The military REQUIRES that their rifles impact within the limits of the target provided in the milspec. The standard for same are set forth in the milspec.


LMT's email is disheartening.

What sight radius and style of rear sight on your LMT upper?

For the carbine length with an A2 rear the POI shift is .42" at 25 yards, or 1.7MOA. That works out to just over 7 clicks to a correct a 3" errror at 25 yards.

[ETA]dammit, knew that didn't sound right. I'm not taking benadryl anymore. That is too f*cked up posts in less than one day. .42 is the change in ELEVATION, not windage. I'm sticking to alcohol from now on.

For the carbine length with an A2 rear the POI shift is .17 at 25 yards, or .68 MOA. That works out to almost 18 clicks to correct a 3" error at 25 yards.

It would be .13 (1/8") at 25 yards for a 20" barrel. [/ETA]

I like my irons to be as centered as possible (within 3 clicks in cool) bc it speeds up target aquisition. Windage has never been a problem with the shooting that I do.

corrected original post good catch JLM allergies are beating my azz.

FWIW, the 100 yard M4 target is 16" wide, the 100 yard M16 target is 11.6"  wide.

The spec for both the M16A2 and M4 the standard is 12 clicks +/- 5 when zeroed at 100, 11.6 and 11.77 respectively. For the M16A1 and A1 sighted carbines the standard is 6 clicks +/- 5 when zeroed at 100 yards, 5.8 and 5.84 respectively.

If you're 3 inches off at 25 you'll be off 4 times that amount at 100.

With a 12" error at 100 yards you'll need just over 17 clicks (sound familiar?) to correct the error.

Both of my copies of the milspec (M16A2 and M4) show the rear sight should be within +/- 5 clicks of center. That 's from MIL-R-63997B(AR) FEB97 and MIL-C- 70599A(AR) JAN94

If the quoted M4A1 standard of 12 clicks is newer than the one for the M4 then more math is needed.

[ETA] I forgot I had a copy of the M4A1 spec and it does indeed say that the rear sight should be +/- 12 clicks for ten shots into a 16" wide rectangular 100 yard target.  I see no addendum, amendments or changes to that (MIL-C-71186(AR) JAN94 spec. That is very rare in the military.

Perhaps this is the "looser accuracy requirement" for the M4 that some have bemoaned.

However, if the 16" and 12 clicks are true, all my previous calcs that have been around for years are now in error and I'm not now in the mood to redo them. Oddly, the M4A1 spec is dated only 11 days from the M4 spec.

For future viewers, there is more information here.[/ETA]

Correct

Quoted: Tweak, should my rear sight look like this:

That's nasty but I've seen worse.

Run the rear sight base up and see if the offset corrects itself. If so the threads on the rear sight base are probably incorrect.

Paladin,

You could determine that by switching in other known good parts.

"Threaded incorrectly" isn't quite the right description, I typed that post in haste on my way out the door.

The top most, rear most thread on the RSB acts as a stop. It stops on the inside top most edge of the hole the RSB sits in. If that thread is mislocated the RSB will push back from the upper and rock to one side.

If you inspect the threads on the RSB and there is a gap between the last thread (upper most) and the bottom of the rectangular portion large enough that the abutted portion of the upper can slip in between the two then you need another RSB. If the web of the upper is skinny then a new RSB won't help.

Disassembly is covered in the 23&P, you'll need a pin punch and nerves of steel.

Apologies for the previous short post, again headed out the door at 1230.

I'll include this here even if you're going to send it back as this thread is now part of the FAQ and someone later on will need the info.


Removing the RSB is simple and there are tricks that make it even easier.

Reinstalling the RSB is a different matter and the right tool makes all the difference. Check with Brownells and order one or break out your Dremel and a busted punch and make one. The tool is the one to used to compress the elevation spring while you reinstall the retaining pin. It is a vital tool.

That said, run the RSB up to a few clicks shy of it's highest point. Don't run it all the way up. Elevating the RSB takes tension off the elevation spring.

Drive out the roll pin with the proper sized punch. I usually drive the pin out from left to right. That way the forward assist will hold the upper off your work surface so that the roll pin can come out without interference. I reinstall it from left to right so I don't have to worry about striking the forward assist hump with my hammer or punch. Don't fiddle about with driving out the pin or there is a chance you will catch the edge of the elevation spring. Once the pin punch starts into the roll pin hole DO NOT withdraw the pin punch or the edge of the elevation spring will block the roll pin hole. Drive the pin completely out of the upper.

Rotate the upper to its normal orientation and catch the elevation spring. Sometimes you have to fish the spring out with a pin punch or dental pick.

Rotate the elevation index to unscrew the RSB. There is a small ball bearing and spring in the front of the RSB. Make sure to catch the ball bearing as the bottom edge of the RSB clears the rear of the upper. Some folks will do this with the upper inside of a large plastic bag. I find it best to remove the spring from the RSB and set it and the ball bearing in a safe place. I use a magnetic parts tray. Unless necessary for repair, do not remove the elevation index or rotate it any farther than necessary to remove the RSB. The only thing retaining the elevation index in the upper is an equally small ball bearing and spring. If you rotate the elevation index after removing the RSB, you will have to re index it when you reinstall the RSB. Neither of these procedures are complex but it’s nice to avoid them if possible.

Inspect your RSB. Most of you will be looking at the gap between the threads and the bottom of the flat, rectangular surface. This gap should not be large enough for the web of the upper (between the elevation index and the RSB) to fit.

When you’re ready to reinstall the RSB replace the small spring and cover it with a dab of grease then stick the ball bearing to the grease.

Reinstall the RSB in reverse. Make sure to start the RSB into the elevation index before turning the index.

Rotate the elevation index until the “window” in the shaft of the RSB can be seen through the retaining pin hole. You should double check the fit by sliding the pin punch through the hole and verifying that the pin punch goes THROUGH the shaft of the RSB.

Remove the pin punch and start the roll pin into the upper. Do not drive the roll pin in far enough for the end to enter the RSB. Drive the pin in just enough to stay in the upper. I usually drive the pin in from the left side. Insert the elevation spring into the RSB through the bottom of the upper. Make certain that the spring does not kink. Compress the spring with the special tool and completely drive in the roll pin. Make sure to center the roll pin in the upper and that the roll pin is UNDER the elevation spring. You do not want the roll pin going though the elevation spring.

Rotate the RSB to its lowest position. The battlesight (8/3, 6/3) setting should align with the vertical index mark on the left side of the upper. The index mark is between the bottom of the RSB and the top of the elevation index. Some makers do not include this mark on their uppers.

If the battlesight setting does not line up with the index mark rotate the elevation index so that it does. Insert and Allen wrench of the proper size through the top of the RSB. There is small hole forward of the aperture for this. Loosen the screw that locks the elevation index and the elevation wheel about three or four turns. Do not completely remove the screw, as it is a pain to reinstall. Rotate the index to align the battlesight setting with the index mark once the screw is loose enough to turn the index independent of the wheel. Retighten the screw. If there is any binding go to the nearest whole click and tighten the screw. Sometimes you have to wiggle the wheel (not the index) slightly in order for the screw to catch.

Run the RSB through it full range of motion and check for binding. Take a look at the head of the lock screw as it comes into view and make sure you didn’t bugger it up and that is fully recessed into the index.

What Dano said.

Check the FAQ under "excess elevation" for more about carry handles and flattops.

You're welcome JLM, let us know how it fares.

If you're running a 14.5" yes.

Not IME.