Can I mark it on a cleaning rod? Yes. Dry erase marker - rinse repeat - same result. Pretty sure the OP isn't toting his Encore to a bench rest competition. And when you're new, having several methods to check the same thing isn't dumb.
Let's agree that what is required is a measurement with an accuracy on the order of 5/1,000th (or 1/200th) of an inch.
The thinnest mechanical pencil you can find (and that will take some looking) is 0.3mm, which is 0.012". (Not at all sure you can find a permanent marker that fine, but let's use it for discussion.) Such a point is, in other words, capable of marking a line about 1/80th of an inch wide. Assuming
everything else is perfect, that's your maximum accuracy potential. That's less than a third of the level of precision required. Moreover, you have to do it twice - once with a cartridge and once without. So it's off by a factor of six.
Now factor in the inherent problems with trying to get the point
precisely up against the muzzle, the inevitable distortion in a marker's point (we've all seen it) that comes with use, etc. Add those on and you cannot possibly get a measurement accurate within a factor of ten.
Perhaps it could be said that all you have to do is measure the distance between one
side of one line and the same side of the other line. Really? What are you going to measure with? A normal ruler would of course be pointless, so you'll have to use a micrometer or calipers. Unlike measuring a real object with calipers however, where the jaws run into resistance, you're going to have to eyeball it - eyeball the difference between the
edges of two lines a mere 1/80th of an inch wide. Sorry, but I don't think so.
Oh, and is the front end of your rod
precisely square?
In short, while this is probably a good way to check general measurements, there would seem to be enough compounding potentials for error that I remain unconvinced that it be relied upon to provide precision measurements.
