Horizontal POI Shift with ammo change, SR15 upper.

[TMS951]

On a recent trip to the range I was sighting in a new optic, a Nightforce 1-4 nxs.

I was trying different ammo's so I could see what kind of poi shift I would get and be able to take this into account. I expected vertical shift, essentially this is what I was looking to record. I do this on my other guns so I know what my hold overs are with different ammo.


As I changed ammo, I started seeing horizontal shift, in addition to the vertical shift I was expecting.

I found it frustrating and thought it was my shooting and kept chasing this horizontal shift thinking the zero was off. After a while I found it was quite consistent and called it a day.

When I came home and researched it I found it was not uncommon, and not my shooting.


So here's my question, what causes this? My research revealed it was "barrel harmonics" and "point in the cycle" the bullet leaves the barrel. This second one had me thinking about the SR15s gas system length, could this be the culprit?


Equipment used:
KAC SR15 mod.0 upper with
Geissele MK.1 mod.1 15" SMR rail
Vltor pinned gas block
YHM 5C1 comp.

Ammo:
75gr. Prvi match ammo, Zeroed with this ammo.
62 gr. Federal XM223SP1 (62gr. gold dot) POI up 3moa, right 1moa
62 gr. ATI M855 up 6moa, right 3-4 moa

Other uppers I have seen vertical poi impact with, but no horizontal using the same ammo.

DD LW 14.5" hammer forged, carbine gas
shaved FSB,
LMT enhanched BCG,
YHM 5C1 comp
DD 12" lite rail.

Noveske 18" SPR intermediate gas,
Pinned Vltor gas block
Vltor 12" VIS
Young NM BCG
Smith Vortex

LWRCi 14.5" M6A2,
pinned battlecomp

The above three all have similar vertical POI shift, but no horizontal POI shift that I notice. The ammo is all from the same batches.


Can this be fixed or changed? Or is it just not a great barrel and I'm stuck with it. It kind of sucks because of customizing this upper and having sunk some cash into it. I was really hoping to build the be all and end all of super reliable, super durable AR uppers, I have done that now, but it turns out to just not be a shooter which is a bit of a disappointment.

[Boba Fett v2]

How much of a horizontal shift are you observing? Is it consistent?

[Failure2Stop]

I've seen stuff shift all over the place.
Yes, I work at KAC, so I understand if you might think that I am skewing data, but all I can do is promise that I am not.

To truly evaluate ammo shift you need a bench and magnification in the 10+ area.
Sometimes shift is due to shooter perception of the sights/target and/or influence on the gun.

Different guns do different things with different ammo. Maintain a log if this kind of data is important to you.



Typos brought to you via Tapatalk and autocorrect.

[TehLlama]

"Different guns do different things with different ammo" - very true, and not all voodoo.

Barrel harmonics, the particular yaw/angle of attach a bullet will leave at, small things about how powder and velocity make the crown of that particular barrel impart different forces - tons of stuff can make small differences.

Also, PPU ammunition isn't that consistent. It's still my absolute favorite stuff, and the 75gr is simply the best value on somewhat accurate ammunition, but it's still 1.5MOA ammunition to me. I'll get some noticeably tighter groups off my Mk12 barrel, but if I'm talking extreme spread expectations, I still consider that stuff to be 1.5MOA ammunition.

Finally, I'd say that some POI shift isn't worth worrying about, and to really get a handle on it, work on more data points for repeatability. A 1MOA shift is pretty reasonable to expect if you're changing bullet shape, weight, sectional density, and draw direction; powder volume, brand, and cut; as well as probably different primers and cases - that's a lot of variables, and even more since you're not firing them necessarily side by side, so environmental factors probably played a role.

To really conclusively say the ammo is causing a horizontal POI shift, load a magazine with twenty rounds, potpourri mixed with ten of each - fire them all, and see if you actually get two distinct group centers before you really feel confident that's it's all hardware.

With that 3-4MOA shift, some of it logically is hardware, and it's not a bad thing (other stuff like wind, lighting changes if it's a North-South range, temperature) can account for that, but some of it is likely just hardware (different bullet construction on the M855 style rounds can absolutely lead to a different sort of fleet yaw characteristic coming from the same barrel.

My Mod0 upper actually seemed happiest shooting the Mk318 overrun ammunition I snagged a couple years back - with the exception of the 5% flyers (ammo related, known issue - or me sucking) that stuff was 1.5MOA ammo or better through that barrel with just a 4x optic. Sometimes barrels are partial to one loading, and 62gr SOST worked really well for that upper. Since then, it's been fed almost exclusively 55gr PPU anyway, because that's been my backup/class rifle.


