Sight Geometry

[Weg Ban]

Having now played with this question until I am totally confused, I'll ask the Forum. Assume a 1911 with an adjustable rear sight. Further, assume the sight has been lowered to it's lowest position. The pistol is then aimed and fired. The point-of-impact (POI) is below the point-of-aim. You then raise the adjustable rear sight however many clicks it takes to get POI = POA at some specific range, say 50'. Further assume that for whatever reason, the particular pistol involved requires the sights to be raised very high (lots of clicks, so the sight is now sticking way up off the slide). An adjustable sight in that position is very vulnerable to mechanical damage, so if the pistol is to be carried, the sight should be lowered (but then you can't hit a target). It seems to me that to minimize this problem you could a) lower the height of the front sight, or b) shim up the base of the rear sight. If the rear sight can't be shimmed (say it's an Elliason), then the only choice is to mill off the front sight. Is it? Is there another way to get a POI = POA adjustment?

[brickeyee]

You have just described why adjustable sights are often frowned upon in guns that may receive hard use - such as a carry piece.
Changing the front site height is one way to get the rear lower in this case.

[robot1911]

Apparently you have plenty of leeway in your present sight configuration, so I'd recommend you lower that front. I'd scribe a mark on the side...say .020, and put that baby in your vice and file it down, filing front-to-back, not across the slide or you'll loosen the sight--assuming it's a stake-on type. Considering you have to raise the rear so much, .020 may not be enough.

Bob

[bearboy]

Just went through that with a 70 series colt.Factory sights 8 in. low at 25yds.put bomar rear sight on. had to file around .0030 off front sight or else leave the bomar rear sight way up.

[Weg Ban]

Good. Thanks for the info. I thought and thought about the problem until I confused myself with little diagrams. Regarding Brickeyee's first comment - I agree, my regular carry is a plain vanilla Government model with fixed sights that required almost no changes to have POA = POI at 50', but sometimes I want to put my Gold Cup on. I've got an old cast iron surface plate left over from a machine shop and a magnetic dial indicator, so I can get the measurements pretty close. I'll give it a shot.

[Hawkmoon]

You could also have a qualified gunsmith check the barrel lockup to ensure that the barrel link is the correct length.

A lot of Springfields come from the factory shooting low. I don't know what the factory does to correct it, but they do if the owners send them back.

[Gammon]

I use Novak fixed sights on my 1911s and install a taller than necessary front sight to adjust for elevation. I bought a small, cheap clamp on vice for this purpose, and padded the jaws to protect the slide. This allows me to use the vise at the range and make small adjustments on the front sight until the gun is zeroed.

[Weg Ban]

Thanks again. Hawkmoon: That's been done on both my GCs and they both shoot low at the bottom of the elevation setting. I will probably do them both (one at a time, so I only mess up one). I don't think I want them to zero at the lowest setting, I just want to get the setting a little lower so the Elliasons don't look like giraffes on the back of the slide.

[Hawkmoon]

I agree -- ideally, you would set the rear sight a couple or three or four notches up, so you have some modicum of adjustment left to compensate for different distances and different ammo.

[John]

How high you have to adjust the rear sight for it to shoot POI=POA? I mean can we see a picture of it? Something tells me it's not just a sight issue.

[Weg Ban]

I am not at home right now and the sight cards for both GC (which are similar, one a Ser 70 and one a Ser 80) are in my range bag which is at home. From memory, one is 42 clicks and one is over 40, maybe 41 or 42. I can do pictures of the Ser 70 when I get home (a day or two). Both have been checked by a gunsmith I consider competent and are correctly setup. At 42 clicks up, the Elliason sight is really vulnerable - I shoot and drop them back to rest before casing them.

[John]

Please do, there is something wrong with this! 42 clicks is more than half the height of a bullseye target at 25 meters.

[Weg Ban]

Still out, but I had another thought (dangerous, I know, but maybe this one won't get lonely and wander off). Assuming no mechanical problems and if it is just a sight issue, rather than trial-and-error filing on the front sight, why couldn't you use a micrometer to measure the height of the rear sight when zeroed. Then, knowing that measurement, lower the sight to say 8 clicks up and measure it again. Subtract the height at 8 clicks from the height at zero and have a competent machinist mill off that much from the front sight (no file, no wobble, no scratches, no vice at the range). Since all you are doing is shifting relative heights from the back to the front of the sight radius, the geometry ought to work. Right? Spot blue/black the milled surface. Then go to the range to "proof" it.

[Herb Clark]

Simple math - To determine change in sight heighth necessary: multiply change wanted (at a particular target distance) in inches times sight radius and divide result by target distance (in inches). For instance you are shooting four inches high at twenty five yards and your sight radius is 7 inches: 4 X 7 divided by 900 (25 yards converted to inches) = .031" or about 1/32". Therefore you need to lower rear sight .031" or raise front sight .031".