I am using a PO 3.5x21 calibrated in 5.45 on my Arsenal 106 in .223. I was wondering how close it would be as well. Unfortunately, I have not had the chance as of late to test the BDC past 100m. I think it would be pretty close using 55grn ammo, but I am no expert in the differences and similarities between the two calibers.

Any ballistic experts out there that can answer this?

 

They are virtually the same out to the ranges that matter. About 400-500 yards or so. When I got my PO scope a few years ago I spent some time on the ammo makers' websites trying to determine their bullet drop charts and while not scientific, they seem to be pretty close to each other. I figure if I need to aim at anything more than 5 football fields away I'm either in the wrong spot or using the wrong ammo

 

Primary Arms and others generally rate their BDC reticle optics for 5.56/.223 as 5.56/.223/5.45 reticles. IIRC, Rob Ski has commented and demonstrated on video that the 5.56, .223 and 5.45 can all use the same reticle out to 600 yards, which is at the edge of all 3 cartridges effective range.

 

I suspect they're close enough to where the quality of ammunition will probably affect it more than that actual BDC reticle itself.