Really, just pick the ONE cartridge you care about being accurate with, and get a solid zero with that. Right now my Mk12 upper is actually set up for 75gr PPU, just because that's what I shoot most through it, while my Recce upper is set up for Mk262. My shorter HD type carbines are all zeroed with IMI M193 - but I don't much care about deviation when running other rounds, because when I care about accuracy (relatively speaking) through that upper, those are the rounds I intend to use.

[WS6]

62gr Bonded tactical has ALWAYS shifted laterally for me out of ANY gun I have shot it, whether I was shooting LE223T3, or FBIT3.

[mtdawg169]

I often get some horizontal shift in the 1-2 moa range when shooting different loads through the same gun. It happens in my SR15's and my Noveske barreled SPR.

[waffentomas]

I have a bear load that I shoot through my .308 bolt action hunting rifle that requires 8 clicks of right windage at 100 yards. It's a 150gr CT Failsafe.

My normal elk load is a 180gr WW Failsafe.

I never would have thought there could be so much difference a few years ago, but after wasting a lot of ammo testing it due to my stubborn disbelief, I finally gave in to it.

Tom

[aguila327]

Out of curiousity, can you give us some numbers as far as the POI shift?

I have the same issue on one of my AR's but its negligible, (in my case) but I have a .308 bolt gum that amazes me. It shifts inches to the right and up when certain ammunition.

[littlejerry]

I have a Colt 20" barrel that is zeroed for PPU M193. When I shoot Federal Fusion/Gold Dots it requires 4 clicks to the right. When I shoot Brown Bear it requires 5 clicks to the left.

Vertical spread is almost non existent.

[steyrman13]

I have seen this with other calibers with different weight bullets. It could be that barrel would shoot 75-80 grain fine but the lighter the bullet the more it will shift. Try some 55 out of it and see how the poi is. It could be the true twist rates of your barrels are different.

[Joe Mamma]

I have also seen horizontal shift in a 5.56/223 with different ammo. It was significant (a few inches at 100 yards). I never would have believed it if I didn't experience it myself.

It bothered me. But I stopped thinking about it when I realized how much time and money it would take me to figure out exactly what is going on.

Joe Mamma

[maw1777]

This is normal I've seen it in bolt guns and semi's

[TMS951]

Ammo:
75gr. Prvi match ammo, Zeroed with this ammo.
62 gr. Federal XM223SP1 (62gr. gold dot) POI up 3moa, right 1moa
62 gr. ATI M855 up 6moa, right 3-4 moa


All of this was done at 100 yards, with a bipod and a bag in the back.

The 75gr. Prvi was shooting about 2 moa groups, and the XM223SP1 was about 3 moa groups. The ATI stuff opened up to an easy 6 moa group.


I guess it was unfair of me to so directly blame the KAC gas system and didn't really mean to make that accusation. I was more interested in if the gas system could come into play.

I guess since this happens with bolt guns its more just about the barrel than the gas system and movement of the action? Or does the movement of the action come into play as well?

I read a post on another forum where the gun in question was a SCAR L. Interestingly enough he was using a YHM 5C1 as well, switched to a Vortex and the phenomenon lessened. I don't really want a open bottomed or open ended muzzle device on this gun is my only issue. But i am tempted to try my vortex off my SPR on this upper.


What I am really after here is the very technical side of why this happens, the mechanics of it.I am really just after better understanding and more knowledge. I have accepted it is the gun and I will live with it, and luckily I only put a couple hundred rounds through it before coming to this conclusion.

[T2C]

I have seen video of rifle barrels whipping as the weapon is fired and I found it interesting. An old school gunsmith told me that you had to think of a barrel as a tuning fork while making changes to a rifle to enhance accuracy.

Could your shift in POI with different rounds be due to how the rifle recoils with a slight difference with different rounds? Could different amounts of gas being pushed through the gas system have an effect on POI? Do you think the barrel whips slightly different with each round?

A high speed camera like one used at some firearm factories would be handy, but way out of most of our budgets.

[Jippo]

Barrel is vibrating in all axis when fired. Muzzle moves in horisontal and vertical axis while the bullet travels inside the bore. Different loads cause variations in tthis vibration and thus hit in different places as they exit the muzzle is in other phase of the vibration wwhen the bullet exits the muzzle. By luck two loads can have same POI, but that is 'luck'.

