Change in point of aim - causes

[BrenGun]

Hello AH

My 1st post - so please if I have omitted anything - my apologies ....

I am looking for some advice on changes in point of aim.
Recently I have been to our shooting range on 3 different occasions.
Each time with a different caliber - 270, 300H&H & finally a 375H&H

I noticed that all 3 rifles were shooting 2" - 3" higher than where I had them set. The group was still there. Left & right = no problem.

I am a hunter not a target shooter.
All my rifles are set at - 2" - 2.5" high at 100m
I try my best not to shoot paper - rather a gong at 200m & 300m. (clearly should adopt this more ...)
Ammo - my loads for the rifles

What could cause this?
Me 1st up. Could my eyes be changing their close sighed focus? 48yrs old no glasses.
Time of day? (hot all 3 days - temp 25'C - 30'C) BUT the last time I shot paper would have been in the winter so cooler by maybe 5'C - 10'C
Time of year? (summer) BUT last time would have been winter, with all 3 rifles.

I dont want to fiddle too much, so any input or advice would be welcome.

Regards

 

[Ray B]

If I understand your situation, You have three rifles, all sighted to hit 2-2.5" high at 100m; then on three different occasions you fired the three rifles and all hit about 5" high from point of aim (3 inches higher than previous point of impact).

Since all rifles had the same change I would conclude that it wasn't something that happened to the rifle. since there were three different cartridges, probably not a change in the cartridges.

This would leave two possibilities:
1. the change in temperature was such that it causes a change in impact and since all cartridges were equally affected, it could have been a result of increased powder temperature leading to changes in velocity. or
2. The change in temperature resulted in you having more/less clothing on which resulted in a different holding of the rifles- this would be my first thing to check. Try reshooting the rifles, being very aware of what clothing you have on, how you are holding the rifle. Also, if from a bench try this:

Right before you are going to press the trigger, close your eyes, take a breath then reopen your eyes and see if you are still holding the same sight picture; then after you shoot, your rifle has recoiled and bounced back down to the rest, look through the scope/sights and see where the sights are. This will give you an idea of how your body alignment is affecting the groups.

 

[mark-hunter]

@BrenGun

First change of POI:
- weather and climate. If you zeroed rifles on cold winter day, then the rifles will shoot visibly higher on warm summer day, visible alreday at 100 meters

- AT 300 meters, other things come to play. (this is begining of medium range, for long range shooters)
- Mirage (most probably will give you higher point of impact)
- Wind - drift to side down the wind
- spin drift, with more effect with some wind (right handed twist, brings the bullet to the right in calm weather condition, with wind from left, spin drift will bring bullet down and right, and right wind will bring the bullet left and up)

All above will be very well visible if:
- you make perfect zero at 100 meters, re-click the scope to 300 with calculated bullet drop.
- make balisitc tables, based on exact BC, and chrony measured velocity
- comparing results with actual clicks
- not making above calculations, just using the hunters set up +2 inch hight at 100, but still shooting with well zeroed rifle, will make you notice the change of POI, but calculation will give you exact answers. You can use hornady ballistic software on their web site, if you dont have chrony, just deduct average 20 m/s from factory ammo declared velocity.

I have noticed that if we are speaking about visible POI change, (and I shoot on paper, regularly, which gives the best reference on hits): then based on last three factors, POI can change day after day.

If you dont shoot too often, then good habbit would be to write down in rifle log, how you zeroed the rifle. This is to avoid situation when not too certain how it was zeroed earlier.

And all above, for hunting purposes at normal distances up to 200 meters, is very much irrelevant. With well zeroed rifle, you will be able to hit vital zone, of average size game.

 

[BrenGun]

Thank you very much @Ray B & mark-hunter

There is a huge amount of relevance in both your answers. I understand that wind & heat make a difference but did not think it would be hugely relevant at 100m.
Zeroing and setting a scope to a hunters 2" compared to actual clicks based on a chrony & BC also makes sense - as I have noticed changes on different days. I just put it down to me.

I do shoot fairly regularly and therefore have noticed much of what you have explained so clearly for me.
I do keep a file with my targets and dates. I mark the shots as well.
I do shoot from the bench only from 100m and on paper. So I will take note of what I wear (my Uncle got his SA colors for clays and it was imperative he wore the same kit. From Cap to socks and shoes)
I dont have to go to that extreme but I understand the concept.
I am going to adopt the aiming procedure as well, of looking through the scope closing my eyes and looking again.
Thank you

 

[mark-hunter]

Just by the fact, that you have noticed the differences in POI, it makes a good basis for further improvements.
I know many many plain simple hunters that never noticed any difference in POI on winter/summer/mirage/small wind - conditions etc.
So, I am sure you will refine further in this.
And, btw, welcome to the forum.

