Sphinx Sub Compact Sight Question

[dross]

I'm looking for a set a night sights for my Sphinx Sub Compact. I'd like to use XS Big Dot sights, but they only offer them for the Compact model, and they say specifically they won't work in the Sub Compact because the point of impact will be off.

Here's the thing - I measured the sights on both pistols and as near as I can tell, they both use the same height front and rear sights.

Any idea why the impact would change, if the stock sights are the same height?

[rhart]

Shorter sight radius?

[dross]

Shorter sight radius?

Exactly. A shorter sight radius should change the point of impact. But that doesn't answer my specific question. If the stock sights are the same height, why would aftermarket sights the same height change the point of impact any differently?

[DF_Hammack]

Exactly. A shorter sight radius should change the point of impact. But that doesn't answer my specific question. If the stock sights are the same height, why would aftermarket sights the same height change the point of impact any differently?

I think you misunderstand the physics of sighting. A shorter sight radius does not change the point of impact; it makes it more indeterminate. As the sight radius shortens, any error is multiplied. A short sight radius is bad. The longer the sight radius, the better.
What changes the point of impact is the relatioinship between the placement of the front and rear sights, relative to the bore. Proper sighting requires aligning the sight axis with the bore axis. As for the height of the sights, taller sights require more paralax correction. Paralax is the angular difference between the bore axis, and sight axis. Shorter sights (not shorter sight radius) require less paralax correction.                                           

[rhart]

Shorter sight radius?

Exactly. A shorter sight radius should change the point of impact. But that doesn't answer my specific question. If the stock sights are the same height, why would aftermarket sights the same height change the point of impact any differently?

I'm guessing here, but perhaps one of the guns required a variation in standard front sight size when regulating the pistol at the factory and this caused the sight height of one of the pistols to end up having the same height as the standard sight on the other. If the variation is on the subcompact, you should be good to go with what's available (assuming the sights are satisfactorily regulated). Of course, there could have been a mix-up at the factory with the sights. I have three Sphinx pistols - one subcompact and two compacts and I have found sight regulation to be hit or miss as all three have different POA/POI points elevation wise.

[dross]

Shorter sight radius?

Exactly. A shorter sight radius should change the point of impact. But that doesn't answer my specific question. If the stock sights are the same height, why would aftermarket sights the same height change the point of impact any differently?

I'm guessing here, but perhaps one of the guns required a variation in standard front sight size when regulating the pistol at the factory and this caused the sight height of one of the pistols to end up having the same height as the standard sight on the other. If the variation is on the subcompact, you should be good to go with what's available (assuming the sights are satisfactorily regulated). Of course, there could have been a mix-up at the factory with the sights. I have three Sphinx pistols - one subcompact and two compacts and I have found sight regulation to be hit or miss as all three have different POA/POI points elevation wise.

If the sights are the same height, but the front sight moves back, it would change the angle between line of sight and the bore, moving the impact down. Wouldn't it?

[dross]

Shorter sight radius?

Exactly. A shorter sight radius should change the point of impact. But that doesn't answer my specific question. If the stock sights are the same height, why would aftermarket sights the same height change the point of impact any differently?

I'm guessing here, but perhaps one of the guns required a variation in standard front sight size when regulating the pistol at the factory and this caused the sight height of one of the pistols to end up having the same height as the standard sight on the other. If the variation is on the subcompact, you should be good to go with what's available (assuming the sights are satisfactorily regulated). Of course, there could have been a mix-up at the factory with the sights. I have three Sphinx pistols - one subcompact and two compacts and I have found sight regulation to be hit or miss as all three have different POA/POI points elevation wise.

That's very interesting and useful information. So you're saying that the sight heights vary on all three of your Sphinx pistols? If that's the case, and they're using different sight heights to make up for individual variation, I wonder how that affects aftermarket sights.

[DF_Hammack]

If the sights are the same height, but the front sight moves back, it would change the angle between line of sight and the bore, moving the impact down. Wouldn't it?

No, moving the sight back will do nothing, except increase the magnitude of error. When you move the sight back, you are moving it in the same axis relative to the bore. In order to change the POI you must change the sight axis, relative to the bore axis. To change vertical  POI you need to change the height of either the front or rear sight.

[DF_Hammack]

Dross, I made an image to illustrate the effect I am talking about...

[dross]

Dross, I made an image to illustrate the effect I am talking about...

