Have any weird angles or dimensions you need measuring and long for a real optical comparator? Here's how I made my Android phone into a pseudo comparator.

I used ON 2D-CameraMeasure
ON 2D CameraMeasure

You'll need a secure phone mount/tripod. I used a cheap suction cup windshield mount.

A light-box will come in handy. The web is full of cheap DIY's.

You'll need a ball joint, I used a ping-pong ball in a shot glass. You'll need a way to affix your item (in this case cheap steel .223) to the joint. I used modeling clay. Something a little stickier would probably work better. Like the no holes poster hanging putty.

Overview shot.

You'll be taking a picture in macro mode. My phone can focus pretty close, but the stock app doesn't do a good job of things. I used a magnifying glass app.

Magnifier

Now you'll want to adjust the pitch and yaw of your item on the ball joint by rolling it around. What you're looking for is a true silhouette. The flat base of the cartridge and the sharp edges of the extractor groove will help you zero in on the image you want. You're trying to align the item's plane of observation with the plane that your camera uses.

Silhouette


Now, load that image into ON 2D.

From there, follow the apps directions on how to take measurements and calibrate your image.

Here's the shoulder angle of the steel 223.
ON 2D Shoulder angle

180-157.58=22.42. Close enough to the spec 23Deg, with shaky fingers and all.

Cheers!

Interesting to say the least.