.22 LR trajectory question

[gunsrfun1]

I've always been lousy with understanding trajectory (if it shoots low/high at one distance, what does it do at other distances), so I am hoping someone can explain this to me.
I have a Marlin 39 I just picked up. It has a Lyman 5D rear sight and a Lyman front bead sight (not the original front sight).
Even with the 5D bottomed out, it shoots high as follows:
2.5" high @ 50 feet
3.5" high @ 25 yards
6" high @ 50 yards
8" high @ 100 yards
Yeah, really.
Obviously I need a higher front sight, but what I am confused about is why it continues to shoot high at 100 yards, since a .22 zeroed at 50 yards should be shooting low at 100 yards, not high.
I am thinking that the reason it still shoots high at 100 yards is because I am shooting high "across the board" so I never have a zero point as a reference.
Is my logic correct?
Thanks

 

[rvn67n20]

Sounds like it to me; I'm guessing that the current zero range is "way out past Fort Mudge."

 

[grovey]

here's something for you to play with..

http://ballisticscalculator.winchester.com/

 

[MErl]

your logic is correct. plot those numbers out and you have a zero at maybe 2 yards. you are really lobbing those bullets into a nice high arch.

 

[jmr40]

Since the bore of the gun is lower than the sights, the bore will always be pointed slightly upward. When a bullet is fired it will travel upward at an angle and at some point will be in alignment with your line of sight. The bullet will reach the highest point of it's arc and begin to drop somewhere along it's flight. Where the bullet crosses the line of sight the 2nd time is the range the rifle is zeroed. Your gun is probably zeroed at 150 yards or so.

With a front sight exremely low in relation to the rear sight your barrels upward angle is quite steep I wouldn't be surprised if it does not cross the line of sight very close, 2 yards from the muzzle may well be right. With most guns this doesn't happen until 25-50 yards. Your gun is still high at 100 yards because the angle of the sights is so extreme that the bullet still has not reached the top of it's arc and started dropping yet.

You run into the same problem with AR's with their sights being so high above the bore and with scopes mounted in extra high rings. Some take advantage of this and use it for long range shooting. It helps cheat a bit and give a little extra sight adjustment.

The problem is not just your front sight being too low, but the that the rear sight is too high. A lower rear sight, or taller front sight will dramatically decrese the upward angle of the barrel. You may never find a front sight high enough to get a 50 yard zero with that rear sight. If you do it will be WAY up above the barrel. Then you may have trouble getting your eyes to line up with the sights with the stock. For most hunting uses this is why it is usually better to have sights/scopes mounted as low to the bore as possible.

 

[gunsrfun1]

Thank you all. Now for the bonus round question.

You have the stats on my point of impact. Now let me provide some more info, in the hopes you can advise on a replacement front sight. We’ll be working from the Marble web site.

Here is the URL: http://www.marblearms.com/frontSights_info.html. (I picked Marbles because I heard that my 39 Century originally came with a Marble front sight, although no one has confirmed that; plus their web site is relatively easy to navigate for amateurs like myself.)

I tapped out the front sight. It’s a Lyman 31MA stamped on the bottom. It measures .445 inches high from bottom of dovetail to top of sight. (That’s how Lyman and Marble measures their sight heights.)

Going to the table at the bottom of the URL I provided, there is no “adjustment adder” beyond 6 inches. And you already know that I am about 8 inches high @ 100 yards. So we have to extrapolate or best-guess. (I only want to replace the sight once, if I can.) The distance between the front and rear sights is 21 inches. If I add the maximum shown (.035) in the table to .445 I get .48 inches. But that is for shooting 6 inches high, not 8.

When I spoke to the tech expert (very nice guy) at Marble last week, I had estimated the sight height at .433 based on measuring it in the gun, and I had only fired at 50’ and 25 yards. Based on that info, he had proposed my going to a .500 inch front sight.

This seems to be a reasonable suggestion, but I am open to your opinions, now that we have more info to work with. I don't want to underestimate the height and then still be shooting high. You can see that they make two other higher heights, .530 and .570.

On another subject, regarding the bead itself, the current sight has a 1/16” diameter white bead. My Lyman 5D rear has a .05” peep hole. I am thinking of going to a fiber optic, but not sure. Open to your opinions on that too, as well as your preferences on 1/16” diameter bead vs. 3/32” diameter bead, ivory vs. fiber, etc.

As always, thanks in advance for your opinions and guidance. This gun is very accurate (groups) so far, so I'd like to get it set up right.

 

[ngnrd]

What is the distance between your sights?

 

[gunsrfun1]

Distance between the sights is 21 inches.

 

[ngnrd]

Sorry. I missed that in your other post...

Anyway, for an 8" correction at 100 yards with a 21" sight radius, you need to add 0.047" to your front sight.

0.445 + 0.047 = 0.492 <----- that's your minimum front height needed to get dead on at 100 yards.

Since you currently have some upward adjustment on the rear right, and no downward adjustment, you want to go taller to still be able to adjust your elevation.

If you install the 0.500" sight, you will be shooting 0.5" low at 100 yards, and will be able to easily correct that with the rear sight elevation adjustment. But, if you ever change ammunition, you may not have enough adjustment remaining to compensate.

If you install the 0.530" front sight, you will be shooting 6.5" low at 100 yards. You will have sufficient elevation adjustment in your rear sight to make this correction.

If you install the 0.570" front sight, you will be shooting 13.5" low at 100 yards. You may still be able to make that correction with your rear sight, depending on how much elevation adjustment there is.

So, it looks like either the 0.530 or the 0.570 height would work. If it were me, I'd pick the 0.530" height, in order to allow a reasonable adjustment while still keeping the sights as low as possible.

 

[gunsrfun1]

NGN - Thanks. The .530 sounds like a good choice. I did fail to mention that I plan to zero it at 50 yards. Not sure if I implied I was going to zero at 100 yards, sorry.

I don't think that should change your advice, but just letting you know.

 

[ngnrd]

It may make a difference... let me run the numbers.

 

[ngnrd]

OK...

For a 50 yard zero, the numbers do change just a bit.

The 0.500" front sight height wouldn't even get you back down to zero. It would still be shooting an inch and three-quarters high.

The 0.530" front sight would shoot 1.25" low at 50 yards. Certainly do-able, but not a whole lot of adjustment remaining. A change in ammo may have you fighting for enough elevation adjustment again.

The 0.570" front sight would shoot 4.75" low at 50 yards. In this case, raising the rear sight just under 1/16" should get you back to zero, and leave plenty of adjustment - both up and down. And it should still work fine if you want to change to a 100 yard zero.

So with a 50 yard zero, I'd probably opt for the 0.570" front sight.

 

[gunsrfun1]

Great, thanks again.