FAIL... confusing POI/POA between stages

[akula88]

Last weekend’s medium range rifle shoot was the most challenging in my(our) limited/novice trigger time in (medium-range) rifle competition.

My rifle supposedly had an initial zero at 41mtrs for a 200mtr return zero. At 100 meters it was at around 2” high POI over POA. My home range only has at most 110 meters (with a slight 10-deg angle). I have to rely on these figures to compute for 150 and 200 POIs. We had no access to a longer range.

Equipment : 16” BCM middy 410 SST 1/8 with VTAC-TRX 11”, Vortex PST 1-4x in Bobro Mount, Geiselle Hi-Speed, Bobro gen2 bipod. Ammo – 69gr SMK with 22.7gr VV N133.


We had to drive about 7 hours to the competition range. It had a large wide bay divided into 4 competition stages, while there was also narrower one that was offered for zeroing purposes. In between was a hill about 12 meters high. I reconfirmed my scope’s zero with a handful of table shoots (using the bipod) and it was hitting about 2-3” high at 100 and 150 meters POI over POA. An 8x8 steel plate at 200 was gong’ed. Temperature around 92-98-degF.

NOW – going into my first stage – it had a horizontal log placed as a barricade. It’s height was awkward so its either you shoot at high prone using handguard as support, or my bipod in unstable position on top of the log. I chose the later. When it was my turn, I had difficulty hitting the 150m plate array, so I decided to switch to the 185-200 plates. To my dismay, I had to hold about 2” up high (12 o’clock) above the 8-10” plates. POI is already in downward trajectory when they are supposed to be DEAD CENTER or still upward at this distance. When the 150m plates were engaged, I still had a 12 o’clock hold. I thought it was just me and bipod ‘harmonics’ but… my second stage which was done with a 30-rdr mag as support (bipod is forbidden in this stage), similar POI/POA differentials happened. Most of my teammates also noticed the same confusing trajectories. It seemed that bullet velocity was slower in this bay. I had to make a 3-clicks (.2 MRAD per click) to correct the disparity for the other two stages

Anybody had similar experience where each bays have different bullet drop profiles? Is there some wind 'vortex' sucking down our bullets at that particular bay?

[sinister]

Your bullets are going exactly where you're aiming for where the scope is zeroed.

You don't understand your cartridge's trajectory so you don't know where to (offset) aim.

If you were shooting at standard Army F and E-type targets you'd be killing them, but you're shooting smaller targets and the trajectory and off-set are hurting your feelings.

The 1-4 scope should be more than enough (even overkill) for 100-200 yard shots.

[Cameron]

I had a hard time following.

So you have a 200m zero but have not zeroed your rifle at 200m?

You zeroed it 2" high at 100m?

You checked zero at both 100 and 150m where your POI was 2-3" higher than POA at both ranges? Was is 2" high or 3" high at 100 or 150?

You found your POI was approx 6-8" low at 185-200m on the first string? (having to hold 2" high on 8-10" plates)

Was there are large change in elevation or temperature from when you ran your dope?

I would say you probably didn't have a true 200m zero, and the 2-3" high at 100m is actually a 1MOA difference between 2" and 3" so at 200m that means at least 2" of POA/POI delta the unknown zero at 100m.

I would suggest zeroing your rifle at an actual known range of 100m and re-running the numbers for your dope.

Cameron

[akula88]

The ammo (69SMK) was actually zeroed at 41meters (POA-POI). 100 meter POI was confirmed at around 2.5" up over POA (at HOME range).

I used PointBlank/6mmmbr.com trajectory to compute for the approximate 200-meter zero, and 150mtr hold-overs. I'm saying around since I can't keep a tight group with my smallish dot.

How I wish I could have actually zeroed my rifle/ammo for that particular competition range, but it was no longer possible on the game day. Zero was allowed on another bay. I zeroed around 8am, and was shooting my first stage at around 10am, probably a plus 10-degreeF temperature differentials.

[Cameron]

I don't think it was a temp difference then, it is definitely a zero issue. A 41m zero is NOT a 200m zero no matter what the table says. If you are running dope off a 41m zero you are going to be way off. Then checking your zero again at 100 and 150 you are running a supposed 200m zero that has only been shot at 41, 100 and 150 meters...

The most telling thing here is that the delta between 100 and 200m should not be more than a few inches and you where off by more than 3MOA at 200m... So you zero and your data were off. What velocity were you getting when you zeroed?

I would zero at 100m on your home range, chrono your load, check the BC for the round and re-run the data. It is really the only thing you can do.

Cameron

[akula88]

Thanks, Cameron.

[Alpha Sierra]

I'm going to say failure to execute marksmanship fundamentals correctly + failure to understand bullet trajectory = FAIL

8x10 plates at 200 meters or closer is not that hard.

[Cameron]

I'm going to say failure to execute marksmanship fundamentals correctly + failure to understand bullet trajectory = FAIL

8x10 plates at 200 meters or closer is not that hard.

Tough crowd in here this morning...

Cameron

[FromMyColdDeadHand]

14 hours in a car (7 both ways?) to run a match, kudos to you. If you confirmed your zero before the match and then during the match you weren't hitting where you aimed, outside of a loose optic mount it sounds like a bit of 'buzzer fever'?

[shootist~]

It’s height was awkward so its either you shoot at high prone using handguard as support, or my bipod in unstable position on top of the log.

...it sounds like a bit of 'buzzer fever'?

I have to agree - getting you out of your comfort zone can be a killer. I've seen as much as 1 MOA difference when changing from a sandbaged bi-pod zero to prone with a bi-pod.

A 40m zero does equal a 200M zero for my 77 gr SMK load at 2,700 fps with a sight height of 2.6" - although I would only trust the rifle when actually zeroed at 200. And 2" high at 100M is correct as well.

Try to replicate the shooting position on your home range and see what happens. A chronograph check would be advised as well.