Suppressors Large POI shift with can: Downwards?

1

1911fan

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Nov 18, 2006
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Lewiston Idaho
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#1
It's been my understanding that suppressors give freebore boost and usually increase velocity a bit. To my thinking, this should give me a higher POI compared to unsuppressed. This is the opposite of what is happening.
Rifle is a Remington 700, .308 Rock heavy fluted barrel w/factory threading, factory trigger set to around 2 lbs, NF 5.5-22 on 20MOA rail, HS PSS stock and mag system, screws set to 69 in-lbs front and rear. Load is 175gr Sierra HPBTMK over RE15 at around 2700fps. Can is YHM 3400 screw on.
At 100 yards I have to come up 2.0 MOA from unsuppressed, at 200 yards it's an additional 2.25 MO from unsuppressed, and it gets more noticeable the further out I go. I haven't had a chance to record additional increases needed at longer ranges though I know as distance increases so do the needed additional comeups compared to unsuppressed. I havent shot it past about 1150 yards suppressed, but at that distance (IIRC) I added 10+ minutes to my unsuppressed elevation.

My theories:
A) Suppressor weight at the end of the barrel is now causing the bullet to leave the barrel at a downward point in normal oscillation.
B)I need to bed the rearward 2-3" of barrel to prevent the action shifting due to can weight.

Is this normal? What does your .30 caliber setup do? Ideas?

1911fan
 
#2
Re: Large POI shift with can: Downwards?

I think you're seeing the standard shift due to hanging a weight on the end of your muzzle. You don't mention it, but how long is the barrel? The YHM can isn't light, it will definitely cause more sag than a few fps increase in boost is going to make up for.
 
#5
Re: Large POI shift with can: Downwards?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JSTARSZ</div><div class="ubbcode-body">as long as the POI shift is repeatable you will have no problem predicting the outcome of the shot. </div></div>

Yep. Repeatable shift is the key. Know your shift and it isn't that big of a deal. It would be nice to not have POI shift, but the reality is that when you hang a weight on any barrel all bets are off.
 
#6
Re: Large POI shift with can: Downwards?

+1 for extra barrel weight causing your POI shift.
Your accuracy should be spot on though.
Just record the shift and adjust accordingly.

TI cans like the TBAC 30P-1 do a good job of minimizing this due to the lighter weight coupled with the freebore velocity boost effect.
 
#7
Re: Large POI shift with can: Downwards?

I believe that freebore boost from the current generation of suppressors is very small if even there at all. In my own suppressors and those of other manufacturers we've not seen any real differences in velocity with or without the can. I'm guessing that its due to more effective control of the gasses and perhaps the more rapid decrease in pressures. I have seen some impressive boost results from some older suppressors like the Ciener stuff seen back in the 80's.
POI shift occurs with any can installed on the barrel and the weight can make a huge difference along with other characteristics of the suppressor. Some suppressors will shift the POI not only down but also to one side or the other.Trying suppressors of different weights will usually move the POI differently though not always as expected. I have seen some cans that cause the group to move upward though that is pretty uncommon. Probably damping the barrel vibrations to a point where the barrel is moving upward when the bullet exits. . .
Trying a number of suppressors of the same type on a rifle will usually show somewhat different shifts as no two cans are identical as no two rifles are.
Long whippy barrels are usually more drastically affected by the can than a short stout barrel.
In any case, the effect of the installation of the suppressor is normally quite repeatable when using the same can on the same rifle. Make note of the change required in your scope(or sights) in your logbook as it is normally a very consistant change.
My .338 suppressor for my Desert Tactical SRS rifle weighs 47 oz and moves the POI 1.8" down and .4" right at 100yds based on 100yd zero with the factory muzzle brake installed. This was the result of about 100rds of testing. 7 clicks up and 2 left put the target back on center all the way out to 600 which is as far as we've been able to shoot so far with targets we could check. (been to a few longrange MG shoots but who knows if all those hits are yours or not??)
Just did a bit of 250yd shooting yesterday with the can installed and without. It hits the same spot with the same corrections.

Hope that helps
Frank
 
#8
Re: Large POI shift with can: Downwards?

Just as long as the POI shift is repeatable like everyone else has stated. I have two complete sets of data, one supressed and one unsuppressed.
 
#9
Re: Large POI shift with can: Downwards?

I haven't seem point of shift on reflex cans... Could this be because the can has 2 contact points on the barrel wich makes it much stronger and not like pasta when shot
 
#10
Re: Large POI shift with can: Downwards?

Nice responses, thanks guys! I'm going to go with the weight of the can causing a downward POI shift and build two data sets. I'd started on the suppressed data set without really realizing I needed to.
biffj, it's interesting that seven clicks up and two left recentered POI out to 600 yards; mine seems more logarithmic.
I'll just have to shoot it and see.
Has anybody with a downward POI shift tried bedding the barrel near the action? I might try that.

1911fan
 
#11
Re: Large POI shift with can: Downwards?

I just shot my HVT QD on my 700 308 18 inch barrel with barrel tensioning device attachment. I was about 9/10 of a mil low and centered. It seemed pretty repeatable. I fired 3-5 rounds took the can off and back on and did it again. I shot about 90 rounds. The first couple of times I did this the first round was way low and to the right, the second round was closer and the third was back in the bull. About half way through this problem seemed to disappear. It was a brand new can so I am wondering if this was some type of break in that the can shoots better dirty, or being a QD after I reinstall it does it further seat itself with a couple of shots.
 
