Making Plastic Front Aperture Sight Inserts

It's been a while since I made an airgun post. As I mentioned in my previous entry, I recently bought a Diana 75 T-01 10M match air rifle. It only came with one front aperture sight insert, of 3.5mm diameter.
I was having a dickens of a time getting a good sight picture so I noodled around on the internet for information and found that most recommend around a 4mm aperture for 10M airgun. I could have bought either metal inserts or a set of plastic inserts, but I figured I'd save some money and have some fun making my own plastic inserts.

Some squares cut out of 1/8" thick lexan. Acylic might be slightly better optically, but Lexan is what I had. I drilled with a #32 drill bit (.116", just inder 3mm) for a #4-40 screw hole.

Mounted several on a mandrel with a #4 screw

I turned them down to 17.5mm (.689") diameter, which seemed to be the size that fits the front sight mounted on the Diana. Sizing of inserts is confusing, will the common 18mm inserts fit my rifle? I don't think they will, unless that refers to the thread root diameter. But I digress.

Sorry for the blurry picture, I snapped these rather quickly. I mounted the disc in the soft pie jaw chuck, and drilled and reamed to 5/32" dia. although the hole came out smaller due to the springiness of the plastic, .154" (3.9mm). Anyway, I'll have to make a bunch of different aperture sizes to see what works best for my rather amateur shooting ability.
I then countersunk the hole with a 90 degree countersink, figuring that it would transmit the dark tube reflection as a ring, by the same action as a periscope prism.

The finished insert, with the metal insert beside it.

As you can see, I get a nice dark ring, without the added distraction of the little arms on the metal insert sight. I snapped off a quick 5 shots to test and shot the smallest group thus far. So it works! All told it took about 20 minutes to make the one from start to finish, with several blanks left over.


UPDATE 2/28/08
I did a little more futzing around, and made some new inserts based on my experiences. One thing I found is that if the insert thickness is such that it is under the aperture insert slot of the front sight tube, light will enter the plastic insert, spoiling the black ring. The obvious fix is to get some thinner plastic, but for now a piece of electrical tape over the slot takes care of the problem. I didn't notice until I shot with the shop lights on.

I turned the new inserts about .006" under the 17.5mm (.689") size that I did the first one. It seems to center well and is much easier to get in and out. I also chamfered the edges as the blanks were on the mandrel, using a vee cutter, as seen above.

First op. was to gently and lightly chamfer one end of the hole, both to provide centering for the next op. and to debur.

Then I drilled with a 4mm bit at relatively high speed, so as to get a round hole (hard sometimes with a drill bit as they want to make a Reuleaux triangle.

Then I flipped the disc and put the 90 degree countersink in with a 90 degree center drill.

I'll put all this up on a dedicated web page eventually...