My coment is first off, what is a custom lapping bar? I use centerless ground drill rod in the appropriate diameter. 30mm, 1" or 32mm which ever is applicable.
Centerless ground drill rod is fairly cheap and usually comes in 3 foot lengths so you wind up with plenty of lapping bars. Last time I bought a stick I believe it was 25 bucks and unlike mild steel rounds, the OD is precision, usually parallel within 0.001 and diamterically 0.001 plus it's hardenable (air or oil casehardening).
MSC sells it, Enco sells it, Alro sells it, Contractors Steel sells it.
Anyone who pays for a 'custom lapping bar' is not informed. No offense, it's just so much cheaper to be informed. Likewise with lapping compound. Midway sells it in nice convenient tubes but so does MSC and Enco, is nice tin cans ir more than you'll ever need.
I'd line up the rings by fitting the rings loosely on the rail, then slipping the bar through both rings and then toequing the rings to the bar itself and then rotquing the rail mount fasteners, in that order. The I'd loosen the tube mounts, pull the bar, apply dykem blur (or red) to the inner diameters of the rings, slide the bar back in snug the rings and rotate the bar. The loosen the tube mounts and pull the bar and note how the Dykem was removed by the bar. That will rell you if the rings are in alignment and it there are high spots.
If the rings are true and the Dykem is rubbed off evenly, I'd be looking at the rail, the receiver/rail mounting holes or the rail to receiver torque.
If I misspelled some words, excuse me, I'm a temporary gimp. Tipped a tendon in my haqnd....
