New to the site, and have a question.

[swar45]

hey guys I'm new to the site, i have been lurking for the last couple weeks.

my question, is based off the 50/200 yard zero. from what i understand...or at least read, a 50 yard zero should also be on at 200 yards. my problem is that, well its not for me. i have a S&W M&P 15, carbine length, running a EOTech holographic sight.

alright, the question here, that i can't seem to figure out...i zeroed it at 50 yards...most the time where i shoot is only up too 100 yards. there is a 200 yard range, which is about 45 minutes away, and i have just recently learned about, well a few months ago. anyways...like i said...rifle was zeroed at 50 yards. went to the 200 yard range, and the point of impact was no where near the point of aim at 200 yards. well i got curious about this and started doing some reading, and from what i have learned, at least read, often times the 50/200 yard zero isn't always achievable. a lot of factors come into play, including barrel length and ammo (the ammo i was using at 200 yards was the same as i zeroed with, PMC XTAC green tip 62 grain, 5.56). basically my question is, is that accurate? being that even just perhaps the barrel is only 16 in (correct me if I'm wrong please, i believe thats what it is), the shots are going to be different and the 50/200 thing isn't going to be accurate? the poi was low, was using the silhouette targets. aiming at the head, put the poi around the mid section of the silhouette.

 

[ChattanoogaPhil]

No. It's your shooting.

I've got a 16in barrel and shoot 62 grain. Bang on steel at 200yds.


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[swar45]

What could I have been doing wrong? Was shooting from a bench with sandbags. After checking and seeing the poi low the first time I aimed higher the next round and was hitting headshots

 

[swar45]

zeroed at 50 using a range finder. Optic wasn't adjusted. range not being 200 is a possibility. But couldn't be that far off. So since I you really were just a smartass and didn't very little to answer my question, from the position I stated above, what could I of been doing wrong?

 

[swar45]

If the range was correct and shooting from a bench with sandbags, what would be a common issue that I would be doing wrong? I'm always looking to learn and improve. And I apologize if i took it wrong but that's the way you came off

 

[Rpg]

Shoot groups of 5 at 100 yards holding dead on.

I suspect a shooting issue, not an ammo or barrel issue.

 

[rwsmith]

Try sighting at 200.....

The idea is that the bullet is rising when it goes through the 50 yard plane, continues to rise, then descends until it hits the 200 yard plane. WITH THE RIGHT load and bullet this will be more predictable. 50/200 may not work all the time but 60/120 might work for a different load combination. You have to know the profile of the bullet, it's weight and velocity to make such a claim. There are external ballistics calculators that plots trajectories and can show you a close combination to get where you want to go. If you have a blunt bullet, it will slow down faster than a pointed bullet and the "what goes up must come down rule" still works, but maybe not at 50/200 but some other combination. Also realize that the RATE of drop changes as the bullet flies. A trajectory for a rifle bullet isn't a perfect rainbow, but the farther and slower a bullet goes, the rate of drop increases.

 

[swar45]

I really wish I had the targets still so I could look. I know it was about the average silhouette. I'm thinking you may be right about the range it was zeroed being off. Unsure through. I will have to make my way out there to double check. I just rezeroed last week and my first group was way low at 50 yards. But then again I also removed and reinstalled the optic along with installing a free float hand gaurd (coming from drop in), installed a new muzzle brake, and changed the standard a2 front sight gas block to a low profile. Also moved the eotech up further towards the barrel from where it previously was. Not sure the effect all that would have. I know moving it forward will effect it.

Also it is an eotech 516. Single dot

 

[robrossk]

BTW - welcome to the forum. Sounds like you have made several adjustments so hope you can get it zeroed in easily. My M&P15 is completely stock and it tings away.

 

[ChattanoogaPhil]

The idea is that the bullet is rising when it goes through the 50 yard plane, continues to rise, then descends until it hits the 200 yard plane. WITH THE RIGHT load and bullet this will be more predictable. 50/200 may not work all the time but 60/120 might work for a different load combination. You have to know the profile of the bullet, it's weight and velocity to make such a claim. There are external ballistics calculators that plots trajectories and can show you a close combination to get where you want to go. If you have a blunt bullet, it will slow down faster than a pointed bullet and the "what goes up must come down rule" still works, but maybe not at 50/200 but some other combination.

We're not talking about unknown ammo here.

X-Tac 62gr has a published ballistic coefficient of .304 with a muzzle velocity of 3100.

Here's the chart with a 50yd zero.
Right on the money within an inch at 200yds.

I really wish I had the targets still so I could look. I know it was about the average silhouette. I'm thinking you may be right about the range it was zeroed being off. Unsure through. I will have to make my way out there to double check. I just rezeroed last week and my first group was way low at 50 yards. But then again I also removed and reinstalled the optic along with installing a free float hand gaurd (coming from drop in), installed a new muzzle brake, and changed the standard a2 front sight gas block to a low profile. Also moved the eotech up further towards the barrel from where it previously was. Not sure the effect all that would have. I know moving it forward will effect it.

