POI shift on carbines from prone to positional

[rugbydave]

This isn't a topic I've really seen addressed here but there has been quite a bit of discussion on it for a while stateside. Simply put, most carbines will show a shift of POI when transitioning from prone with a bipod/rear bag to shooting off bags or supported positional (barricade and/or tripod) shooting. This applies to carbines with "free float" handguards as well as non-free float designs. All of the talk around this down south is to do with ARs (obviously) and how best to mitigate it, monolithic or semi monolithic uppers being the best answer but you can make good improvements with larger barrel nuts and stiffer handguards. Even though we can't legally shoot our AR pattern rifle any more, this still all applies to Ravens, Cryptos, Canadian AR-180s, Tavors, etc. I'm curious how many guys check for this kind of POI shift? It's only something that comes into effect if you're shooting your carbine the way you'd shoot a precision rifle (ie bipod and rear bag) and also shooting supported positional or tripod, for run 'n gun setups it's not really a factor. It's something I have the guys in my scoped carbine classes test for as it's a piece of data that is very important to have so you can decide how to deal with it.

As a general rule, all (AR and AR180 pattern) carbines tend to exhibit a POI shift down when zeroed prone but shot off bags or a tripod/positional. The 2 methods for dealing with this, once you've figured what the shift is, are to either zero between the 2 different POIs or to zero prone but understand how much elevation to add when shooting supported positional. The first method works very well with systems that exhibit minimal POI shift (say 0.2mrad) as long as you engage targets that are say 0.3 mrad tall. The =/- 0.1 mrad gets lost in the height of the target. For rifles that exhibit more shift than that, I prefer to zero prone and add the offset to my dope when shooting supported positional, so if your offset is 0.4 mrad down and my dope for 300yds is 1.1 mrad but I'm shooting off a support bag, I'd dial/hold 1.5 mils.

I'd be curious to hear others thoughts on this topic and/or see what you get as you test for this offset.

The pics below are from some targets shot with a BCL Siberian:

The first pic is 5x 10rnd groups shot at 100yds.
Top left target is checking zero (which was off) with bipod towards end of handguard.
Top right is checking zero after making the adjustment from T1.
Bottom right is after moving the bipod all the way back to the mag well.
Middle target is shooting off a bag at the mag well.
Bottom left target is checking zero a final time with bipod towards end of handguard.

As you can see, the offset down from shooting off bags at the mag well is aprox 0.2 mrad while the offset down from shooting with the bipod all the way back is just about 0.4 mrad.




The 2nd pic is 2x 10rnd groups shot at 100yds.
The left target is shot seated off a bipod (and wasn't my finest shooting, haha)
The right target was shot after to verify zero (prone with bipod out front and rear bag)
As you can see, the offset is approx 0.2 mrad

 

[deadman]

This is something I didn't explore when I was testing various ammo a few weeks ago, I know you suggested it at the time but i felt i had exhausted testing with my barrel. I will def be digging deeper into this when my rifle is finished.

No going back now, my old barrel is sold and currently in CP limbo.

If my PRI order makes it through customs I'll have a free floated handguard to try again. Hopefully mid December, would be great to spend my holiday putting it together and getting some rds downrange.

If all works out and I can make it shoot well, I'll be spending time collecting data on shooting position variables. I need a coyote rifle that weighs less than 18.5 lbs!

 

[zupermann]

I have major shifts with using slings, and with different positions. My understanding is that some competitive types have different zeros for different positions. I used "kentucky" windage/elevation to compensate. For example, my sitting slung POI was 3" down and 3" Lt from my prone bipod zero!

Was more pronounced on light barrelled rifles. And even free floated rifles had this tendancy. Couldnt figure out why until someone suggested that the differing shooting positions affected ones' follow through, hence affecting POI.
It sounds credible to me !

 

[rugbydave]

This is something I didn't explore when I was testing various ammo a few weeks ago, I know you suggested it at the time but i felt i had exhausted testing with my barrel. I will def be digging deeper into this when my rifle is finished.

No going back now, my old barrel is sold and currently in CP limbo.

If my PRI order makes it through customs I'll have a free floated handguard to try again. Hopefully mid December, would be great to spend my holiday putting it together and getting some rds downrange.

If all works out and I can make it shoot well, I'll be spending time collecting data on shooting position variables. I need a coyote rifle that weighs less than 18.5 lbs!