 

[Fixfire]

Not sure if already stated, but I notice when sighting in my rifle, it may take a number of shot to finalize - thus heating the barrel. Before a hunt, perhaps a month or two later, I’ll revisit and take a shot or two just to be sure. Always seems to shoot higher - Cold barrel? Drives me crazy.

 

[Berettaco]

Good morning @BrenGun, what are you using for a rest? Bag, tripod, shooting rest, etc? The reasons stated above could be the answer but another reason might be cheek weld and positioning. Looking at the all the variables that one would seem to be the most likely

 

[Mark A Ouellette]

If all three rifles have the same Point of Impact (POI) change, what is a common factor? The shooter!

A firearm is moving in recoil as soon as the ejecta (projectile, wad if any, and powder/expanding gas) start moving down the barrel. If one presents a different "opposition to recoil" the POI will change.

I had a 100 yard POI change of over a foot when moving from a lead sled to shooing off the sticks with a .458 Lott. Go figure?

Analyze your shooting position for possible changes,

 

[PARA45]

Maybe, maybe not. By any chance are you using the same powder?

 

[Rubberhead]

Short version of a longer story: I missed two deer in a two year period. I don't usually shoot more than 100 yards so I was baffled by the clean misses. Then it hit me, I missed both deer when shooting straight in front of the same stand. The shooting rail on that stand is about a foot closer on the sides than shooting straight ahead. I was resting the gun on the barrel instead of the forestock. The slight upward pressure directly on the barrel was causing the gun to shoot high. Next deer from that stand, I made sure to rest on the fore arm and bang, flop.

 

[Fixfire]

Short version of a longer story: I missed two deer in a two year period. I don't usually shoot more than 100 yards so I was baffled by the clean misses. Then it hit me, I missed both deer when shooting straight in front of the same stand. The shooting rail on that stand is about a foot closer on the sides than shooting straight ahead. I was resting the gun on the barrel instead of the forestock. The slight upward pressure directly on the barrel was causing the gun to shoot high. Next deer from that stand, I made sure to rest on the fore arm and bang, flop.

Same has happened to me. In the heat of the moment on an Elk hunt I didn’t realize the barrel was resting on the sticks - miss! Significant muzzle jump resulted. Worked out for the good, the next day I shot a much larger bull. It’s now the first thing I process in my progression prior to pulling the trigger.

 

[Shootist43]

BrenGun, you have received a lot of good information. As I was reading the posts starting from the top, I began formulating my list of possible reasons. IMHO Mark A Ouellette has probably hit the nail on the head. You need to control the forend of your firearm. POI will change if shot off different types of rests. Attached is a link to an article by an accomplished "Long Range shooter i.e. Nathan Foster that explains what I think is your issue.

Hold that Forend!

 

[Desperatezulu]

Hi BrenGun,

Lots of details needed (but still missing) to come up with an internet solution
As has been mentioned, the operator is usually the source of error, rather than the mechanical tool. The regularity of your deviation in results points to a technique/set-up issue.

1. Are you using the identical set up at the range across the various trips - bench/stool, x-bags or rest etc?
2. What ammo are you using - factory or reload? Are you using the same ammo across the various range trips (from the same batch)?
3. It sounds like you're in SA and if you reload, you probably used Somchem powder - my own experimentation indicates that it's quite temperature sensitive - so that's another possible source of error. Of course if you've used different batches of Somchem then its inconsistencies could cause havoc too.
4. Do you chrony your shots? That would quickly tell you if you've got big velocity deviations which could explain things.
5. Do you have silencers on your guns? Were the silencers fitted on all occasions?
6. You didn't perhaps check/adjust zeros shooting from a vehicle? The different height versus the target can upset things.
7. Summer vs winter could mean big humidity swings depending where you live, which might cause warping of wooden stocks - unlikely to cause such a consistent deviation across 3 rifles though. And you may have synthetic stocks which would rule this out.

