I just got a new A-1 upper, and the rear sight has two apertures that look damn near the same size.

What is the difference?  Niether are marked or anything.

-Spaceman

Your right they do but I was told on this board I shouldnt use the A2 aperture to replace it so on a drill press using slightly larger bits every time I bored out the 2cnd arperture until using the final .200 bit wich is the same size as the larger A2 aperture in its accurate and works great.I was told I ran the risk of not drilling it out properly but using slightly larger bits 4 to be exact before the final one,little by little and taking my time things worked out great for me.

Why shouldn't the A1 aperture be replaced with the A2 aperture?

Spiff,

Nice threadjack in progress huh?

The apertures are different heights over the bore. One should have an "L" marked on it. The "L" should be visible (it's large) stamped into the metal around the windage screw under the aperture. You have to rotate the aperture away from you to see it.

Should look like this
[img]http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/3-22.9/image1566.jpg[/img]

An M16A1 is near zeroed at 25 metres using the "L" ([b]L[/b]ong range) aperture. After near zeroing the rear sight is rotated back to the unmarked aperture. This creates a 42 metre near zero and a 250 metre far zero. Flipping the sight back to the "L" aperture extends the far zero to 375 metres.

[b]Let's not get into a discussion of zero ranges and methodologies, it's all academic folx[/b]

The A2 sight works in similiar fashion to the A1 in that the two apertures are at different heights over the bore. Again like the A1, shifting between the two apertures on the A2 rear sight will shift the POI vertically. This compensates for the round's high trajectory from 0-200 metres when near zeroed at 25 metres.

The A2 rear sight (just the fiddly bit, not the whole shebang) makes better sense on a close range rifle and is a recommended modification.

[edited to add comment in bold and change pic to a better one]

Who says the A1 aperture shouldn't be replaced with the A2 aperture?

I've done it for a long time as have many of us on the boards.  An even better solution IMHO is the Express Sight same plane rear aperture but its a $30 part vs the $8 A2 aperture.

On a carbine the A1 aperture is pretty much pointless - the A2 makes much more sense.

Not to "hijack" the thread BUT, I used the larger aperture on my A2 sights to close sight my rifle at 25M, should I have used the smaller one instead?  I thought the larger aperture was for low light use and the smaller aperture was for general use but that they were both set up to hit to the same point of aim, just different sizes. Please enlighten me to this matter.  

Read model927's post a little closer, I think that's typo.

TNFrank,
If you're not using sights/rifles set up in the same configs as the Army then none of the above applies. All bets are off.

There is a vertical offset, ~6MOA with the 20", between the 0-2 ap and the unmarked ap. That corresponds well to the +5" to +6" over LOS that the bullet has at midrange when zeroed at 25 metres [b]when fired from an M16A2[/b].

Zero at the range you expect to engage at based on METT-T using the ap you expect to be using then check POI at intermediate ranges and record the offset between POA and POI.

I have a 20" bbl in AR15-A2 config. so should I have sighted in at 25M with the smaller apt.?  I'm hitting point of aim now with the larger apt. at 25M. Talk to ya' later. TN.Frank

TNFrank,

I'm not going to get into zeroing methodology here. IIRC, I posted something to that effect earlier in this thread. It's one of those topics that brings out all the experts, at least all the ones that spend more time shooting off their mouths than they do shooting off their rifles.

You do what works for you, I'll do what works for me and as long as we can both hit what we aim at no one can complain....much. [:D]

If you want a list of most of the zeroing methods then Forest has a page of them at his MD-AR site. It's fairly comprehensive.

For that matter here is the new Army M16/M4 marksmanship manual so you can see how Sam does it.

[url=http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/3-22.9/toc.htm]FR 3-23.9 at RDL[/url]

thanks guys, this helps a lot.

one thing, my aperture is NOT marked with an "L" or any other letter, it is plain.

also, the sight holes are tapered, and the big end of the hole faces rearward towards the shooter.  I am considering turning it around to reduce glare in the sight picture.  should i do this?

thanks

-Spaceman

The taper should be facing toward the shooter to reduce glare, if you turn it around you get more glare thru the sight.  I'm going to put a few rounds thru my gun with both apts to see where each one hits.  If what I read earlier is right then I should have used the smaller on to sight in with and not the larger, I didn't know that they were drilled to different points of aim.  Ya' learn something new everyday.  Talk to ya' later.  TN.Frank

Everyone I've seen has the wide end of the aperture facing AWAY from the shooter's eye. Possibly an attempt to increase FOV.

