Last year I bought a Sig556 ER.  It sat for a while and I have finally started taking it out.  I really like this rifle but I'm getting really frustrated with it.  I took it out last weekend to the range to zero in a new scope.  I was able to get great groups with a 6X scope at 50yrds.  When I pushed the target out to a 100 yards my groups ballooned so large I was not even hitting the target..  Any ideas on what is going on?

Thanks!

What ammo were you using?

What kind of ammo are you using? If you're using 55 gr. ammo, I'd recommend switching to heaver rounds like 62 or 75gr.