could anyone give me a rough estimate on the maximum EFFECTIVE distance of a 55gr XM193 Federal 5.56 coming out of a high end 16" 1/7 barreled piston ar15??

20 yards less than from a DI AR.

Seriously though, you can hit past 4 if you're good.

Unless you got a high-powered scope, a spotter with an even higher powered spotting scope, and match-quality reload versions of the XM193, I think its gonna be hard to even see if you can get that distance, IMHO.  

i believe 2700 fps is the fragmentation threshhold but im not sure what distance that turns out to be. i think Molon has the info posted in the hornady tap thread.

I have read posts from warfighters in SW Asia regularly getting hits beyond 500 yards with the M4 and M855 that put down the bad guy.
All the data we have on the wounding mechanism of this round and the range that it will reliably fragment within are about stacking the odds in your favor for possible defensive shoot situations.
What it consistently fails to do is restate the obvious facts that punching holes in someone's brain housing group will typically be terminal, regardless of neck, max TC, or retained weight of the projectile.

Only hits count, regardless of what bullet you are shooting, the weapon it is coming out of and the range it travels before impact.

Over and over and over again we get to read the same oversimplified question, spiced up with irrelevant details that make it clear the OP has no idea what he is even asking that makes me appreciate and respect the mods in this forum and some of the people who routinely answer the OP's question instead of explaining why the OP is asking the wrong question and pointing them to the content in the FAQ.

Does rate of twist have anything to do with the "effective range" of a 55 grain bullet?
Will a piston instead of a gas tube impact the "effective range" of the round?
Will a miss within the consistent fragmentation range of M193 stop a bad guy better than a hit beyond that frag range?
Will a "low end" rifle wielded by someone who trains constantly and keeps his cool under pressure be less effective in battle than a "high end" rifle wielded by someone who never trains and freaks out when the pressure is on?

Compare the ballistics of m193 and 22lr and 22mag ballistics.  For example, m193 @ 500 yards vs 22lr and 22 mag at muzzle.
A 22mag at point blank, with soft points, is nothing to sneeze at!  
At least thats they way I think of 223 "effectiveness" at long distances (long to me).