View Full Version : Sights: L1A1 front sight question

November 01, 2002, 23:19
Does anybody know exactly how far (up or down) one complete 360 degree turn of the front sight will move the point of impact at 100 yards on an L1A1? Thanks.

November 01, 2002, 23:41
The L1A1 front sight has a thread pitch of 40 TPI.

So, one revolution is one-fortieth of an inch, which equals 0.025".

At 100 yards, the point of impact moves one inch per each .006" movement of the sight. (very helpful to remember that .006" movement of the sight equals one m.o.a. ... irrespective of the distance).

So, the answer should be: " about 4 inches."

November 01, 2002, 23:46
Thanks, gary.jeter. That will save me some guesswork in sighting in this weekend.

November 02, 2002, 11:35
The extracts from the British manual indicate one complete turn moves MPI up or down 8 Inches @ 100y.

My copy of Candian manual C-71-113-000/CF-001 indicates one complete revolution of the Canadian foresight is 4 MOA, or ~ 8" @ 200y!
Can someone in the great white north, or w/ a real C1A1, verify this?

The British had at least seven sizes of foresights, whose heights varied by 0.5 mmm each. This allowed pretty finite adjustment to suit individuals and tolerances in rifle manufacture.

Mr pogo
November 02, 2002, 14:58
One full turn of front sight moves the MPI vertically 1/2 inch at 25yds, 2 inches at 100yds, as per User Handbook for the Rifle, 7.62mm, L1A1.

November 02, 2002, 17:54
My Stevens book says 4in at 100m. Pg 164 sec 2
Made a mistake the first time and gave info for post instead of blade

November 02, 2002, 17:58
Thanks for everybody's input. Now, however, I'm REALLY confused. I'll let you know what mine does.

November 02, 2002, 20:26
It is ABSOLUTELY IMPOSSIBLE to move the point of impact any distance other than 1 inch at 100 yards if you move the sight .006"

I have not actually measured the thread pitch of the inch-pattern front sight. I simply took Gunplumber at his word where he posted at http://www.falfiles.com/forums/showthread.php?s=&threadid=21952

Now, do the math and prove me wrong. Better yet, prove Jon Yagla wrong, because I'm no good at math. John is on my rifle team, and he is real smart. Jon works for the Navy doing things like calibrating tandem artillery pieces on warships, and designing guidance systems for ballistic missiles. Jon was the only REAL rocket scientist I could locate for consultation when I was trying to figure out the values of the clicks on my rifle sight. He was very helpful, and he provided the following straightforward explanation on how to "figure it out yourself."


Objective: move point of impact one inch at 100 yards.

Step 1
Measure sight radius.
FAL sight radius is 22 inches. (Note that the sighting radius of a FAL changes as you adjust the sight for range, but not enough to matter for the current application.)

Step 2
There are 3600 inches in 100 yards.

Step 3
To calculate the amount of movement needed to move the point of impact on the 100-yard target, we divide the sight radius (22 inches) by the firing distance (3600 inches).
22/3600 equals 0.0061

Step 4
Therefore, 0.0061" lateral movement of the sight (front or rear) will move impact one inch on the 100 yard target. So, one number you can stick in your pocket is: .006" sight movement yields one inch change on the target at 100 yards.


Mr pogo
November 02, 2002, 21:15
From the British manual of arms, 1959-
Infantry Training, Volume I
The 7.62mm Self Loading Rifle and Bayonet

One half turn of the foresight will raise or lower the MPI about 3/4 inch at 25 yards or about 2 3/4 inches at 100 yards.

Hmmm, seems the British had conflicting manuals back then :eek:

November 03, 2002, 09:26

The info I reported was for the X8 trials guns, essentially Metric FAL-Canada rifles. cheif61 (chief :?) reports the correct info: Each complete turn moves POI up/down 100mm at 100m. The same is true for the backsight: one-half turn of an adjusting screw moves POI 50mm at 100m.

I believe Gary reported it correctly. Mea Culpa! :embarass:
I'm still curious about the Canadian report.

November 03, 2002, 09:31
No problema Teodoro.

A fella can pull his hair out trying to remember all these "manual" references.

For practical riflery, all you ever need is a cheap caliper. For AR's, FAL's, M1A's, and all other rifles of approximately like sight radius, the number to remember is .006".

.006" sight movement always equals ONE INCH at 100 yards.

November 03, 2002, 09:57
:biggrin: I said 8" the first time too then caught it and removed it. The name misspelling (cheif) I didn't catch when my internet was set up until I got home and I never went back to change it.

November 03, 2002, 20:00
According to the book one complete turn of the foresight will give you 14.3 cm (5 1/2 ") @ 90 m (100 yds).
One complete turn on the rear sight will give you 10 cm (4 ") @90 m.

These correspond with the size of the grid squares on the zeroing target which was developed before the switch to the metric system up here, hence the dual measurements.

I will have to go out next weekend and actually try it out to see how close the measurements come to what is in the book.


November 04, 2002, 09:02
4 MOA at 90m (98y) according to that copy of the CF document. Strange.

4 MOA ~ 4.1" @ 100y. :confused:

muktuk joe
November 20, 2002, 15:08
Thanks for the lesson, should save a lot of time zeroing in for the first time with this system.

December 06, 2002, 22:17
Just done a search and found this thread ......... excellent ... just what i wanted.

My L1 is shooting 10" high right now at 100 yds .... and that's with rear at lowest .. so, now I know what to expect.

I appreciate finding all this info. Thx guys.

December 14, 2002, 14:44
Originally posted by EMDII

......The British had at least seven sizes of foresights, whose heights varied by 0.5 mmm each. This allowed pretty finite adjustment to suit individuals and tolerances in rifle manufacture.

I don't believe this is accurate, the whole purpose of the L1A1/C1 front sight design was to eliminate the requirement to have different front sight sizes, as required for the Original FN design. Extracts from British/Canadian/Australian TM's seem to confirm this.

December 14, 2002, 22:34
It may be in reference to the different types of foresights, both day and night, that the Brits came up with.

The C1 and C1A1 had only the one size through out.

I have no info on Aussie rifles:wink: or Brit to go from, but do have all the Cdn tech manuals.

December 15, 2002, 13:02
That info came from the Stevens' book. The Skennerton manual reprint indicates only two NSNs for a foresight blade:


Foresight assembly, Trilux

Ain't military catalogues grand :?

The section on 'The L1A1 Rifle Today' has the following at the section entitled 'Sight Adjustments'.

- one-half turn will raise/lower MPI 50mm at 100m, or 12.5mm at 25m.
- Same is true for the backsight adjustment screws: one-half turn moves MPI right/left 50mm at 100m, or 12.5mm at 25m.

I believe the 'several' foresights comes from the FAL-Canada X8 trials guns used by the British in their pre-manufacturing trials.