I have a new AP-4 from DPMS with 16.5 inch barrel. I'm shooting targets at 100yds and can barely keep everything within an 8inch diameter...even with using an Eotech 512. I've also got a Rock River match grade trigger in it. Is this world of short barrel carbine accuracy...or is there a few easy things I could check out.  (My Colt HBAR 6700 w/21inch barrel can shoot a gnat off a paper clip from the same distance...with same 55gr ammo)hinking.gif Any tips would be appreciated.

You cannot generally expect great accuracy out of a 16" tube with mil-spec factory loads.
I have shot 16" Wilson Barrels, 16" DPMS and finally ended up with a 16", 1-8 Douglas HBAR to get the kind of accuracy I wanted.
It shoots better with 60 grain Black Hills than 55 grain mil-spec....and evens stabilizes 77 SMK's very nicely.
My Bushy 16" Patrol Carbine, with it's chrome lined bore...shoots 55's nicely too....but it's a 1-9 and is not intended for match grade shooting.

I suspect M-4 Clones with 16" barrels do well for Mil-Spec accuracy needs of combat use. But I found to get a very accurate 16" tube requires a custom barrel with hand-loads.

I know somebody will say I'm full of it...they got a cheapy, short-barreled upper that shoots 1/4 minute accuracy all day. You may have a bridge to sell too, but have you seen my Florida Swampland Special just waiting for you?  

It's either you, or something is BADLY out of spec. That is a 3 inch or better setup, usually.

Are you firing offhand at 100 yards and getting 8 moa?

That's not bad. In my book, firing offhand and easily hitting a 10" circle at 100 yards is good shooting.

If you are getting 8 moa from a rest at 100 yards, that's not so good.

I figure 4 moa from a rest at 100 yards is decent with iron sights or EOTech.

HS1

If you have an EOTech, at 100 yards from a supported position, you should be shooting 1 - 2 MOA with that gun. Make sure your sights are tightly mounted to the weapon and that you're shooting consistent lots of ammo from the same manufacturer.  The truth would be at 25 meters where you should be stacking rounds with either the holosight or irons.

Your dot alone is 1MOA.    You can't expect even 2 MOA, but 8 definitely is excessive.  It is most likely more you than the rifle.  What is your accuracy at 25 yards?    It takes more care to stabilize a 16" barrel AR at a rest than a 20" barrel rifle unless you have a Dissy because you can have the rest further away from you on a longer barrel rifle.   Any movement on your part is magnified with a shorter barrel.

same accuracy of a 20" barreled rifle at 350 yds..14.5 to 20".5.56 is a nice flat shooter.