Any Rhyme or Reason to POI Shifts?

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warthog97
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Any Rhyme or Reason to POI Shifts?

Post by warthog97 » Fri Mar 29, 2019 1:05 pm

Hey all,
Had the chance to shoot 300 BLK suppressed and unsuppressed and 308 Win suppressed and unsuppressed. I discovered that 300 BLK sub sonic only had about an inch of POI shift at 100 yds. I figure that because there was such little energy released with that load that the barrel just doesn't vibrate all that much? The 308 Win has about 2-3 inches of POI shift, but the 300 BLK supersonic load I shot had about 4-6" of POI shift between unsuppressed and suppressed. I was astounded at the difference. Do you folks find that the POI shift is similar between similar loads (ie similar POI shifts for 300 BLK supersonic loads) or is each load going to have it's own POI shift that I need to keep track of?

Thanks!

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oodnoodn
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Re: Any Rhyme or Reason to POI Shifts?

Post by oodnoodn » Sat Mar 30, 2019 6:59 pm

I've found POI shift to be gun related & not load related. Your observation of different POI shifts with sub & supersonic loads is interesting: I haven't seen that.

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plant.one
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Re: Any Rhyme or Reason to POI Shifts?

Post by plant.one » Sat Mar 30, 2019 7:42 pm

people actually shoot their guns unsuppressed once they have suppressors? :shock: :shock:

wow. learn something new every day :mrgreen:
Reloading info shared is based on experiences w/ my guns. Be safe and work up your loads from published data. Web data may not be accurate/safe.
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TRshootem
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Re: Any Rhyme or Reason to POI Shifts?

Post by TRshootem » Sat Mar 30, 2019 11:11 pm

My 20" .308 has a heavy barrel profile and is stupid accurate. I do have about 1.5" higher impact with the can on than without. It's consistent enough with several loadings, so,my dope sheet uses the suppressed data. It works.

alamo5000
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Re: Any Rhyme or Reason to POI Shifts?

Post by alamo5000 » Mon Apr 08, 2019 1:48 am

warthog97 wrote:
Fri Mar 29, 2019 1:05 pm
Hey all,
Had the chance to shoot 300 BLK suppressed and unsuppressed and 308 Win suppressed and unsuppressed. I discovered that 300 BLK sub sonic only had about an inch of POI shift at 100 yds. I figure that because there was such little energy released with that load that the barrel just doesn't vibrate all that much? The 308 Win has about 2-3 inches of POI shift, but the 300 BLK supersonic load I shot had about 4-6" of POI shift between unsuppressed and suppressed. I was astounded at the difference. Do you folks find that the POI shift is similar between similar loads (ie similar POI shifts for 300 BLK supersonic loads) or is each load going to have it's own POI shift that I need to keep track of?

Thanks!
POI shift is a system issue. Keep in mind there are a lot of variables. Ammo is one. Naturally supersonics will have a different natural point of impact than subsonic even when shooting unsuppressed. IE a 110grain super vs a 220grain sub at 100 yards won't have the same zero.

Aside from the natural point of impact differences brought on by velocity, bullet weight and distance there are other factors that create or decrease POI shift.

One of the biggest ones is the attachment method of the suppressor to the rifle. This is why I like taper mounts better than others. You get consistent baffle alignment every time. If your baffles get turned or aren't in the same orientation it will change things.

The muzzle device can also cause poi shift even with the can on the gun. When I first got my first can several years ago I mis-installed the muzzle device. The gun was very accurate but the suppressed to unsuppressed shift was like 5 inches or so. Then I took the muzzle device off my gun and installed it with the right orientation (it needed to be timed) and just that alone using the same ammo, same gun and everything took my POI shift from 5 inches to about 1 inch.

Long story short the muzzle device, even inside the can cause the confusion zone (ie area between the crown of the barrel and your blast baffle) to treat gasses differently. In other words, read the instructions for your can when you get it. :lol:

So called "QD" systems can sometimes have a natural amount of wobble so your gun could be a 2MOA gun no matter what you do because the can shakes when you pull the trigger.

For direct thread cans they have a tendency to walk off the gun, meaning that you tighten them up but over time recoil can cause them to come loose which would cause the timing (or orientation) of the baffles to be different shot to shot.

There are a lot of things that can contribute to POI shift. If you shoot with a long pencil barrel and a 1.5 pound suppressor the more you shoot the more shift you will see... heat causes things to open up so to speak, but if you are shooting a short 9" long barrel with a light can it's a different subject all together.

alamo5000
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Re: Any Rhyme or Reason to POI Shifts?

Post by alamo5000 » Mon Apr 08, 2019 2:03 am

I don't want to nerd out too bad, but actually yeah, yeah I do... LOL :mrgreen:

Assuming your can has proper lock up on the gun and is very repeatable, OR, that your baffles aren't getting moved around (there are user serviceable cans out there).... assuming you know all that and control it and zero for it...

A lot of the POI shift will have to do with simple physics of a gunshot. If you haven't studied out ballistics charts they will tell the tale. The farther you go out the more time the round has to lose energy and gravity has to work on pulling it to earth. At 50 yards the shift of different rounds might not be so bad, but at 100 yards it becomes exponential. It gets even more crazy when you shoot farther out (say 1400 or 1600 yards or whatever). All that stuff gets magnified a thousand times over the farther you go.

I like to do precision and long range shooting so the quality of ammo will be your biggest factor. Assuming you have consistent handloads using different grain weights of bullets, you will need to keep track of what ammo impacts where. Say if you shot 10 rounds of Hornady 208 grain subs off of a bench with a perfect aim, and then you shot 110 grain rounds from someone else you will need to note down the POI shift for each bullet and just dial it up before you hit the range. Each different round might shoot a 1/2 MOA group, but the shift will be different for each one bare muzzle and also suppressed too.

It just depends on how accurate is accurate enough, and how consistent you want to get. Believe me, trying to do those one hole groups is a whole other rabbit hole to go down.

If you don't want to go all scientific about it just get good at zeroing your rifle.

bearcatrp
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Re: Any Rhyme or Reason to POI Shifts?

Post by bearcatrp » Thu Apr 11, 2019 4:51 pm

Barrel harmonics affect POI. When you add weight to the front of the barrel, your going to see the shift, depending on weight of the suppressor. On my 6.5 Grendel, tried 5 different bullet with and without suppressor. 1 went 2 inches at 11 o'clock, other 4 went 3 inches at 4 o'clock after zero. My sandman s has a bit of weight. Would presume if I had a titanium suppressor, being lighter, would be less. Just part of owning a suppressor.

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