Measuring headspace with crimp die

I use an RCBS mic gauge to measure spent rounds for headspace when I'm reloading 223.

I've just started reloading 300aac and there's no mic gauge for that.

So here's my idea... When I drop a spent round into the factory crimp die the shoulder comes to rest on the split collet.

The base of the round is slightly recessed into the die at that point. If I measure that distance I have a defacto measurement of the distance from shoulder to base on the cartridge.

If my resized brass match that distance minus a few thousands then I'm in the right ballpark with headspace aren't I?

I've never heard of anybody doing it this way, but I can't see why it would be less accurate than the mic gauge as long as I have a good measuring tool.

Am I missing something?

You could buy a headspace gauge in 221 fireball for maybe 25$ us . Take a go gauge.

Hi, one way to measure head space is to fire a primer in an empty case the primer will back out of the case then you can measure how much it is sticking out and that will tell you how much head space you have you may have to load a wad of some sort to generate enough pressure in the blackout but for most cases just the primer will do the trick .. if you are not having head space problems why measure it Rob