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  #1  
Old 09-11-2019, 02:45 PM
sdcromer sdcromer is offline
 
Join Date: Jun 2012
Location: Clinton, South Carolina
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Question Sighting in "L" shaped rear sights?

I've got carbine with the original L shaped rear flip sights and an un-filed front sight. The FM says these are for 150 and 300 yards. I've been running some ballistic calculations with the JBM online trying to find a BSZ distance that will correspond to the bullet crossing back over the line of sight at 150 yards.

What sight height should I use in these calcs?
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  #2  
Old 09-11-2019, 03:22 PM
RedSpecial RedSpecial is offline
 
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If it’s an original carbine the front sight was filed at the factory for zero. Irwin did have different height front sights, but I don’t think anyone else.
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  #3  
Old 09-11-2019, 07:22 PM
6 Ring 6 Ring is offline
 
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For a new un-filed front sight, If you measure from the bottom of the sight to the top of post, You should get something near 1.110". If you measure side to side of the front sight you should get something around 0.650". Subtracting half the side to side distance from the top to bottom distance, you should get something under 0.785". which would be center of bore to top of front sight un-filed. If your front sight has been filed you will get something less.
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  #4  
Old 09-11-2019, 07:27 PM
meplat meplat is offline
 
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Here dat pic

http://forums.thecmp.org/showpost.ph...45&postcount=6

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  #5  
Old 09-14-2019, 05:50 PM
.Steve. .Steve. is offline
 
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Calculating from sight heights doesn’t do much for a particular carbine.

Take it out and shoot it at 150 yards.

Every 1/100th front sight change equals 2.5” at 150 yards.

Why, follows:

From the sticky above:

The Why plus the short version in Bold.

Front Sight Reduction To Zero The Rear Sight Slider

The question frequently arises along the lines of: "My carbine shoots too low on its sight settings. I want to cut a gnat's whisker off my carbine front sight to make it shoot to zero at 200 yards set on the 200 mark instead of the 300. How do I do it?"

There are very accurate ways to calculate and measure the front sight height reduction/cuttage needed. Setting up the calculator is simple. All you have to do is construct the formula and solve it. It amounts too little more than turning around the “How much do I have to raise my rear sight” equation.


The bullet movement on the target paper desired in inches
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Goes over the range in inches


(That would be 7200 inches at 200 yards and 3600 inches at 100 yards.)

After making those two entries, they equal =


The unknown front sight cuttage needed
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over the sight radius (being 21.5" with a carbine set on 200)


Solve the equation and you have the exact amount to cut off.


Easier yet is knowing that for each 0.01" or each 1/100"

the change is:

At 200 yards, 0.01" = 3.35" of impact change on the target.

At 100 yards, 0.01" = 1.67" of impact change on the target.

If you divide those inches of change into the amount of change needed, you know how many 0.01” or 1/100” to reduce the front sight height by. (Oh, well, I guess I also told you the easy way to do it.)


How to measure and shorten the front sight is fairly simple. Take a solid square end flat sliver of really hard wood. Stand it up against the vertical face of the front sight. Press it against the sight and tip the top to the muzzle. That should leave a distinct impression in the wood showing the top of the face of the front sight. Repeat that wood impression in 8 or 10 slivers. Trust me, you will need them as you check repeatedly. (Packages of frozen popsicle sticks are perfect.)

File the front sight down keeping it level and tapered to the front. This is best done by taking a 3x5" index card, cutting to the center, cutting out a barrel size circle, putting the cutout on the barrel by the band, and then looking at the front sight from the muzzle end against the white card. Most fellows would unload the carbine first, lock the action open, and perhaps put a chamber flag into the chamber before looking down the muzzle line. Positioning in a cleaning vise so the carbine holds still helps. Use Magic Marker frequently to show where you are cutting. Focus on the top of the post for levelness. Lightly touch the sides of the sight and the vertical face to eliminate burrs before looking.

Reuse (make a second impression) a wood sliver and note the change in the impression line made. You will quickly see two lines. You cannot reuse a sliver a second time to check because the lines blur and the slot made becomes rounded rather than two crisp lines.

Set your micrometer jaws on the change desired and compare the impression lines to the gap between the micrometer jaws. When the gap on the stick matches the micrometer gap, that is your desired change.

Two things.

(1) Do this with/for ammo you will have a lot of. Putting back the metal on the front sight is a lot of work if you decide to change ammo or your case of LCX456 runs out and you start using Pusan489.

(2) Shoot a bunch of 10 shot groups before you decide the changes needed. Only when the group centers are in the same place with your shooting should you even consider changing the front sight.

As a quick screening device about where a carbine will shoot in relation to its slider settings at that slider set distance, look at the front sight, but from the side, left to right or right to left. Note that there are holes in the two front sight wings. Using the situation where a shooter wants a point of aim/point of impact sight adjustment style, the following would apply and is very often quite correct is assessing the sights.

If in looking, you line up the top of the holes/circles straight across with each other and then see that the top of the front sight post is materially above the top of the holes, most likely it is a new front sight that has never been cut down to zero that carbine. As such it is too tall and will cause the carbine to shoot low. It can be shortened.

