POI vs. POA Data - HK/FA91 Bipod vs. Sling
*** Note: - The purpose of this thread is to quantify changes in POI for the changes in stress generated by different mounting positions of a Bipod, for bench or improvised support shooting (such as a fence), and different attachment points for Sling supported shooting the AppleSeed way. The rifle was zeroed at 25 yds, then I twisted the elevation control about 8 clicks up and the windage control about 10 clicks left so that the POI would be "in the white" for easy visibility from the shooting position.
Also note that the MOA numbers shown on the scans are not group size, they are the number of MOA shift in POI between the mounting and attachment conditions for each scan. ***
I hate not knowing the why and the how and the how much of things, so I decided to generate some data on the difference between POI and POA for some configurations of sling attachment and bipod mount locations.
All data is at 25 yards, sling has moderate to snug tension (sorry, I don't have a good way to actually quantify sling tension).
2 configurations are tested as shown below -
1) POI/POA shift for FRONT Bipod mount vs. FRONT Sling attachment
2) POI/POA shift for MID Bipod mount vs. FRONT Sling attachment
An additional configuration, MID Bipod mount vs. MID Sling attachment will be tested when the weather clears, and when I find my plastic hand guard (I don't want to drill holes in the wood hand guard).
Shown below is how the Bipod and Sling stud are both attached to the FRONT of the hand guard, which is in turn attached to the barrel:
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_FRONT_BIPOD_FRONT_SLING_s.jpg)
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Here is the difference in POI from bench/FRONT Bipod and FRONT Sling attachment in sitting position:
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_Sling_vs_Bipod_FRONT_FRONT_s.jpg)
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Shown below is how the Bipod is mounted MID position (almost directly under the receiver), and Sling stud still attached to the FRONT of the hand guard, which is in turn attached to the barrel:
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_MID_BIPOD_FRONT_SLING_s.jpg)
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Here is the difference in POI from bench/MID Bipod and FRONT Sling attachment in sitting position:
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_Sling_vs_Bipod_MID_FRONT_s.jpg)
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Interesting to note that the difference between POI with FRONT and MID Bipod mount is only 1 1/2 MOA.
Most of the difference in POI between bench/Bipod shots and Slung up shots is due to sling tension, and is approx 4 MOA.
It makes sense that the barrel is merely bent upwards by the Bipod, and does so less for a Mid mount than a Front mount.
Apparently, the combination of downward force at the Front Sling attachment point, and upward force at the Mid hand/arm point bows the barrel quite significantly.
Next on the list of things to do is to attach the sling as re-ward as possible, and see if the change in POI is reduced....
Or is it changing the harmonics during lock time or rising on the bi-pod. Changes would be different for every rifle. All I need to know is it will be different.....O.L.
I'd just put the sling where I wanted it, run a quick IMC, and call it a day.
On my scope/bipod rig I have a GI sling attached to the rear of the bipod. My scope is attached via quick disconnect and I have a padded cheek riser that's held in place with velcro straps. To shoot irons I just take that crap off.
I zero my scope. I zero my irons. My bipod/sling mount never changes. Since the sights are independent of each other they both stay zeroed as well. No shift in POI/POA since "in theory" I'm aiming at the same place no matter which sights/rest I use.
It looks like you used the same sight for bench shooting and sitting.
Because of that, you probably changed something else - your cheekweld. Since you're on the bench and then sitting, it's quite possible your face moved a little bit and that contributed to your POI shift moreso than the strain on the barrel. Looking at those two comparisons, they're basically the same groups. If you'd repeated this test a dozen times and got the same hair-splitting results I'd say sure, it's your mount.
But, looking at the two groups, you're high left on one, low left on the other. I'd make a quick IMC change to get my windage squared away and then I'd install a kisser button of some type to insure my cheek was exactly in the same place whether I fired from the bench or the ground. I'm willing to bet my Appleseed paycheck you're just slightly off and that's what contributed to the change in POI.
Quote from: slim on March 25, 2012, 09:52:25 PM
It looks like you used the same sight for bench shooting and sitting.
yes.
Quote from: slim on March 25, 2012, 09:52:25 PM
Because of that, you probably changed something else - your cheekweld. Since you're on the bench and then sitting, it's quite possible your face moved a little bit and that contributed to your POI shift moreso than the strain on the barrel. Looking at those two comparisons, they're basically the same groups. If you'd repeated this test a dozen times and got the same hair-splitting results I'd say sure, it's your mount.
But, looking at the two groups, you're high left on one, low left on the other. I'd make a quick IMC change to get my windage squared away and then I'd install a kisser button of some type to insure my cheek was exactly in the same place whether I fired from the bench or the ground. I'm willing to bet my Appleseed paycheck you're just slightly off and that's what contributed to the change in POI.
Hmm, well the POI change with Front Bipod vs. Mid Bipod mount location is interesting, but the main point is to quantify and find a way to reduce change in POI for Bipod vs. Sling.
