Suppressors Help me understand POI shift

TOPO-sniper

TOPO-sniper

Sergeant
Full Member
Minuteman
Dec 2, 2004
242
4
League City, TX
#1
My new AAC Cyclone thread-on can is giving me a 4 MOA POI shift, seems excessive if it was purly a harmonics shift. Any better suggestions on "why"? I have a request in to AAC regarding this but am curious on the cause. Here is a test target showing unsupressed, AAC Cyclone POI & Gemtech HVT POI for comparison:

100 yard target
25-07-09_1533.jpg


24-07-09_1459.jpg
 
#4
Re: Help me understand POI shift

BR: Oh, and do not see any evidence of baffle strikes (good point). Wouldn't a baffle strike(s) open up the group?

DK: mine seems to be repeatable too, just curious to the "why" it is so large. What length barrel are you using with the Cyclone? I think it is interesting the HVT shift is almost along the same axis but just to the other side of center.
 
#5
Re: Help me understand POI shift

the reasoning behind POI shifts are the fact that the can now becomes your muzzle crown... the gas isnt coming out of the barrel exactly the same way as with it unsupressed. THATs why the impact is different ... its repeatable because with the suppressor on there its the same every shot. Thats how the supressor is changing the gas before the bullet leaves the end of the can, its got it slightly different pressure points.
 
#6
Re: Help me understand POI shift

what outsydlookin75 says......

on the strike thing.....i have had 'em strike the exit hole and leave a linear mark.....

but and however....unless you got a mandrel jocked up in a lathe and measure the run-out on the exit hole on the business end of the end-cap....my bet is the boolet is not exiting absolutely concentric......i have seen this concenticity check done on several different cans.....they are all over the place....even caught one before it was used that had a 38 thousands run-out.....it would have been catastrophic.
 
#7
Re: Help me understand POI shift

I was told that POI shift also had alot to do with the thread job. I haven't checked my Cyclone yet with factory ammo...Only oddball subsonics that were hard to keep track of.
 
#8
Re: Help me understand POI shift

Outs, looking for a slightly more technical answer. I understand the basics, but is "pressure points" the only driver?

1) Can weight: changes barrel harmonics - am I just catching the outside of the swing. Would changing the barrel length +/- one inch change the POI? (if anyone in Houston has a threaded barrel & wants to meet for some range time it would be an interesting test to try this can on another rifle. I do not see the same POI shift magnitude/difference in my 300 Whisper, is it the reduction in gas volume alone or also the absence of the shock reflections?).

2) Baffle concentricity: Is it just exiting gas after the crown nudging the bullet this way or will the shock reflections from the bullet inside this system do this? Fighters have very complicated inlet designs to take supersonic flow down to subsonic at the engine face, again curious how supressor innards react to these.

The HVT has a large cutout in the baffles to meet some prior military water drain time requirement or something to that effect, I need to look at where that orientation is compared to the POI shift of that can.

Just curious, anyone have/seen CFD modeling of any of this?

Thanks
 
#9
Re: Help me understand POI shift

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TOPO-sniper</div><div class="ubbcode-body">DK: mine seems to be repeatable too, just curious to the "why" it is so large. What length barrel are you using with the Cyclone? I think it is interesting the HVT shift is almost along the same axis but just to the other side of center. </div></div>
PAI 5R with a 24" tube...couldn't tell you why the large POI shift, but some intelligible answers look like they were given.
 
#11
Re: Help me understand POI shift

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TOPO-sniper</div><div class="ubbcode-body">

1) Can weight= changes barrel harmonics

Thanks </div></div>

this
 
#12
Re: Help me understand POI shift

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 762SD</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TOPO-sniper</div><div class="ubbcode-body">

1) Can weight= changes barrel harmonics

Thanks </div></div>

this </div></div>

You know the weight difference is only 1.6 ounces, I dont think that is going to cause that much of a difference in barrel harmonics. I could be wrong but even if I was the Cyclone is actually lighter .....

HVT is 24 ounces, the Cyclone is 22.4 ounces.

and TOPO, the idea of putting your can on someone else rifle isnt going to show anything, their threading isnt going to be the same as yours so the can may not line up exactly on their rifle as it does on your rifle giving a different POI shift.

I could see that if the barrel harmonics were messed with somehow that you would have a pattern and not a decent group that you have.

If you look at the AAC SPRM4 it allows for different indexing points ... why? To minimalize POI shift. With a thread on can basically I hate to say this, either you chase the POI by cutting and rethreading the barrel to see if you cna get it to be minimal with that can on. OR you just zero for with the can on or with the can off and you make a note of where it shoots with thecan on or off in your log book.
 
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