 

[Fixfire]

BrenGun, you have received a lot of good information. As I was reading the posts starting from the top, I began formulating my list of possible reasons. IMHO Mark A Ouellette has probably hit the nail on the head. You need to control the forend of your firearm. POI will change if shot off different types of rests. Attached is a link to an article by an accomplished "Long Range shooter i.e. Nathan Foster that explains what I think is your issue.

Hold that Forend!

Thanks for this article!!! I was never comfortable or adapted any shooting techniques that didn’t utilize firearm control. Just doesn't seem natural. I’ve always utilized the sling wrap using a military sling as well.

 

[mark-hunter]

@Fixfire
Cold shot, some inches in unexpected direction (looking like flyers) made me crazy for some time.
It turned out that I did not clean barrel thoroguhy or properly, and had left some oil inside, this caused first shot stray. Its hard to make barrel dry just by using patches or rags.

Once I started drying the barrel before shooting, by using spray break cleaner, this made barrel dry, and first cold barrel shot was centered. No more flyers.

@Rubberhead
resting the barrel on hard object before shot, changes the barrel harmonics. In such case shots will be much higher.

 

[Rubberhead]

resting the barrel on hard object before shot, changes the barrel harmonics. In such case shots will be much higher.

So, resting it on something soft like a padded shooting rail, won't change the barrel harmonics? Why is it always high when the harmonics are changed? It seems like if it were barrel harmonics the elevation would be affected by where the barrel is rested on something hard and could go either up or down depending on where in the sine wave harmonic the barrel touches. I'm probably overthinking this, though.

 

[C.W. Richter]

If PV=NrT, then T=PV/nR and P=nRT/V Pressure (resultant velocity) is dependent upon Temperature. Elevation has same effect (many African countries are 1,000s if not +5,000 ft in elevation from where many coastal shooters live.) If you handload, you have the choice to select less temperature-sensitive powders. I've experienced some longer range misses when the sight-in was checked at the range at 45F, only to take a shot of a lifetime at -14F, and miss, due to what you say plus a split second opportunity for the shot. Also opposite effect going to tropical locales, but you can correct for this by sighting in on the hottest days (90sF) at home. T is important. If you check your sight in in diff. weather, take note, and make appropriate corrections prior to shooting. That's what I do NOW!

 

[C.W. Richter]

BrenGun, you have received a lot of good information. As I was reading the posts starting from the top, I began formulating my list of possible reasons. IMHO Mark A Ouellette has probably hit the nail on the head. You need to control the forend of your firearm. POI will change if shot off different types of rests. Attached is a link to an article by an accomplished "Long Range shooter i.e. Nathan Foster that explains what I think is your issue.

Hold that Forend!

When i see people NOT holding the forend, I cringe, when seeing all of those poorly placed shots in videos/tv shows. Esp. w/ larger caliber DG rifles!

 

[mark-hunter]

Why is it always high when the harmonics are changed?

The answer that I use to answer myself this question:

Harmonics in free floating (or free movement) barrel work wavelike in all directions. left, right, up and down.
If you put the barrel on hard object below, harmonics or vibrations of barrel are neutralised in lower 180 degrees of vibration sector, and becasue of forces reaction to hard object they are increased in upper 180 degrees sector. Barrel will "jump" higher, and shoot higher. (significantly)

This you can try on the range. See what will you get.

If you rest the rifle on soft object, on the forend stock, barrel harmonics will not be affected because of free floated barrel - which is not touching the stock (which is common standard in modern factory rifle).
In a blind, if nothing else is available, put your hand under the forend stock, when resting. This will be sufficient, and acceptable best practise. You can use a blanket, or a sand bag as well, or backpack.

Soft object below, on resting forend stock, will partly only compensate vertical part of recoil movement, can only improve accuracy, (better put is, it will allow free and normal natural recoil of rifle) if not for any other reason, then for subjective reasons of the shooter, but in that case it does not affect barrel harmonics.

 

[sgt_zim]

I do not believe temp/humidity is your issue, especially at hunting distances of 300 yards or closer.

Bullets will sail a bit in warmer weather, but you really don't start seeing much effect until you get out to say 500 yards.

I just ran a test through a ballistic calculator
160 gr bullet at 2900 fps with BC of .45, 100 yard zero, 90ºF = 49.4" of holdover at 500 yards
same variables as before, except tempt at 40ºF = 51.5" holdover

The effect will be greater in low BC bullets, and less in high BC bullets. But the change is still quite small, even considering different BCs