??? Point of impact changes when shooting AR prone instead of seated ???

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I sighted in my AR rifle at 50 yards with Aimpoint PRO and Burris 3x magnifier from seated position at bench. Target was essentially in-line with the rifle bore, or just slightly lower (~4 inches?). Shot groups were fine, small and compact, reproducible mag after mag. Then, when I went prone and fired same rifle, ammo, target (now target was "uphill" relative to my being prone on the ground) the groups were tight, compact, reproducible mag after mag but were about 3 inches below the point of aim. Went back to bench seat and used same rifle, ammo, target and shots were again all POA=POI. Went prone again, tried to be very careful, shots were 3 inches tight group below POA.

What am I doing wrong? Do you have to sight in separately or differently for prone vs. bench rest seated? Just for info, I also shot 100 yards under same conditions, and with bench seated shooting, the 100 yard shots were a little higher at 100 than they were at 50 yards (that is to be expected). At 100 yards from prone, the 100 yard shot group was essentially POA=POI whereas the prone 50 yards shot group was low. In other words, the shot group was lower by about same amount at 50 yards and 100 yards when shooting prone compared to bench seat.

What am I doing wrong?

Rifle is a 5.56/.223 Bushmaster AR 14.5 inch barrel+flash hider from mid-2000's (before BM was bought by Freedom Group/Remington). Ammo is home reloaded 55 gr FMJ-BT over Win748 powder with previously loaded but fully processed brass.
 
It's not uncommon to have a poi shift based on shooting position. The cheek weld you get from sitting to prone often shifts. Also changing from bench shooting to prone, I would guess, would also include stability of the rifle and your follow through. Are you using a rear support on the bench? Are you also using a rear support when prone? On the bench are you using the same front support as you are in the prone position?

Consistency is the key, and I would guess that you could eliminate some of the variables from one position to the other to minimize some of your point of impact shift, but you *probably* will have some amount of poi shift.
 
Were you using a sling or bipod going prone? If you were maybe pressure on the handguard doing it. Do you have slob between the upper and lower?
 
It seems likely that the 3x magnifier is causing or increasing parallax, so that a slight shift in your eye location behind the optics is causing the aiming point to shift. The optics are all probably regulated to work this out at 100 yards, but not closer. Remove the magnifier and try again.
 
It's the change in alignment with your hold and eye.
When you are shooting from the rested position the gun is probably lower in the shoulder and you are most likely leaning over the stock to some extent.
From the prone position it's a straight alignment.
My guess is you need to raise your resting point when shooting from the bench.
 
Is the barrel free floated?

Are you resting the barrel/handguards on sandbags or a rest on the bench?

Do you use the same rest prone, or do you use your support hand or sling?

If your barrel is not free floated, changing how you support the front can and will change your POI. Cheekweld changes matter, but not nearly as much as going from say a sandbag to using a sling for support, or from a hard rest to just using your hand.
 
I'm willing to bet that it's all about how you hold the rifle in each position.

When prone you've got a lot more weight bearing down on your forward support hand and likely on your trigger hand as well. Plus your head is heavy and in an awkward position so you're likely bearing down with your cheek weld harder.

All of this means the rifle won't kick up as much during the recoil. Especially if when shooting seated you rely in a forward bag for support so that it's free to jump upwards.

Just for giggles try something for yourself. Shoot from standing using two widely different stances. For the first one lean WAY back so you look like one of those awkward newbies that leans back to counter balance the rifle. And I mean really exaggerate it. Shoot a group from that stance. Then shift your stance so you're leaning WAY forward. Get your back to a 45 or even lower angle. Like you're leaning over to shoot under a low barrier where your back is almost at the same angle it would be for prone.

I'd be willing to bet that your second group is noticeably lower than your first. And it'll be due to how you are supporting the rifle and your arms plus head in the different positions.
 
Are you using the mag as a monopod when prone? I've seen some ARs shoot high when the mag was used as a monopod
 
Not at all uncommon to have POI shift when changing positions. Every competitive shooter I know keeps a dope book with these adjustments included. It is caused by changing positions causing inconsistent cheek weld and eye placement behind the sight.
 
My money's on barrel support as the primary issue.

The AR design leaves POI subject to influence by where the front end of the gun's supported.
If you were to start at the bench by resting the flash hider on a sandbag, zero the gun there, and then move the sandbag back progressively inch by inch as you fire additional test groups, you'd see a corresponding shift in impacts relative to POA.
That's even when you move the support back under the handguards on a conventional AR.
It will decrease in severity the farther back you move the support.

There's enough barrel flex built into the design that the AR is much more vulnerable to changes in front end support variations than other designs that offer stock support to resist upward pressure on the barrel.

The two most obvious examples of this are the affect in impact shift created by zeroing without a bipod, then shooting with a bipod, and using the old traditional military 'sling-up".

Upward pressure created by the bipod farther out toward the muzzle than you'd normally hold by hand actually bends the barrel relative to the receivers, causing elevation shifts.
Side pressure created by the sling pulling the barrel to the left will cause horizontal impact shifts.

When you zero a NON-FREE-FLOATED AR at the bench, always support the front end as close to the receivers as possible.
The farther forward you support that barrel, the more flex you'll get.
A free-floated barrel doesn't do this, as long as it's supported on whatever handguard you use & not on any part of the barrel.

Denis
 
My money is on cheek weld. Stretch that turkey neck far forward on the stock in all positions for a repeatable platform of support regardless of being seated, prone, or standing
 
I'd be willing to bet that your second group is noticeably lower than your first. And it'll be due to how you are supporting the rifle and your arms plus head in the different positions.
Absolutely correct. And the lighter the rifle/more felt recoil the lower the poi will be. Noticed this deer hunting this fall with my 5lb 308. I shooting the game off hand standing versus bench and or prone sight in. I am going to resight shooting offhand at 100yds.
 
Not uncommon. Loui Awerbuck used to ask specifically in his rifle class if you zeroed it from the bench instead of prone for just this reason. I don't know why it changes.
 
cheek weld, I'd bet.

Me, I, I lay the corner of my jaw solid to the stock, and slide down to the natural stop on my cheek bone, and ( on an AR) check my distance from my nose to the charging handle (for me, if I rock my head slightly, it touches).

That's MY cheek weld- you may develop your own, but it's doing it just the same, every time, regardless of position that counts. you'll find some postures make it easier to get the sweet spot, and you may have to adjust your weld so you can make it just the same no matter how you are shooting.
 
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