I have a piece of precision ground .300 round stock I special ordered to use as a gauge. I did this before Geissele came out with their tool to do the same thing. If I had it to do over, I would buy the Geissele tool. If you're only going to do it once, a lot of NFA dealers and gunsmiths can check it for you for cheap, I know a couple of guys who do it for $25 and $30.
I've never seen a 30 cal cleaning rod. If you're talking about a cleaning rod made to use on a 30 cal rifle, no, that won't work, as there is far too much play (upwards of .05) between the lands and the rod.
Having a baffle strike is a big deal. I waited over 9 months and paid upwards of $1400 for my 30 cal suppressor a long time ago. If I put a bullet though it, I have to send it back to the mfg, and HOPE they can fix it, otherwise I get to buy a whole new can, including a new stamp, approval process, transfers, shipping, etc.
I should have stated in my original post, if I'm putting a 30 cal can on a 556 gun, I don't check that, but you're looking at over .075 clearance between the two.