#12
Re: Large POI shift with can: Downwards?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jakhamr81</div><div class="ubbcode-body">I just shot my HVT QD on my 700 308 18 inch barrel with barrel tensioning device attachment. I was about 9/10 of a mil low and centered. It seemed pretty repeatable. I fired 3-5 rounds took the can off and back on and did it again. I shot about 90 rounds. The first couple of times I did this the first round was way low and to the right, the second round was closer and the third was back in the bull. About half way through this problem seemed to disappear. It was a brand new can so I am wondering if this was some type of break in that the can shoots better dirty, or being a QD after I reinstall it does it further seat itself with a couple of shots. </div></div>
Interesting, HVT with low POI. Let us know if it changes or you keep getting the same results.
 
#15
Re: Large POI shift with can: Downwards?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 1911fan</div><div class="ubbcode-body">
biffj, it's interesting that seven clicks up and two left recentered POI out to 600 yards; mine seems more logarithmic.
I'll just have to shoot it and see.

1911fan </div></div>

I suppose I should clarify....the 7 up and 2 left recentered the POI at 100yds. I still have to move up for longer ranges but the amount is the same as without the can. It just needed to be recentered to account for the suppressor. Removing the suppressor requires that the original correction be removed to recenter with the brake. Its been very consistant. I've not done much shooting without the can though as it is so much more pleasant to shoot suppressed.
Hope that makes more sense.

Frank
 
#16
Re: Large POI shift with can: Downwards?

The YHM can uses a baffle stack composed of slanted angle oblong washer style baffles- they create an aggressively angular crossjet of gas across the bore. The bullet is in the bore while that is occurring.

Someone who formerly won the international sniper competition was talking to me about suppressors and told me that suppressors with eccentric porting in the bore seem to exhibit a tendency to impart instability to projectiles which increases parastic drag and with it drop at distance and the need to add come ups to established dope.

Surefire and Ops Inc have a baffle system which is ported outside of the bore axis and he mentioned not having these issues with their suppressors.

The guy did studies on bullet flight with radar at yuma proving grounds and I would have a tendncy to believe him.
 
#18
Re: Large POI shift with can: Downwards?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RollingThunder51</div><div class="ubbcode-body">Wouldn't know where to begin..

If come up is all he had for a problem with his suppressor, he didn't have a problem.
</div></div>

I disagree. If a suppressor is availible that does not require every range to have a uniquely different come up the suppressor that does is not ideal.

If the can requires 1MOA up at all ranges that is simple, if it requires a different amount of dope to be added to each further range, you have then introduced a third element into shooting. You have to dope wind, estimate range, and then consult a table for the suppressor.

If the guy has years of experience on M118 and now his come ups are all different that's like introducing a new caliber.

Now his holds for movers just changed because the bullet is not getting to the target in the same flight time.

The more you give the shooter to calculate, the more likely he is to miss.

<span style="font-weight: bold">Suppressor or no suppressor- In the tactical environment I'll take the suppressor every time. Give a smart shooter a choice between altered an unaltered ballistics and he'll choose the suppressor that provides unaltered external ballistics. </span>
 
#19
Re: Large POI shift with can: Downwards?

With respect.

The issue isn't about unaltered ballistics. It is about conclusions as to the effects of some baffle designs and the suggestion that the suppression performance of other designs represents either good sound suppression or a reasonable chance of accuracy after a couple of rounds. Add to that other tactical requirements (like FA) and the subject of either asymetric or symetric baffles, proper diffuser design, proper post baffle chamber design, and a host of other issues come into play.

The can that 1911 is using may be effecting his accuracy for some of the reasons meantioned (weight and their baffle design). But not all asymetrical baffles are either monocore, nor angle disk, the newest are gas path core cut bezzle. The idea that Surefire and Ops Inc are inherently the right end solution leaves me not knowing where to start.
 
#20
Re: Large POI shift with can: Downwards?

my gemtech gives me exactly 1 mil drop in poi so don't feel bad, but it doesn't bother me because its totally repeatable.

The weird part is you don't have consistent change in poi that is something i would test out at 600yards if you have the range to do it. that would tell you alot.
 
#21
Re: Large POI shift with can: Downwards?

POI shift can be caused by many factors.

Adding weight to the end of the barrel has already been mentioned. If that is whats causing the shift, you will most likely just need to live with it.

As also mentioned some suppressors are more likely to cause shift, and others less. Symmetric vs Asymmetric baffles, bore diameter, etc. Mounting becomes an issue too. Some suppressors like the Gemtech Halo & AAC Omni may not mount the same way each time. Some suppressor mounts are not good for repeatability.

Muzzle threading makes a huge difference. I have ADCO thread my AR15 barrels. I have had upwards to 4-10MOA POI shifts with factory threaded barrels. Putting the same Surefire 556k can on the same mount on threads cut concentric to the bore(by ADCO) I have zero POI shift.

In the OP's case, he is lucky. Having a POI shift that is straight down is easy to remember and compensate for. I always seem to end up with a POI shift off at some awkward angle.
 
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