Bear with me for a moment. You're showing the bore and the line of sight as parallel, but they're not, they intersect. The bore is actually tilted slightly up relative to the line of sight.

As you move the front sight back, you move it under the line of sight enlarging the angle.

[DF_Hammack]

Dross, I made an image to illustrate the effect I am talking about...

Bear with me for a moment. You're showing the bore and the line of sight as parallel, but they're not, they intersect. The bore is actually tilted slightly up relative to the line of sight.

As you move the front sight back, you move it under the line of sight enlarging the angle.

No, I am showing that moving the front sight to the rear changes nothing, but that raising the rear sight lowers the POI. It logically follows that if you raise the front sight raises it the POI. Moving to the front sight to the rear increases the apparent size of the front sight, but it is still the same height.

[dross]

Dross, I made an image to illustrate the effect I am talking about...

Bear with me for a moment. You're showing the bore and the line of sight as parallel, but they're not, they intersect. The bore is actually tilted slightly up relative to the line of sight.


As you move the front sight back, you move it under the line of sight enlarging the angle.

No, I am showing that moving the front sight to the rear changes nothing, but that raising the rear sight lowers the POI. It logically follows that if you raise the front sight raises it the POI. Moving to the front sight to the rear increases the apparent size of the front sight, but it is still the same height.

Look at your bottom image. It's the most realistic, even though very exaggerated. It shows the line of sight intersecting with the bore, creating an angle. This is how sights are set up. When aimed, the bore is tilted slightly up to compensate for bullet drop. The bullet, coming from the bore pointed slightly up will cross the line of sight at some point, rise to its apex, then drop again across the line of sight. This is why there are actually two zeros.

Look at the bottom image again. If you move the front sight back, it will be below the line of sight. This would cause you to raise the bore even more to create a good sight picture, and it would raise the impact point of the bullet on the target.

What you're describing would be correct if there was no bullet drop and the bore and bullet flight ran parallel to line of sight. Then it wouldn't matter if you change how close the front sight was to the rear.

Your description would work if the firearm shot a laser out of the bore, because the laser would't drop and there would be no need to have the bullet trajectory intersect with the bore.

[rhart]

No, I am showing that moving the front sight to the rear changes nothing, but that raising the rear sight lowers the POI. It logically follows that if you raise the front sight raises it the POI. Moving to the front sight to the rear increases the apparent size of the front sight, but it is still the same height.

I think you've got it backwards a little bit. Raising the front sight lowers the POI. Raising the rear sight raises the POI.

[Walt Sherrill]

Could it be that XS knows something that we don't know? 

Are the factory sights really the same for both the subcompact and compact model AND are interchangeable?  A quick call to Kriss/Sphinx might give you an answer.  Explain that you want to use BIG DOT sights for a compact on a subcompact, and see what they say.   But if they are physically the same sights, there shouldn't be any difference.  Barrel length and sight radius should be non-issues.

If the factory Compact and Subcompact sights are interchangeable and are effectively the same sight, nothing will have really changed if you install "compact" XS sights on a subcompact gun.   You'd have to use different  "Kentucky" windage with a shorter barrel than with a longer one, when shooting at near or distant targets -- but most ammunition loads and gun sights are set up to zero at about 20 or 25 yards.  (I don't think I'd be spending a lot of time shooting at 20-25 yard-distant targets with a subcompact; I'd be practicing at closer distances.)

As you can see by looking at the images below, how you position the BIG DOT in the "V" of the rear sight can be different than when using a smaller dot.  But if you align the TOP of either sight with the target you might be OK.  Using the WHOLE DOT rather than the tritium center can have a big effect on the point of impact at greater distances, but not much when the target is up close -- and in dark situations you may only see the tritium dot as it glows.  You'd have to find a sighting method that works for you.

I have a Sphinx SDP, and it has a black rear sight and a small dot front sight.  I tend to line up the top of the sights and try to center the front sight's dot in the rear notch and aim over the top of the sights.  It works for me.  I've not found a recommended sight picture for the Sphinx SDP.

The only XS dot sights  for a SPHINX (SDP compact only) are shown below.



I've been tempted to get a set of their smaller --  standard -- night sights for several guns, but I get put off by their higher prices.  But if you really want a BIG DOT front sight, a higher price is something you just live with.

[twowheels]

The sphinx rear sight is a novak cut.  I put a height adjustable rear novak single vial tritium sight on my subcompact and had tooltech make me a front sight ($$$).  I can get the POI I want with this setup.