Canadian Airgun Forum

Make your own IOTA (Indoor Optical Training Aid) for people on a budget.
Disclaimer: Support folks making products like these! I'm pretty broke at the moment and couldn't afford the $60 for this. Seems like the manufacturer is a pretty good guy and I by no means condone taking business from him if you're in a place to give it. If my financial situation turns around I'm planning on buying one from him. See the link in the text body for information on purchasing.

I don't get to the range as often as I should, and training indoors (dry firing, positioning) can be problematic as far as getting a good sight picture goes. Via this post I learned about the IOTA, an aperture-reducing device that attaches to your scope and lets you get a good sight picture at as little as 11 feet. This is awesome for training inside, especially if you can get a slideshow of targets on your a backlit monitor (the device reduces aperture and as such limits light entering your scope. Having a well lit target is essential). Finished product.

Required materials:

Opaque, soft plastic. At least 60mm square.

Snips, diagonal cutters, or shears. I used all three, but primarily used the diagonal box cutter. Use whatever works for you.

Calipers or ruler with millimeter markings.

Permanent marker or felt-tipped pen.

Scope cap (mine's Butler Creek).

Instructions:

Find a suitable piece of plastic or card stock. The IOTA uses what looks like opaque black plastic. In my case, I used the lid of an ice cream container. The plastic is opaque, dark, and pliable. It's soft enough to be cut with scissors. Ideally find a plastic numbered 1-3 (inside the little recycling symbol. These numbers represent different types of plastic, and typically the lower you go the more pliable and less prone to shattering the material will be).

Measure the outer diameter of your scope's objective bell. Use calipers for this if you have them. My scope's bell is 49mm (SWFA SS 10x42M). For the sake of simplicity I'm using 50mm throughout this post.

Draw a circle on a piece of thick paper or card with a diameter equal to that of your objective bell. You can do this with a compass or you can lock your calipers at your given diameter (e.g. 50mm), draw a dot on the paper or card, and use your calipers jaws as a compass by holding one jaw on the dot and dragging the other in a circle to score the workpiece. Be gentle if you're using the calipers, as I imagine this could damage the tool if you use too much force, and TBH I wouldn't recommend it on your $200 calipers if that's what you've got. This is an example of what your dimensions will look like. And I know my drafting skills are rusty.

Cut out the circles with scissors or an Xacto knife. You can then use this to test for tight fit inside your scope cap.

Once you have a working diameter for the insert transfer the dimensions to your opaque piece of plastic. I ended up discarding the paper and just using the calipers to score the plastic. I found that this method gave me a more precise line, but do whatever works for you. Either way, check your marks with calipers or your ruler.

Score your inner circle. So here you need some basic math. The center hole has a diameter 1:3 to that of the larger circle (e.g. if your large circle is d=50mm you want your inner circle to be d=16.667mm). Extrapolate that ratio to your bell's dimensions. For example, my large circle was d=49mm, so my inner circle was d=16.33mm. Note: If you are using calipers or a compass to mark the dimensions you'll need to convert to radii. For the large circle set your calipers to 25mm for a d=50mm bell, and 12.5mm for the inner circle. Ultimately, you just want your inner circle to be 1/3rd as large as your outer circle. Once you have your lines marked or scored the plastic will look like this.

Cut! Time to cut out your plastic. I used diagonal box cutters for the profile of the larger circle. Follow the score marks (or marker lines) as closely as you can, erring on the outer edge of your mark. You can always cut the thing down but you can't really add material back. For the inner circle I punched a hole and then worked a small pair of snips around the score. If you have a 50mm bell you can probably get away with a 5/8" drill hole. Haven't tried that though. Finished product (again).

Test fit (again). Take your workpiece and try to fit it inside your scope cap. If it slides in and lies flat without too much force you cut it correctly. Most likely you'll need to remove some material though, so just go back and trim a tiny amount of plastic off the circumference of the circle. Test fit throughout. You want the plastic just big enough that it fits snugly inside the scope cap and stays put, but not so big that you're warping the dimensions of the cap.

Optional. Deburr your hole. You can use a knife to deburr the plastic. Scrape your blade on an angle around the rough edge of your cuts. You want the cutting edge facing away from direction you're dragging the blade. It may take a few passes, but it should do the trick. Bonus points if you have a professional deburring tool.

You're done! Fit your homemade IOTA in your scope cap and place it on your rifle. Set your parallax to the lowest setting and adjust the focus. Try it out on a target across the room with a lamp pointing on it.