Point of Impact shift - Red dot

[shawnfergie]

Hey Hey,

I have a Primary Arms M4 AA sight from 2010. I have had it on my MP15-22 ever since I got it.

I went shooting last week and noticed if I move the sight from one rail position to another that my point of impact would move about 10 - 15 MOA.

I placed the same PA red dot onto my MP Sport which has an aluminum rail and no MOA impact change between different rail sections(at lease it was close enough for my meager skills).

I didn't have the red dot on the handguard rails at all, just the receiver.

Has anyone else seen this issue? I'm thinking I have small burrs on the underside of the rail sections when I mount it to another section I get this dramatic movement.

 

[Kayback]

You'll probably find there is a shift in the 5.56mm but because the error doesn't affect the faster, "flatter" round it doesn't appear as much.

There will be a POI shift if you change the position of the sight.

It is probably just more apparent on the .22 because of it's rather arcing trajectory.

KBK

 

[Desert Five-0]

What distance were you shooting. Unless you were shooting a decent range it was probably not due to bullet drop. With red dots or other optics any time you remove them and later put them back on there will be some change. Some mounts do better at this than others. The ones that claim to hold a zero usually stay pretty close, however usually are off by a small amount. The difference may not even be noticed in a red dot or a shorter distance shooting.

If you want your optic to hold a zero leave it on the gun.

 

[w0ady]

ya the distance you were shooting at is important. id lean towards the different calibers being the cause more than the gun or mount.

 

[rraisley]

IMHO, the aluminum rail will not result in much POI difference because it is a precision machined item. The plastic rail on the 15-22 is moulded, rather than machined, and will naturally have variations. That's not to say it can't be accurate when removing and replacing a sight (I have a QD mount for both my red dot - also a PA M4 - and my scope, and find both return to the same zeros when remounting), but you pretty much have to put them back into the same location along the rail to do so. Just find out where you like them, and remember (or mark) the locations for remounting.

BTW, I feel a QD mount does this better than a standard mount, because the pressure applied is always the same, while when tightening screws, it will vary more.

 

[Maddmax]

BTW, I feel a QD mount does this better than a standard mount, because the pressure applied is always the same, while when tightening screws, it will vary more.

True on the screw mount. When I have to remove my Streamlight off my FS M&P 9 PRO you can see it big time when remounting. Normally I can get a re zero off my "Iron" sights at it's set 25 yard distance just by tightening the screw more. I'm going to try that on my AR15 with the red dot at a measured set distance for the iron sights and bring the dot to it for a re zero.

 

[shawnfergie]

I was shooting both guns at about 30 yards. Now I zeroed in the red dot for the 15-22 and then moved the sight to another position and had the impact change.

I then placed it on my MP15 Sport and then rezeroed again, then moved it to another rail position and it stay just about the same (as far as I could tell).

 

[Kayback]

Yup that sounds right. at 30 the .22 will have a marked arc while the 556 will still appear flat. Try it with the 556 at 100 or 150 and you'll see a bigger difference.

 

[Kayback]

Just out of interest, with a 50 yard zero at 100 yards the .22 is 10 inches below the POA. With the 5.56mm and a 50 yard zero it is only 10 inches below at 350 yards. At 100 it is still above the POA.

You're basically comparing apples to beef, and wondering why the sight works differently on each system. There will be a POI shift if you move your sights after zeroing them, that's why you don't do it. It will be more pronounced on the .22 because it isn't as fast as the 5.56mm.

KBK

 

[shawnfergie]

No I'm asking people if they see that if they move their red dot on their MP15-22 from one rail position to another that the impact point shifts like mine.

I only use the MP Sport with the same red dot to ensure that the red dot or red dot mount isn't faulty or loose.

I in no way need to compare 22LR vs 556 / 223. I know that the points of impact would be different between the rifles. I'm just checking that my Sport rail stays the same with POI no matter where I mount the red dot vs the MP15-22.

 

[Kayback]

If you move the sight the point of aim changes, so the point of impact will appear different. If you mounted the rifle in a rest you'd see the POI doesn't actually change, only your POA.

The fact you are using it on a low powered round like a .22, which has a trajectory like a rainbow, will exacerbate the issue making it much more noticeable.

The 5.56mm will ALSO change, but the problem will not be as apparent because of the velocity/trajectory of the round.

If you could launch the .22LR at 3000fps, you wouldn't notice the difference either.

If you run the 5.56mm out to around 100-150m you'd see a similar shift.

Shooting it at 30m means it hasn't even crossed the sight line yet so moving the sight won't change much.

If you want an actual test you need to borrow another .22LR or shoot the 5.56mm at a longer range.

It sounds like everything is working as it should be.

KBK

 

[shawnfergie]

By moving the sight one rail section I would see that much difference because of all the info you posted above or because my plastic rail has variations in it?

 

[Kayback]

Most probably because of the sight movement.

I honestly doubt the molding on the receiver has that much variation in it.

How long has the sight been on the rail in the original position? I have heard people mention the polymer possibly cold flowing from the pressure the mount puts on it. Maybe you are right and there is a variation in the rail.

KBK

 

[Beuford T. Rimfire]

3" of impact change by moving the red dot? If moving to a different rail position effects your hold or cheek position in any way that might have an effect. Was the dot centered when you were shooting in each position? At 30 yards, if the dot wasn't centered, you may just be picking up a parallax shift. But even 3" sounds like more parallax shift than I have ever experienced.

 

[shawnfergie]

I would say I had the same hold from one rail position to another. I will check co-witness to my irons and change it to see if I can tell a difference from one spot to another.

 

[Majorlk]

By moving the sight one rail section I would see that much difference because of all the info you posted above or because my plastic rail has variations in it?

This is why many of us have put a 0.5" riser on the receiver and put sights on that.