Also it is an eotech 516. Single dot

Sounds like you've made a lot of changes and need to start from square one.
Zero at a known 50yds and go from there.

 

[swar45]

Yea. I did. These changes were made after the trip to the 200 yard range.

 

[sgtsandman]

My recommendation would be to make sure everything is square. They have magnetic bubble levels that are pretty cheap to make sure the rifle is level and the optic is level to the rifle.

Next, if you have access to a gun vice shooting rest to lock in the rifle and remove the human factor as much as possible, zero your rifle then. Then if possible, check at the 200 yard range using the shooting rest.

Like stated above, the bullet you are using may not match the 50/200 in ballistics. It also depends on what load, brand, and bullet your rifle likes. What one rifle likes is not going to be the same for the next, even if they are the same brand, model, and twist rate.

Since you did a lot of changes and moved things, your zero is going to be off from before. Take it one step at a time and be patient. It will take a while to figure out the ballistics table specific for your rifle.

 

[ChattanoogaPhil]

Y'all are getting into the weeds. Trajectory differences of common ammo for these guns just isn't that great, guys.

Here' a couple common types

55gr
Ballistic coefficient .270
Muzzle velocity 3240

62gr
Ballistic coefficient .307
Muzzle velocity 3020

Not an inch difference at 200yds with a 50yd zero.

 

[Rastoff]

OK, first of all, the concept of a 50 yard zero being the same as 200 yards is a misnomer. As you can see in the charts above, a 50 yard zero means it will be on zero again at about 225 yards not 200. So, if we're talking about real precision, it will be off at least an inch at 200. For defensive shooting that's a non issue.

So, before we go further, what is the precision you're looking for? Exactly how far off were your shots at 200 yards?

Some other stuff to consider:
Were you shooting off a rest when you zeroed at 50?
Are you sure your optic was mounted solidly? (I just had this problem with my .308Win)
When shooting at 200, are you using proper technique? Breathing, hold, trigger control?

 

[cyphertext]

What size groups were you getting at 50 yards? What size groups at 200 yards? The PMC X-TAC is not match ammo and 4 MOA is not that all unusual...

So at 4 MOA, that 2 inch group at 50 yards opens up to 8 inches at 200 yards.

 

[cyphertext]

If the range was correct and shooting from a bench with sandbags, what would be a common issue that I would be doing wrong? I'm always looking to learn and improve. And I apologize if i took it wrong but that's the way you came off

Just because you are shooting from a bench with bags doesn't mean that your technique was good. Sight alignment, trigger control, and breath control all come into play...

 

[WR Moore]

But then again I also removed and reinstalled the optic along with installing a free float hand gaurd (coming from drop in), installed a new muzzle brake, and changed the standard a2 front sight gas block to a low profile. Also moved the eotech up further towards the barrel from where it previously was.

OK, just about everything you did will potentially affect point of impact. The muzzle brake (or whatever you have screwed on the end of the barel) can change the bullet impact greatly. So can loose fasteners on your optics.

Now then, your original problem was shooting low at 200 yards. Assuming your original sighting in at 50 yards was accurate, you still need to check your point of impact out yonder. Any errors in the 50 yard zero will be magnified by the additional distance.

At the same time, any errors in your trigger technique will also be magnified by distance. It doesn't matter if your rifle was on sandbags, a trigger yank will still drop your point of impact.

FWIW, with a true 50 yard zero, your bullet should be about 1 inch high (of the theoretical center of the dot) at 200 yards if your technique is good and the distance measurement accurate. Go for the distance zero and the short range stuff will take care of itself-if you aren't yanking the trigger.

 

[ChattanoogaPhil]

OK, first of all, the concept of a 50 yard zero being the same as 200 yards is a misnomer. As you can see in the charts above, a 50 yard zero means it will be on zero again at about 225 yards not 200. So, if we're talking about real precision, it will be off at least an inch at 200. For defensive shooting that's a non issue.



So, before we go further, what is the precision you're looking for? Exactly how far off were your shots at 200 ?

For purposes of this discussion, 50/200 is just fine.

The OP is talking about aiming at the head and hitting mid torso. I don't think we're quibbling over a fraction of an inch on a ballistic chart at 225.

 

[swar45]

At the time I was at the 200 yard range the stock flash hider and a magpul moe hand gaurd were on there. Standard a2 front site gas block as well. It was zeroed using one of those shooting rests that are typically used for zeroing. What I'm thinking is the range was off when I zeroed. I say that because after I seen the target the first time I started using hold over, estimated by looking at the target again through the eotech and holding over the difference in poi from poa when I was aiming at the head. Sure the eotech was solidly mounted. I check it every time before and after I shoot. this was before I moded the hand gaurd gas block and muzzle device.