If in looking, you see the top of the front sight post is about even with the top of the holes, then it has been cut down some. That location means it will probably shoot close to the distances set on the slider. Nothing to do but shoot it and see how it goes.

If in looking, you see the top of the front sight post is well under the top of the holes, again it has been cut down, but probably too much. Too short of a front sight will make it shoot high at the distance set on the slider.

That is a lot harder problem to solve. The US Army and the Italians welded up the top of too low front sights during rebuilds and then machined them off to look as an original new sight would have looked. It is sort of hard to do unless you intend to refinish the metal. Welding and grinding is messy and obvious unless done before a refinish. The resulting sights often had what appears to be metal missing from the shooter side of the front sight post. Rather than be perfectly square, there would be little holes. I had never heard of this until recently.

In the event a shooter is looking for a lollipop point of aim/point of impact is higher style of adjustment, then the front sight in each example would be ever so slightly lower.

If you have a carbine with a front sight cut too low so that USGI spec ammo shoots too high no matter what you do, don't give up completely or start hammering on the front sight in frustration. Some of the commercial ammo are 100fps+ slower than USGI and shoot lower. Find some commercial stuff that is slow or slower and it may well drop into the zero you want. Be sure of your impact points with good 10 round groups before you do anything. (3 shot groups are a joke and exist only because magazine gun writers can't keep more than three shots close together with most rifles, 5 shot groups need a bunch of groups for statistical analysis, and 10 shot groups provide good data very quickly.)
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  #6  
Old 09-15-2019, 07:05 AM
ibm1jh ibm1jh is offline
 
Join Date: Dec 2011
Location: North TX
Posts: 290
Default IMmaterial, not LESS Material

Quote:
Originally Posted by .Steve. View Post
Calculating from sight heights doesn’t do much for a particular carbine.

Take it out and shoot it at 150 yards.
Every 1/100th front sight change equals 2.5” at 150 yards.

Why, follows:

(2) Shoot a bunch of 10 shot groups before you decide the changes needed. Only when the group centers are in the same place with your shooting should you even consider changing the front sight.
AMEN! In my case, filing a front sight is COMPLETELY useless and a waste of time. (My) problem is not too much material, but too little muscle. I am extremely lucky if I can put all 10 shots in a group ON PAPER at 100 yards! even with a benchrest and shooting aids.

Basically, my problem is always being able to keep the sight on target long enough to pull the trigger, or jerking the trigger, or wind, or something. Calculating 1/100th of an inch is completely absurd for my shooting. I have long since given up on shooting match competitions and trying to get sub-inch groupings of more than 1 shot. But I try to remember 2 things:

1) These are battlefield weapons - they were hardly meant to be sniper rifles. They were designed for down 'n' dirty jungle warfare at less than 10 yards combat. 100 yard shots (with carbines, with predictable consistent hits) were probably a little rare. I dunno, but it seems like these are more street fighting weapons, not 1-mile snipers.

2) The little things are meant to be FUN! Whenever I get frustrated trying to shoot a group on paper, I have to stop and just shoot a mag without even looking at the grouping to remember - this is for fun and relaxation, not a contest. I know some people are in for match competitions, but I just want to enjoy and appreciate what American industry was able to achieve in a very few years of production.

Good luck on the calculations, but I would just shoot the thing. If it consistently hits low, aim higher. If it consistently hits high, aim lower. Thanks.
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  #7  
Old 09-15-2019, 07:37 AM
amd65 amd65 is offline
 
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“...1) These are battlefield weapons - they were hardly meant to be sniper rifles. They were designed for down 'n' dirty jungle warfare at less than 10 yards combat. 100 yard shots (with carbines, with predictable consistent hits) were probably a little rare. I dunno, but it seems like these are more street fighting weapons, not 1-mile snipers....”

Nonsense. 100yds is not a 1-mile sniper shot. The Carbine certainly was not designed for 10yds or less.
Keeping all 15 rounds on a paper target at 100yds is not difficult if the Carbine is up to spec, and the shooter has basic marksmanship skills. Sightpicture, breathing, trigger pull.
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  #8  
Old 09-15-2019, 09:16 AM
Rich/WIS Rich/WIS is offline
 
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The carbine was intended for troops who would normally be issued a pistol or whose job was not front line combat. The few carbines I shot were perfect for that role, accurate enough to hit an enemy well beyond 100 yards and a few were much better.
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  #9  
Old 09-15-2019, 11:58 AM
JimF JimF is offline
 
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Quote:
Originally Posted by Rich/WIS View Post
The carbine was intended for troops who would normally be issued a pistol or whose job was not front line combat. The few carbines I shot were perfect for that role, accurate enough to hit an enemy well beyond 100 yards and a few were much better.
Right on, Rich! . . .

As I’ve often said, it was the MILITARY that turned the original mission of the carbine around . . . .from a defensive weapon, to an offensive weapon!

Talk about “mission creep!” . . .

It sure didn’t take long for the full-auto version to appear . . . . or the M3 “sniper” . . . neither version conducive to “defensive” tactics!
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  #10  
Old 09-15-2019, 03:13 PM
BQ97 BQ97 is offline
 
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You're both incorrect. The M1 Carbine was developed for front line combat troops.
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