Question About Zeroing an Ar-15 with iron sights

[bloodandgutsmurphy]

Hi guys and gals,

I bought a used Colt 6940. I fired a few rounds at 25 yards and the gun was shooting 12 inches low. The groups were tight, windage wasn't an issue.

I used my ar-15 front sight adjustment tool. After 300, yes three-hundred, clicks the gun is still 4-5 inches low.

My 20" HBAR was shooting low, but it didn't take 300 clicks to adjust the elevation.

This issue with my 6940 is occurring with multiple types of ammo if anyone is wondering.

Now to my question, is it okay to file down my front sight, should I take it to a gun Smith for filing, or purchase a new blade? It is marked with a capital "F"

Thanks for taking the time to read my post

 

[Leo]

File away, If you go too far, it is only a few bucks to replace it

When getting a basic 300 zero at 25 yards, ammo type really does not make enough difference to measure.

 

[rosejm]

Wondering if the front/rear sights are matched properly...
Maybe this one's built for a carry handle?

Are they both fixed? Or what are we working with...

I also find it difficult to believe 300 clicks on the front sight only moved the PoI 8" @ 25y (32 MoA)
That means the front sight has .10 MoA clicks! On an iron sight!

 

[bloodandgutsmurphy]

The rifle is a complete Colt 6940 with a detachable carry handle.

I assure you sir 300 clicks. I just adjusted the sight another 275 clicks. I hope that does the job.

This rifle is a police trade in. It may be been used and abused.

 

[Cameramonkey]

The rifle is a complete Colt 6940 with a detachable carry handle.

I assure you sir 300 clicks. I just adjusted the sight another 275 clicks. I hope that does the job.

This rifle is a police trade in. It may be been used and abused.

A total of 143 rotations? (4 clicks per turn@ 300+275) I call shenanigans.
A quick googling says its a 32TPI thread.
32tpi= .032" vertical travel per turn.
575 clicks@ 4 clicks per turn= 143.75 turns
143.75 turns at .032" per turn = 4.456" of vertical travel.

Im thinking the second 275 are the other direction. No way you turned it that may turns the same direction.
(even if its a 5 position post its still 3.5" of travel)

How much have you had to drink, Murph?

 

[indyjohn]

Hi guys and gals,

I bought a used Colt 6940. I fired a few rounds at 25 yards and the gun was shooting 12 inches low. The groups were tight, windage wasn't an issue.

Before you get crazy, shoot it at 100 yds. My first impression on your post is it is zeroed at 300 (maybe 200) yards. Consider this-

 

[bloodandgutsmurphy]

A total of 143 rotations? (4 clicks per turn@ 300+275) I call shenanigans.
A quick googling says its a 32TPI thread.
32tpi= .032" vertical travel per turn.
575 clicks@ 4 clicks per turn= 143.75 turns
143.75 turns at .032" per turn = 4.456" of vertical travel.

Im thinking the second 275 are the other direction. No way you turned it that may turns the same direction.
(even if its a 5 position post its still 3.5" of travel)

How much have you had to drink, Murph?

I'm not joking. 500 quarter turns. The front sight must be broken, I don't see it raising as much as it did on my colt HBAR 20"

 

[rosejm]

25/300 should be nearly identical near/far zeros with any reasonable bore offset.
Same for 50/200.

if the impacts aren't pretty close to aimpoint @ 25y, there's no point in moving further away. Certainly not 5-12" off, anyway.

 

[rosejm]

Well, the front sight should be dropping in its base if the impacts are low (moving the bullets up).
FORS (Front Opposite - Rear Same)

But obviously, CM has a point about total travel based on thread pitch. You should notice that much change visibly.
Perhaps it's stripped out and no longer adjusting properly.

 

[Cameramonkey]

I'm not joking. 500 quarter turns. The front sight must be broken, I don't see it raising as much as it did on my colt HBAR 20"

I agree. its time for a new front sight assembly. No way you're actually doing anything with that many turns. its bottomed out and is just spinning.


And for full disclosure, I just realized after reading deeper into the forum that its actually a 36tpi pitch. But still impossible.

 

[bloodandgutsmurphy]

I agree. its time for a new front sight assembly. No way you're actually doing anything with that many turns.

Can you guys recommend anyone for the job? I don't feel comfortable doing it.

My SKS was shooting straight as an arrow. I was clobbering my steel targets with at 100 yards

 

[1775usmarine]

36yds is the equivalent of a 300yd zero.

 

[rosejm]

No personal experience, but give 219 Armory a call in Valpo next to the Chili Bowl.
I recall some mention of folks having good experiences there.

https://armory219.com/

 

[bloodandgutsmurphy]

No personal experience, but give 219 Armory a call in Valpo next to the Chili Bowl.
I recall some mention of folks having good experiences there.

https://armory219.com/

I haven't had good luck with him in the past.
Who is the AR guru in Indiana? I don't mind traveling

 

[Squid556]

I’m thinking the same about it spinning but not continuing to move the post up/down.

I’d spin it counter clock wise until it comes out, inspect it. Might try a different front sight post. Screw it back in, bore sight it.

 

[Cavman]

Will the front sign post return to a mechanical zero? Where its flush?

 

[tcecil88]

Have you tried bottoming out the rear sight (I assume A2 style)? Run it all the way to the bottom then 2 clicks past. it should say something like 6/3. All the way down is the 300M setting IIRC, then you go past that 2 clicks. Alot of people think that you adjust elevation zero there, but that is for range. I used to be a Rifle Instructor when I was in the Military and I seemed to always get stuck on the zero range. This was a very common mistake. I wonder if the former owner did it that way and then maybe you just missed it. Here is a link to a YouTube vid on the A2 rear sight. I hope this helps.

I agree with the above that you should not be getting that kind of movement out of the front sight post without bottoming it out or it falling out.

 

[Brad69]

Also check the rail position of the carrying handle assembly!

Good hit on the rear sight adjustment for a A2 Infidel!

 

[tcecil88]

It was especially irritating because we were supposed to be assigned weapons so our individual zeros were maintained, but we very rarely got our assigned rifle. When we hit the range we usually had to start all over. The smart troops knew their adjustments from Battle Sight zero and adjusted them accordingly. They were the ones that hit the zero range, shot 3 rounds to verify their zero, maybe make a click or two adjustment and verified again. They then went down to the qual range and attempted to qualify one time and was done. I have literally spent hours working the zero range and then had to come back the next day just so I could zero and qualify. Made for a long range trip.

 

[DDadams]

I haven't had good luck with him in the past.
Who is the AR guru in Indiana? I don't mind traveling

@AllenM on here does fantastic work all around.

I'd go to him. Avon IN 15 min out from the circle in Indy.