Hopefully your PRI handguard makes it through Customs, I really was tempted to get one of their handguards and the upper rail and do a MK12 build on a Crypto. I have a sneaking suspicion that the upper rail you see on the Mk12 mod 0 would help mitigate the handguard flex torquing the barrel nut. I ended up getting the Craddock Precision version (steel barrel nut instead of Al) of the CMT arca rail for a build with a Krieger Hbar but the receiver set and some other parts are stuck in CP limbo... ugghh. Hopefully CP is legislated back to work shortly and I get my stuff. Would love to wring out that setup before Christmas.

 

[beltfed]

I'm of the belief that any changes in poi from positional shooting is down to changes in:
a) shooters sighting (be it canted sights, stock position or parallax), and
b) changes to the barrel harmonics from interruption of mechanical systems / attachments to the barrel (even if the barrel is fully floated).

In 100% of the cases it is down to shooter error and not an equipment issue.

 

[rugbydave]

I have major shifts with using slings, and with different positions. My understanding is that some competitive types have different zeros for different positions. I used "kentucky" windage/elevation to compensate. For example, my sitting slung POI was 3" down and 3" Lt from my prone bipod zero!

Was more pronounced on light barrelled rifles. And even free floated rifles had this tendancy. Couldnt figure out why until someone suggested that the differing shooting positions affected ones' follow through, hence affecting POI.
It sounds credible to me !

Different shooting positions can definitely affect recoil management which will translate to a POI shift on target but some of the positional shifts seen in high power type shooting are from the sling applying torque on the handguard. From my understanding, all these poi shifts are caused by torque being applied to the barrel nut via the handguard. That's why monolithic and "semi-monolithic" (ie the handguard bolts up to the upper receiver instead of to the barrel nut) suffer much less from POI shifts.

 

[rugbydave]

I'm of the belief that any changes in poi from positional shooting is down to changes in:
a) shooters sighting (be it canted sights, stock position or parallax), and
b) changes to the barrel harmonics from interruption of mechanical systems / attachments to the barrel (even if the barrel is fully floated).

In 100% of the cases it is down to shooter error and not an equipment issue.

These POI shifts are actually due to the handguard acting as a lever and applying pressure to the barrel nut more so when shooting prone wth a bipod than resting on bags. It's nothing to do with shooter error. I probably need to go back and clarify that when I was talking about positional shooting I meant "supported positional" ie shooting off a support bag or a tripod.

 

[tiriaq]

My experience with Service Rifle:
Prone - monopod on the magazine, support hand on the ground wrapped around the magazine.
Sitting, Kneeling - knee between the grip and the magazine, support hand around the magazine well.
Standing - support hand around magazine well, support elbow on magazine pouch.
In all cases, no pressure on handguard or barrel. Sight elevations set according to confirmed zeros, 500m to 100m. No noticeable zero shift in different positions.
No sandbags, bipods, tripods or other artificial supports.

 

[FarmerDanz]

A compensator that directs gas up or in one direction only can induce harmonics.

For a 556 rifle I would go with a comp that directs gas back only, not up, and does it equally so harmonics are kept to a minimum.

 

[rugbydave]

My experience with Service Rifle:
Prone - monopod on the magazine, support hand on the ground wrapped around the magazine.
Sitting, Kneeling - knee between the grip and the magazine, support hand around the magazine well.
Standing - support hand around magazine well, support elbow on magazine pouch.
In all cases, no pressure on handguard or barrel. Sight elevations set according to confirmed zeros, 500m to 100m. No noticeable zero shift in different positions.
No sandbags, bipods, tripods or other artificial supports.

Yeah in the unsupported positional shooting you do in Service Rifle there's no pressure on the handguard and thus no POI shift. This kind of POI shift only comes into effect when using a bipod and/or supported positions off bags/tripod.

 

[call2arms]

Yeah in the unsupported positional shooting you do in Service Rifle there's no pressure on the handguard and thus no POI shift. This kind of POI shift only comes into effect when using a bipod and/or supported positions off bags/tripod.

And sling.

 

[Frank2368]

One interesting thing to bring up is that my understanding is that this only occurs with scopes or technically any sighting system mounted independently on the receiver. This phenomenon actually doesn't happen with iron sights because the front sight would move the same amount angularly as it is attached to the barrel. Thus any pressure applied to the barrel would not affect POI because the sighting system is moving parallel to it.