It's not clear to me (and yes, I'm a noob) why any small difference in cheek weld would shift POI to a greater (or any) degree, than the effect of loading/bending the barrel from Front Bipod mount to Mid Bipod mount. But it should be easy to isolate out any possible difference due to cheek weld by first shooting with the Front mount position, then at the Mid mount position, and having NO cheek weld at all. If POI changes the same amount as shown above, then we will know that cheek weld makes little or no difference. I think I'll give that a try and see what happens.
But, as I said, the
big shift in POI is between Bipod mount, and Sling/Arm shooting. The next test I have on my list, is to move the Sling attachment point as re-ward as possible. If the grouping moves upward, more toward the Bipod POI, it will pretty much prove that Sling tension, especially at a forward part of the hand-guard and it's related barrel-bowing, is the dominant factor in POI shift. No?
Sure, I'll bet my Appleseed paycheck against yours :)
I agree totally with Slim on this one.
What I also see between what I believe were your 'hand held, slung up' groups would be a superior NPOA on the 4 moa group.
I would spend my time continuing to shrink my group size with the sling. Appleseed does not teach supported shooting, rather as oladcock put it, learn to shoot the rifle w/o all of the fussing YMMV.
Your 4 moa group is a good one and demonstrates that you are capable of shooting to Rifleman standard w/o the bipod. Emphasize that accomplishment in what you are doing, continue to better that to where you can do it with every rifle you pick up.
Best of Luck to you,
I edited in a Note to the first post in order to clear up the purpose of this thread - "The purpose of this thread is to quantify changes in POI for the changes in stress generated by different mounting positions of a Bipod, for bench or improvised support shooting (such as a fence), and different attachment points for Sling supported shooting the AppleSeed way. The rifle was zeroed at 25 yds, then I twisted the elevation control about 8 clicks up and the windage control about 10 clicks left so that the POI would be "in the white" for easy visibility from the shooting position."
"Also note that the MOA numbers shown on the scans are not group size, they are the number of MOA shift in POI between the mounting and attachment conditions for each scan."
Sorry, I should have been clearer.
I enjoy testing and evaluation and once upon a time did it professionally in many disciplines. If one is trying to isolate cause and effect of a variable, one must control and isolate that one variable. There are so many variables involved it's easy to conclude it makes a difference...You did that. Why is a different story cause you can't isolate bending, from harmonics, from cheek weld, from paralax, from bi-pod movement. All of the above is the answer to why POI shifts. This is why one should always test loads and zeros the EXACT way you'll be shooting. Any deviation can and will change POI's.....O.L.
Good points OL. Barrel harmonics are fascinating and complex, generally more-so than the similar behaviours of the radio and radar systems that I do for a living. For now, I am going with the simple-minded assumption that cheek-weld, bipod movement, and differing masses attached at different points along the hand-guard are affecting only (or mostly) the bending and bowing of the barrel, and transient induced barrel motion is little effected.
Of course, any data points that I find that don't conform to that simple bending viewpoint will immediately render my simple minded assumption pretty much useless :-X
When weather/time permits, I am going to do some shots with varying cheek weld, as well as bench support w/o bipod, using the gun vise and sand bags instead. These additional measurements should isolate out these other factors (or perhaps verify them as the doiminant factors!).
Yep, one of my life's was around radio wave propagation, later mixed with aerodynamics and other phyical objects! :) Especially the cheap .22's and light barreled AR's we play with are effected by all these things. My wife is left handed and consistantly we see with her 10-22 being zeroed to a gnats butt, the POI will shift 6-8 MOA in the hands of a right-hander, all due to sling tension cause it doesn't change much at all when no slings are used.
The slings depending on where attached and how soft the stocks are, change the contact points a lot!
I unknowingly used to zero or test loads off a bench or bags. All I did was find what the rifle liked without the human attached. The best loads on the bags won't necessarily be the best slung up. We are part of a "package". If I'm a 2 MOA shooter and my rifle/ammo is 2 MOA, my groups will be 4 MOA. If I better the rifle or find better ammo, that brings it down to 1 MOA, my groups are now 3 MOA. It's all relative, now if I can get my rifle under 1 and me under 1, now we're talking! :)....O.L.
Configuration for First set of today's tests - Front and Mid Bipod and Vise - Vise mount shown below - 2 shots each Bipod with cheek weld, 2 shots each on vise with no contact other than trigger (plus final verification shot # 40):
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_ON_THE_VISE.jpg)
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Configuration for Second set of today's tests - MidSling attachment (courtesy of Bubba) - 11 shots mixed between Sitting and Prone:
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_MID_BIPOD_MID_SLING_s.jpg)
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Here is the difference in POI data:
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_Sling_vs_Bipod_MID_MID.jpg)
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First off, the top dot data shows that there is in fact a difference between shooting w/o cheek weld and shooting with cheek weld. Slim and ola, where should I send my next months AppleSeed check? :bow:
It is around 1.5 MOA or so between cheek and no cheek weld. So, I still contend that barrel bend from the strain of the sling, bipod, is what is mostly responsible for the 5 1/2 MOA shift in POI shown in my first post.