Spiff,
Perhaps your rear sight is installed backwards (I've seen it done) so check the front part of the sight for the "L". I do have a distant memory of unmarked A1 rear sights tho, never know what may have made it out of some parts houses.

Think of it as a funnel for your eye.  It goes from larger to smaller to draw your eye into the hole. Plus the U.S.M.C. Tech. Manual has it put that way so that's good enough for me.  Talk to ya' later.  TN.Frank

P.S.
I just checked my A2, the small hole has the "funnel" back toward the shooter but the big hole has the "funnel" out toward the target so I guess it's anyones guess. LOL.  

yea, If my hazy memory serves me correctly, i read somewhere a long time ago that some manufacturers had the large end toward the eye and some had it away from it.  I believe it was mentioned that Armalite specifically turned their apertures with the small end towards the eye as it gave a more consistent sight picture.  Now this is one of those distant fuzzy memories that may or may not be true, so take it for what its worth.

i am bothered that my sight isnt marked with an L, i suppose the best way to tell which sight is long and short is to shoot the thing.  i doubt it even matters as i wont be switching around anyway.

-Spaceman

Uh, look at the pic I posted TNFrank.
[img]www.adtdl.army.mil/cgi-bin/atdl.dll/fm/3-22.9/image1566.jpg[/img]
You can see that the flat side of the sight is facing the shooter's eye. That's a pic of an honest to goodness M16A1.

Or, you can take my word that every A1 rear sight I have in the shop (over 20, in bins or installed) has the flat side facing the shooter's eye when installed correctly, i.e, with the "L" facing to the rear. I've seen plenty incorrectly installed, a friend of mine even purposely installs his with the large end of the opening toward his eye. He prefers that sight picture. His guns, his sights, I can't disagree with him.



Which manual and what page please? I have the USMC A2 manual but not an A1 manual. Relying on military manuals is hit and miss, check out this diagram.

[img]http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/3-22.9/image1660.jpg[/img]

The Army must be short on numbers since they had to repeat them. [:)]

Here's another error.
[img]http://www.geocities.com/offroadaviator/A1Zero_files/fig102.jpg[/img]

It substitutes a pic of the old Low Light Level Sight (using PM 147 or H3 inserts in the front post) for one of the standard A1 ap. Note the 7mm aperture on the unmarked leg of the sight.

The article that picture is from, [url=http://www.geocities.com/offroadaviator/A1Zero.html]Zero an M16A1[/url] is a direct copy of the Soldiers Manual of Common Tasks (SMCT)Soldier Training Publication (STP) 21-1-SMCT OCT90 I have sitting on my shelf. The task gives the incorrect zero range (contradicting the -10 and the 23-9) for the M16A1 in multiple places.

A quick scan of FM 23-10 "Sniper Training" 17AUG94 will uncover tons of factual errors on top of generic typos and poor editing.

You see, few manuals get changes written to them until they're completely rewritten. In the case of the latest FM 23-10, some are rewritten much to the readers detriment. Those manuals that do have changes written and issued for them (the latest 23&P for the M series is up to C6 IIRC) aren't updated before printing, instead a packet of changes is attached. These changes never seem to make it into the civilian world and bad info gets disseminated. Ever wonder why everyone has a different answer for "proper" barrel nut torque? Now you know.

FWIW, my copy of FM 23-9 JUN74 shows a close up of the rear sight on page 153, Figure C-2. The wide end of the aperture is facing toward the target.



That is a correct part, the wide end of the 0-2 aperture faces toward the target and the wide end of the unmarked aperture faces toward the shooter's eye.

Pic below.
[img]http://www.geocities.com/offroadaviator/A2Zero_files/fig107.jpg[/img]


[bored kode]

thanks tweak.

-Spaceman