Never tested this though because I'm not a great shot with irons. Would also be interesting to test out for irons experts.

 

[beltfed]

These POI shifts are actually due to the handguard acting as a lever and applying pressure to the barrel nut more so when shooting prone wth a bipod than resting on bags. It's nothing to do with shooter error. I probably need to go back and clarify that when I was talking about positional shooting I meant "supported positional" ie shooting off a support bag or a tripod.

If a floated hand guard is installed on a barrel nut, it should in theory and in practice have no measurable affect to the mechanical accuracy of the rifle whether the weight of the rifle is rested at or about the rear of the hand guard where it is affixed to the barrel nut or whether that bearing is cantilevered out further toward the muzzle end of the hand guard (assuming the barrel nut is properly torqued and properly bearing the weight of the barrel and there is no interference from internal mechanisms within the hand guard imparting force between the hand guard and the barrel (like a bending moment forcing the hand guard into the gas block as an example).

I think in most of these cases where a poi shift is visualized it is due to the latter action. This was very prevalent with the Swiss Arms series of rifles when shooting from the bi-pod.

Whether sighting alignment issues, differing follow through, or mechanical interference, it boils down to shooter error.
On that, we'll have to agree to disagree.

 

[rugbydave]

One interesting thing to bring up is that my understanding is that this only occurs with scopes or technically any sighting system mounted independently on the receiver. This phenomenon actually doesn't happen with iron sights because the front sight would move the same amount angularly as it is attached to the barrel. Thus any pressure applied to the barrel would not affect POI because the sighting system is moving parallel to it.

Never tested this though because I'm not a great shot with irons. Would also be interesting to test out for irons experts.

Yeah, that sounds possible but I dunno how many people are shooting prone with a bipod and rear bag with irons?

 

[rugbydave]

If a floated hand guard is installed on a barrel nut, it should in theory and in practice have no measurable affect to the mechanical accuracy of the rifle whether the weight of the rifle is rested at or about the rear of the hand guard where it is affixed to the barrel nut or whether that bearing is cantilevered out further toward the muzzle end of the hand guard (assuming the barrel nut is properly torqued and properly bearing the weight of the barrel and there is no interference from internal mechanisms within the hand guard imparting force between the hand guard and the barrel (like a bending moment forcing the hand guard into the gas block as an example).

I think in most of these cases where a poi shift is visualized it is due to the latter action. This was very prevalent with the Swiss Arms series of rifles when shooting from the bi-pod.

Whether sighting alignment issues, differing follow through, or mechanical interference, it boils down to shooter error.
On that, we'll have to agree to disagree.

Sure, we can agree to disagree but you're also disagreeing with a large body of testing stateside, including from some guys in the USAMU. The reality is that flat top receivers have a certain amount of flex (which has been demonstrated on high speed video) and that a handguard that is attached to a barrel nut imparts a certain amount of force on said barrel nut when a bipod is attached towards the end of the handguard. That same force isn't applied when the handguard is rested on a bag or attached to a tripod back at the magwell. You can claim it's shooter error all you like but I can tell you that I don't execute the fundamentals any differently when I'm shooting prone with bipod and rear bag than I do when I put a bag under the handguard at the magwell and use a rear bag. It's pretty obvious that the whole main POI shifts noticeably. It's also very repeatable. Here's some reading if you'd like to have a read through. The OP of the thread just retired from the AMU: https://www.snipershide.com/shooting/threads/ar-positional-poi-shift-test.7221976/

What you're talking about with the Swiss Arms bipod is similar but is exacerbated by the non-free float handguard (so is obviously also present in ARs that are non-free float). A free-floating handguard lessens the effect but it is still there, to what degree depends very much on the means of attaching the handguard to either the barrel nut or receiver.

 

[Frank2368]

If a floated hand guard is installed on a barrel nut, it should in theory and in practice have no measurable affect to the mechanical accuracy of the rifle whether the weight of the rifle is rested at or about the rear of the hand guard where it is affixed to the barrel nut or whether that bearing is cantilevered out further toward the muzzle end of the hand guard (assuming the barrel nut is properly torqued and properly bearing the weight of the barrel and there is no interference from internal mechanisms within the hand guard imparting force between the hand guard and the barrel (like a bending moment forcing the hand guard into the gas block as an example).

