I tried zeroing the BM with irons last week @ 100yds and found the excercise futile. Even when spotting with a 3-9x scope it was very difficult to see the .223 on paper. What would be the best quality/inexpensive instrument to track progress?

Most of the optics companies make small spotters coting around $100 to $150 in the 20 to 40 power range that would be adequate for your purposes.   Try Burris, for example, although they are not the only one by any means.  These spotters are adequate mind you, not great. Great will cost $1000 to $2000. There are scopes at every price in between.

I had a goofy little simmons that worked at 100 yards...I think it was $40.  I knocked it off the bench countless times, and it never quit working.  

I eventually replaced it with a Nikon Earth and Sky...15x45x60...you can get the whole kit for less than $300.  As jhe888 said, there is a model for every budget.  

I like this one so far.  If you shoot from a bench a lot, I'd recommend that clamp on mount that Sinclair sells...it's very useful.  Gets the scope out of the way, and you won't have to worry about knocking it off the bench. It's a little pricy at $120 or so, but well worth it if you're like me...almost always at a range, shooting off a bench.  

I have YUKON SIBIR spotting scope, 20-50x..........great scope I can easily see 5.45 bullet holes (which makes 5.56 holes seem like .308 holes)  at 150 yards, without a problem

tag

FWIW, I like to use Shoot-N-C targets when zeroing past 50 yards. Much easier, even with average optics to ascertain groups with these targets at those distances. Beyond 50 yards I need glass.
5.56 x 45 leaves such a small hole that regular paper targets are very difficult to discern from beyond 25M for me.

Just my $0.02

I'm gonna have to check out Yukon Spotters. Thanks.

HS1

I had a goofy little simmons that worked at 100 yards...I think it was $40. I knocked it off the bench countless times, and it never quit working.

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Same deal here I got one 4 years ago from CDNN and that thing has been dropped,smacked etc. and it keeps working but it work to the 100yd marker.