2" right would be tougher.
1-2MOA isn't uncommon. You happen to have 2.5MOA. A different powder or bullet might clear things up, or make them worse.

[2manygunz]

Thanks folks. Very interesting details.

My YHM is older and NOT the Ti model, so it is heavy. The barrel is fluted. I'm liking the barrel harmonics/vibration concept as I had a hard time thinking that the can weight alone would "flex" the barrel.

As always, great info here. Thanks again. Additional thoughts welcome.

[JasonM]

I'd be interested in thoughts on this phenomenon, both general causes and specific thoughts on this particular drop. My gut assumption is that the can is somehow taking velocity off the round, particularly seeing as how the results are a dead drop (no left or right). I'm wondering, however, if there's something more subtle at work, particularly as I use the can across rifles and this is the first I've noticed this issue. (Barrel flex from the weight??)

Usually the POI will shift UP for suppressed shots becasue the barrel gas doesn't dissipate or depart from boreline as quickly.
For DROP I imagine that barrel flex is the cause. Your tech notes mentioned many variables, but not barrel profile. A bull or fluted barrel will suffer less than standard barrels.

I've never seen any rule for the direction or amount of shift. Certainly not straight up.

There are solo many factors involved in each particular setup, it's impossible to predict.

OP- you've got 2.5 MOA shift, and you say it repeats. That's perfectly normal (on the slightly high side).

[Emilio]

It's not that a heavy can "bends" the barrel.

Yeah, I didn't say it did. it's harmonics and skinny barrels "can" play the harmonica like a blues player on the Caitlin circuit.


2man your fine if it repeats.

[2manygunz]

If I recall when I dialed up the scope it was 2.3 MOA worth of clicks. Being in site-in mode, I was just happy to be hitting paper. When I put the can on, these little dots started showing up on the bottom of the target. What the…?

[whiterussian1974]

It's not that a heavy can "bends" the barrel.

Yeah, I didn't say it did.

I was clarifying my earlier use of term "flex." I didn't want to give a false impression that I had meant there would be resting flexation. Just harmonic oscillation.

I wouldnt ? YOU Emilio. That would be like peeing on your leg and saying it's raining.

[JasonM]

If I recall when I dialed up the scope it was 2.3 MOA worth of clicks. Being in site-in mode, I was just happy to be hitting paper. When I put the can on, these little dots started showing up on the bottom of the target. What the…?

Again, not significant, and completely normal.

If you were getting baffle strikes or had a really bad mount/thread job you'd see 3, 4, 5, more MOA shift at as close as 25yds.

[JasonM]

If I recall when I dialed up the scope it was 2.3 MOA worth of clicks. Being in site-in mode, I was just happy to be hitting paper. When I put the can on, these little dots started showing up on the bottom of the target. What the…?

What is your experience with shooting suppressed? what hosts, what distances, what cans, etc?