I think in most of these cases where a poi shift is visualized it is due to the latter action. This was very prevalent with the Swiss Arms series of rifles when shooting from the bi-pod.

Whether sighting alignment issues, differing follow through, or mechanical interference, it boils down to shooter error.
On that, we'll have to agree to disagree.

I didn't think it would be the case either, but I've tested it with my Crypto and it certainly did shift, and I used a scope and took time to ensure it wasn't a parallax issue. For me it was about 0.2 mils between bipod at front of handguard as compared to bagged at the back of handguard. And apparently this is quite good compared to many other handguards as shown in the testing in the link Dave provided.



The left was bipod mounted at front of handguard, the middle target is bags at the rear of handguard.

If the handguard wasn't free floated, it would probably be in the ballpark of 2-3 mils of shift. The free floated handguard helps significantly but does not completely mitigate this shift.

 

[Frank2368]

Yeah, that sounds possible but I dunno how many people are shooting prone with a bipod and rear bag with irons?

More of an interesting fact really. Although the point is more that iron sighted rifles practically don't have this issue entirely. They could lay the rifle completely on the barrel, apply significant pressure and there would be no POI shift.

Before I learned this fact I always found it weird that some older rifles/machine guns, off the top of my head M14, RPK, had bipods that mount directly on the barrel, not even on the handguard. Now I realize that it really doesn't matter in their case because they were only meant to be used with iron sights at that time as therefore the POI shift issue never existed in the first place.

 

[zupermann]

Sure, we can agree to disagree but you're also disagreeing with a large body of testing stateside, including from some guys in the USAMU. The reality is that flat top receivers have a certain amount of flex (which has been demonstrated on high speed video) and that a handguard that is attached to a barrel nut imparts a certain amount of force on said barrel nut when a bipod is attached towards the end of the handguard. That same force isn't applied when the handguard is rested on a bag or attached to a tripod back at the magwell. You can claim it's shooter error all you like but I can tell you that I don't execute the fundamentals any differently when I'm shooting prone with bipod and rear bag than I do when I put a bag under the handguard at the magwell and use a rear bag. It's pretty obvious that the whole main POI shifts noticeably. It's also very repeatable. Here's some reading if you'd like to have a read through. The OP of the thread just retired from the AMU: https://www.snipershide.com/shooting/threads/ar-positional-poi-shift-test.7221976/

What you're talking about with the Swiss Arms bipod is similar but is exacerbated by the non-free float handguard (so is obviously also present in ARs that are non-free float). A free-floating handguard lessens the effect but it is still there, to what degree depends very much on the means of attaching the handguard to either the barrel nut or receiver.

Post #9 on that linked forum mirrors my experience.

 

[rugbydave]

I didn't think it would be the case either, but I've tested it with my Crypto and it certainly did shift, and I used a scope and took time to ensure it wasn't a parallax issue. For me it was about 0.2 mils between bipod at front of handguard as compared to bagged at the back of handguard. And apparently this is quite good compared to many other handguards as shown in the testing in the link Dave provided.

View attachment 859436

The left was bipod mounted at front of handguard, the middle target is bags at the rear of handguard.

If the handguard wasn't free floated, it would probably be in the ballpark of 2-3 mils of shift. The free floated handguard helps significantly but does not completely mitigate this shift.

Glad you tested for it. Yeah, from what I've seen/heard, a 0.2 mil shift is about as good as it gets without going to a monolithic or semi-monolithic receiver. Is that with your TNA handguard or the WOA? In your case, I'd probably just zero in the middle and bias my POA slightly up when shooting prone. Nice tight groups by the way, I can't wait for my Crypto to get outta CP jail...

What would be really cool, provided that they don't get prohibbed, would be for one of the Canadian Manfs to make a receiver set like the Crypto but in a semi monolithic design like the Aero Precision M4E1 or the Seekins IMRT-3 where the handguard mounts to the receiver instead of the barrel nut. A buddy of mine has a couple of the Seekins and says he gets no POI shift.

Your point about older rifles/MGs, irons and bipods on the barrel is quite interesting. It's kind of a funny thought that adding an iproved sighting system kind of works against you, haha.

 

[rugbydave]

Post #9 on that linked forum mirrors my experience.

I've heard the same from friends that shot Service Rifle