I recently purchased a used Marlin Model 30 AS that appears to be in very good condition. During my first range visit with it I was using 150 grain Winchester factory catridges. I was able to get the rifle to shoot about 3-inch groups @ 100 yards with correct elevation but all my shots were consistently about 1 inch right of the bulleye regadless of range.

I understand I need to move the REAR sight to the LEFT to move the point of impact to the left. My question is how much to move the sight? Is there a rule of thumb to go by for 100 yard sighting or is it purely a trial and error process. I have visions of over correcting in both directions and ending up in a worse situation that at present. Thanks for the advice.

Sounds like you have good young eyes to be shooting that well with iron sight. I have had to give up that kind of accuracy with my failing eyesight. The best method it to mark both the rear sight and barrel with a 'witness mark" to see how much you are moving the sight. Needing only a 1" correction you will only have to move the sight a fraction, thus without a reference mark aka witness mark it is difficult to tell how much you have moved the sight. A lead pencil mark works well and is easily removed. Again I know of no rule of thumb just tap it a little and check your results but you should only have to move the sight a very small amount. Also be aware that you should shoot the gun at the same time of day to get the same results. If you sight in, in the am with the sun shining on the front sight from one angle and come back in the pm with the sun shining on the opposite side of the sight your groups will have a different point of impact without moving the sight. So in conclusion if this rifle is only 1" off is it worth moving?

Well, if i were you i'd try a few different types of ammunition, if i was still getting the inch to the right with several different ammo selections, id still just leave it alone, sounds like its gonna be a PITA to get it one inch over without over correction, thats just my opinion though.

If you decide to drift the sight over a bit, first place a small srtip of yellow masking tape on the barrel just in front of the sight's dovetail to better hold/show the pencil mark referred to above.



For only 1" off on a game gun - I wouldn't bother, though.

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Brownell's has a calculator on their web site for height adjustment (ie picking a taller front sight). It should work exactly the same for side-to-side calculations. With the depth function on a dial caliper it should be possible to see how much you are moving the sight, assuming a place to accurately measure from. If not - tape or pencil marks, and estimate the best you can.

Thanks for the suggestion gentlemen. The tape & pencil mark worked reasonably well with only one over-correction. Still getting 3-inch groups @ 100 yards but the holes are basically splitting the bull more or less equally...good enough for 65 year old eyes!

Brad

1" at 100 yards is close to "minute of angle". With a 36" sight radius, .010" is very close to the same "minute of angle". Let's just say that your sight radius is 18", and I'll bet its shorter, you would need to move .005" left to make that 1" correction at 100 yards. With a 12" sight radius, it would be closer to .003". 1/64" is roughly .015", so you're talking about a small adjustment.

Put a receiver sight on that rifle to increase your sight radius, and make a clearer sight picture. The better receiver sights have click adjustments in fractions of minutes depending upon the sight radius.