Also anything touching the barrel will change the way the barrel vibrates, meaning e.g. there are no suppressors that do not change the POI of a rifle. Vibrations are the reason why free float handguards are such a good idea: external forces affecting the harmonics don't.

[dwkfym]

It usually vibrates in a triangular pattern. Harmonics are usually tuned to a certain range of ammunition.

Also bores are not true to the center axis of the barrel. Usually you want the barrel to be 'off' in a way that the bullet is moving 'up' when you fire the rifle. I don't know too much about the subject. Its precision rifle stuff.

If it shifts with a different ammo, but it still groups relatively consistently, I wouldn't worry about it too much.

[TehLlama]

The harmonics of the barrel while a supersonic projectile is getting shoved and torqued through it obviously play a role, as described above. Changing the input impulse (from the powder charge), even if just chronologically shifting it will do that, changing the size and shape of the gas tube itself will affect that (since the gas block and tube are part of that resonating structure on a free floated rifle, so it will have its own resonant frequency, in addition to functioning like a cantilevered counterweight on the barrel itself) will have an effect on when, how much, and in what phase the barrel is vibrating. To your specific issue, this should affect accuracy using the same upper, although not too terribly much, but it does help determine what that upper's characteristics will be across various loads fed through it. Shoving a rifle round through a barrel is a very energetic process, and before a lot of that energy becomes heat, it is kinetic energy in the rifle in the form of oddly phased, dissonant vibrations of the barrel and attached components.

Another one is how the construction, material, and shape of the bullet also affect that - consider that the land and grooves are actually shearing against the outside surface of the bullet, all while imparting pretty significant amounts of torque - this is where the fleet yaw variation comes from, and why it varies so much when changing bullet construction/material/weight/length. From the leade forward, that bullet is being crammed against the lands as hard as possible, and deforming against the grooves - two really different bullets (different transitions from ogive to bearing surface, different material, different lengths of bearing surface - tons of stuff). Further, no rifling can be done perfectly, the bore concentricity can't be within microns across the entire distance, the surface treatment (if CL/NiB/other) won't be uniform to the atomic level, and the crystalline structure of the steel in the barrel is still subject to change across the barrel - while the barrel itself is going to stay the same from test load to test load, it will exhibit different behaviours when you're altering the specific curves of the energy inputs (propellant) and/or the internal ballistics on the projectile itself.

Consider the burn characteristics of that powder load - depending on when chronologically different pressure spikes happen, there will be different volumes of gas applying force to the back and sides of the bullet at different times - a good crown is super important on a precision rifle, but all that does is minimize some of the effects of the excess propellant gases and unburnt powder will have on altering the trajectory - it's still going to nudge the bullet slightly, and since it's right at muzzle the angular deflection caused by that is going to be big compared to the comparatively miniscule forces, but that's why minimizing those differences is important, and also why suppressors seem to improve accuracy across different loads.

The Vortex will and won't change things - sure, it affects harmonics and such, but in my estimation the biggest thing the vortex accomplishes is allowing the excess propellant to begin dissipating much sooner, and then the inward-facing triangle (front-on viewed geometry of the hider) allows that excess gas to travel away from the bullet instead of reflecting back towards it. It has a lot less to do with the chiral arrangement of the prongs, and a lot more to do with the fact that the gas is allowed to escape quickly, in a somewhat uniform fashion - the only part I consider valuable about the semi-helical shaping of the prongs is that the FH wants to self-tighten on the barrel, which makes the exact load on the threads at the outside of the barrel by the crown as consistent as possible - on a bull barrel irrelevant really, but on a more sane profile, this might help a tiny bit.


When changing construction of the bullet, I think it's pretty reasonable to expect that the actual yaw angle will change with any given barrel, anything but that is just a happy accident. From FMJ, to Green Tip, to reverse drawn OTM, to the Barnes/SOST style bullets, if you think about the fact that you're taking a small, malleable little thing, and shoving it really hard down a pipe that is intentionally just a little bit to small, and expecting the bullet to deform, and once deformed into the cross-section of that rifling begin spinning at up to ~300,000 rpm - the angle of attack that thing will demonstrate after such a harrowing experience being consistent is a miracle of modern ammunition engineering that it's consistent within one box of ammunition, let alone the fact that it's comparable from one brand to another.

[TMS951]

TehLlama, Thank you for taking the time to write that very informative post. It has answered my questions and then some.

[Dano5326]

When I used to shoot with varying groups.. my red dot had colored tic marks, in vertical & horizontal adjust, for the 62, 70, 77gr ea bunch used.

Horizontal shift is normal.