Also nice to see is that mounting the sling stud away front the front of the rifle, and more toward the receiver, makes the POI shift from using the Mid Mount Bipod to using the Mid Attached Sling close to nil. You can see that shots 29-39 (Mid Sling) are pretty much right over the top of shots 25-28 and 40 (Mid Bipod).
Guess I can put off getting the barrel floated on the ol' FA91 ::)
Quote from: KnowFear on March 27, 2012, 08:37:12 PMFirst off, the top dot data shows that there is in fact a difference between shooting w/o cheek weld and shooting with cheek weld. Slim and ola, where should I send my next months AppleSeed check? :bow:
Just make it out to - "Posterity" and cash it at the next Appleseed. Huzzah!!!
Quote from: KnowFear on March 27, 2012, 08:37:12 PMGuess I can put off getting the barrel floated on the ol' FA91 ::)
I'd find a good spot and stick with it. You really shouldn't need to move it once you have it anchored. And, once you're anchored, you can set your zero and be done with it. For field expedient shooting off an improvised rest, as long as you're laying your bipod (legs folded) on top of whatever you're shooting off of, it should still put the "strain" in the same place.
Since you're testing stuff, would you mind testing one more thing? There's an improvised rest you can do by looping a rope around a tree (pole, fencepost, whatever) and then twisting your barrel though it. In effect, you're "slinging up" to the tree. (As seen on the movie "Savior" with Dennis Quaid. Great movie, by the way.) Can you see if that "strains" your barrel at all? Thanks in advance. And that's one more tool for your tool box.
Doh - my backyard is filled with gophers (well, a few less after the AS class), but no trees!
Will rig up something though and give it a try.
Finished up the measurements today.
Set 1 Mid Mount Bipod, zeroed the scope at 25 yds
Set 2 Mid Mount Bipod group
Set 3 Mid Mount Bipod group -and- did the "Savior" movie "Quaid Sling"
Set 4 Mid Handguard Attachment Sling, sitting into stage 2 AQT
Set 5 Mid Handguard Attachment Sling, prone into stage 4 AQT
Set 6 Mid Handguard Attachment Sling, prone into stage 4 AQT
Set 7 Mid Handguard Attachment Sling, prone into stage 4 AQT
---- moved 3 clicks to the right to center up POI -----
Set 8 Mid Handguard Attachment Sling, prone into stage 4 AQT
Set 9 Mid Handguard Attachment Sling, sitting into stage 2 AQT
Set 10 Mid Mount Bipod group - final sanity check
From IMFDB at http://www.imfdb.org/wiki/Savior - the "Quaid Sling"
(http://www.imfdb.org/w/images/thumb/8/8d/Saviorgalil3.jpg/800px-Saviorgalil3.jpg)
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(http://i154.photobucket.com/albums/s257/KnowFear1111/Final_Test_1.jpg)
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(http://i154.photobucket.com/albums/s257/KnowFear1111/Final_Test_2.jpg)
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Shift in POI summary:
11 MOA - "Quaid Sling" at end of barrel
5 ½ MOA - Forward Mount Bipod vs. Forward Attached Sling
4 MOA - Mid Mount Bipod to Forward Attached Sling
1 ½ MOA Mid Mount Bipod vs. Forward Mount Bipod
1 MOA Good cheek weld vs. Crummy cheek weld
< 1 MOA Mid Mount Bipod vs. Mid Attached Sling O0
(http://i154.photobucket.com/albums/s257/KnowFear1111/Summary_1s.jpg)
Some conclusions I drew from the tests -
...Biggest shift in POI is the "Quaid Sling" - 11 MOA - yikes! Only in Hollywood...
...It helps reduce the shift in POI to move both the bipod and the sling as rearward as possible, less leverage and deflection by both. With my bipod and sling mounted as shown below, POI's are nearly right on top of each other, a far cry from the 5 ½ MOA that I started out with...
...Use a good cheek weld - Not only does it promote consistency, but I also saw a huge improvement in gun control - now that I am paying attention to cheek weld, the gun is coming back down to near center after a shot, rather than ending up on the adjacent shooters target!...
Final configuration, Bipod and Sling as rear-ward as possible:
(http://i154.photobucket.com/albums/s257/KnowFear1111/FA91_Final_Config.jpg)
Fun test - got me some more practice shooting the AS way, learned some stuff by doing the tests as well as by tapping the brain pans of the forum members here, and allows me to shoot off the bench or slung up with only a few clicks of tweeking. Of course a mid mount isn't the ideal location for the bipod for shooting small groups, but it is a tradeoff I'm willing to make to have the POI at nearly the same point as sling shooting.
Hope to give the FA91 another try at shooting the score next Castro Valley event, volunteer duties permitting.