Many of us Smallbore Rimfire shooters prefer the compactness, lightness and general convenience of the low power, fixed focus 'scopes that are available. Typically, one can get better glass and construction in comparison to the more complex Adjustable Objective, (AO), 'scopes in the same price range . . . and aesthetically they blend better with the trimmer lines of the typical .22.
However, although one can find a great variety of such 'scopes in the "very inexpensive, Department store" grade that have their objective lenses set for a parallax-free range of 40 - 60 yards, specifically for the supposedly impecunious smallbore shooter, virtually none of the better quality units are so calibrated. The general trend by all manufacturers has been to assume that only "wealthy centrefire shooters" would purchase their offerings. Hence such better quality 'scopes are almost all calibrated for a parallax-free range of either 100 or 150 yards . . . a bit to far out for most rimfire shooting.
So . . . what to do about it ?
The solution, of course, is to reset the focus to a more suitable distance for our needs. One can send the 'scope back to the manufacturer . . . cross his palms with silver, say $25 ~ $30 plus shipping, and have him do it. On the other hand, a few minutes careful work at home will achieve the same results, at no cost !
If you examine the front end of the majority . . . (Warning ! But not all !) . . . of such fixed-focus 'scopes, you will notice that there is a joint line showing a short distance back from the rim. Ahah ! This is where the forward lens locking ring meets the main bell of the scope. Taking a firm grip on that forward ring try "un-screwing" it. Sometimes using one of those grippy rubber sheets sold for helping housewives undo tight jar lids will be handy here. As a last resort one of the small strap wrenches , such as can be found in Sears Craftsmen tool departments, might be used . . . or even, G*d Forbid, a pair of large curved jaw Channel-Loks. Again - WARNING ! You must not damage or distort this locking ring in loosening it . . . typically it only needs to be loosened less than one turn to free up the lens holder adjustment. Below is a picture of the bell of an older inexpensive Tasco 'scope, with the locking ring completely removed. The tiny slots in the edge of the lens holder for spanner engagement can just be made out.
Now - decide on the optimum distance for your normal shooting. For most non-competition hunters and plinkers a focussing distance in the 35 to 60 yards range will suffice. On the other hand, for example, I do a lot of CMP Rimfire Sporter competitive target shooting. The course of fire, including sighters, entails shooting some 50 rounds at a distance of 50 yards, followed by 20 shots at 25 yards. Now it's obviously not convenient to re-focus the scope between stages . . . so I'd need to pick a focussing distance that will provide minimum parallax errors for both ranges. A quick spot of proportional arithmetic shows us that approximately 43 yards is that optimum distance for this example.
With the scope installed on the rifle, which in turn is "held" in some convenient steady position . . . say sandbags, or a bench rest device . . . take a peek through the viewing end at a target you've set up at the desired distance. I would suggest that the target show a few distinct "lines" or rings that will let you get a good visual reference. Now move your head - and hence your viewing point - slightly up and down, and side to side - while viewing the target closely.
Does the image appear to move relative to the reticle as you do so ? It does ? That apparent motion is known as parallax error . . . it's what causes us to "miss the Bull" because our cheek weld on the butt is not consistent ! Now, by carefully re-locating the postion of the objective lens, we're going to minimise or eliminate that error for our chosen focussing distance.
If, like most centrefire-oriented scopes, the one you have bought is set for a 100 or 150 yard range, then the objective lens will need to be"moved forward" by unscrewing it a little in its mounting threads - normally widdershins, (that's anti-clockwise to you Colonials - lol), will do the trick. The amount to unscrew it may not be very much, and should be done in very small increments with a viewing test after each adjustment. How do I move it , you ask ?
Normally, if you take a close look at the lens holder you will observe a couple of small slots cut in the front edge of the lens holder. These were provided so that the manufacture could use a special spanner to grip and turn the lens holder when he set the original focus. Generally you won't have such a gadget in your posession, so . . . . ! With extreme care, so as not to slip and damage the objective lens, a slight push with the tip of a fine screwdriver on alternate sides of the holder will persuade it to move in the direction you want. It won't move ? You might need to make up your own spanner . . . a fairly simple task ! Take an old hacksaw blade, break off a short length but slightly longer than the internal diameter of the lens holder. Now grind or belt sand the ends of this strip down to a length that will just allow it to fit across the internal diameter of the lens holder and slide nicely into the spanner slots. Use this to coax the holder to rotate in the desired direction. Cutting a slot in a small piece of wood with a similar sized hacksaw blade, and epoxying or Crazy-Glueing in your prepared strip will make it even easier, because your new tool now has a convenient "handle"
When you have adjusted the lens holder to the spot where, no matter how you move your head and viewing position, the target image "doesn't move", then you have set your focus to the range you've chosen as optimum. Now, making sure the lens holder doesn't move while you're doing it, re-tighten the lens locking ring. That's it !
Now go out and shoot
