POI Change?

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mr fixit
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POI Change?

Post by mr fixit » Wed Jun 24, 2015 11:06 pm

Can someone help me understand POI change with a suppressor? What is the cause? I just have a bit of trouble understanding the same gun, same ammo, shooting into the same size group at a different POI.

I'm guessing maybe harmonics?

And how do you minimize it when planning a F1 can?

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doubloon
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Re: POI Change?

Post by doubloon » Thu Jun 25, 2015 6:54 am

From what I understand reading the wisdom of the interwebs a thicker/stiffer barrel can sometimes help but there's no guarantee. Harmonics is the right buzzword.

The suppressor itself is said to inflict some POI change depending on aperture shape/size, baffle spacing/alignment, voo-doo from gases kicking the bullet as it tries to escape the sound/gas/heat vortex that is the suppressor.

When last I had time to go to the rifle range I think I recall a multiple MOA @100yds shift between suppressed and unsuppressed with my ~.9" barrel profile and the Freedom. I have been meaning to get back and test it some more with different ammo. The difference was enough to maybe lose a bet but not enough to miss anything as big as a cat.

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c5_nc
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Re: POI Change?

Post by c5_nc » Thu Jun 25, 2015 8:57 am

mr fixit wrote:Can someone help me understand POI change with a suppressor? What is the cause? I just have a bit of trouble understanding the same gun, same ammo, shooting into the same size group at a different POI.

I'm guessing maybe harmonics?

And how do you minimize it when planning a F1 can?
I don't think it has much to do with harmonics, but more to do with how precise the can is when mounted and the baffle design.

I did a lot of testing on a 11.5" (Daniel Defense) lightweight barrel vs a 12.5" (Compass Lake) HBAR/Heavy barrel and the shifts were similar.

I ran several commercial cans and multiple Form monocores, VSR, FP cans through POI testing. Some of those included modifications to the form 1 cans, meaning for example on the VSR cans the parts may have been rotated in another test, or clips were deepened or a dual clip was added. It did not include "new parts" or materials, but further machining to existing parts or installing the parts differently. The worse can tested was the AAC SDN-6 as far as POI, on this can there was major shift from unsuppressed and suppressed, but also a major shift from 300blk supers to subs, down (which was to be expected) but also about 6moa left between the rounds. The FP and VSRs tested with similar clips were the next worse. The monocores there small shifts. The VSR and Cones that had dual clips were mostly dead on.

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tsands974
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Re: POI Change?

Post by tsands974 » Thu Jun 25, 2015 9:43 am

The change in harmonics is causing the POI to shift when you go from suppressed to unsuppressed, or vice versa. All that extra weight hanging off the end of the barrel absolutely affects the POI. Hang a 2 pound weight off the end of you barrel and see what happens to your POI.

The consistency of your unsuppressed and suppressed groups can be tracked down to how repeatable your suppressor mounting ability is. If you tighten it down snug to say 31 clicks every time, your groups will most likely be more consistent and repeatable than if you have a deviation of 5 clicks or so every time you mount it. Also, a can threaded directly to the barrel is typically more consistent than a can with a QD mount, at least in my personal experience.

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doubloon
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Re: POI Change?

Post by doubloon » Thu Jun 25, 2015 8:08 pm

http://www.youtube.com/watch?v=BDtd2jNIwAU MUSAFAR!
http://www.youtube.com/watch?v=8CrOL-ydFMI This is Water DavidW
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quiettime
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Re: POI Change?

Post by quiettime » Sat Jul 04, 2015 2:44 pm

Harmonics, forces inside the can from gas manipulation, thread squareness and concentricity (to the bore) and also the weight of the can pulling down on the end of the barrel all contribute to POI shift.

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