I was shooting some different brands of ammo out of my 14.5 inch Bushie M4 and I noticed that the .223 rounds grouped significantly poorer than the 5.56 ammo, even from the same company.  I fired Federal M193, S&B .223, S&B SS109, and surplus no name 5.56.  The Federal grouped the best and the the rest of the 5.56 ammo grouped only slightly larger.  However, the S&B .223 grouped almost 2x's as big as it's 5.56 counterpart.  Is this normal?

Different rifles, even some made by the same company to the same specs, will perform differently with various ammo. You may try the same ammo in another AR and find that the results will be reversed. I have a 20" Colt HBAR 1/7 twist that seems to shoot the S&B SS109 better than anything next to match loads. I also have a 16" Bushy HBAR 1/9 twist that seems to have a thing for Federal XM193 and Q3131A.

So while a specific type of ammo may be better on average than another, some rifles may shoot better with something else. That's why if you are interested in accuracy, it's often best to fire various loads and see what your particular rifle likes. My experience has been that the commercial spec loads are often more accurate than the mil-spec loads. And the most accurate one of these I have found (although underloaded too much for me to choose as defensive ammo) is the PMC 55 gr. This round seems to shoot well consistently in all my AR's.

Thanks for the advice Charging_Handle.  I just bought 600 rounds of UMC .223 before I realized that my rifle seems to like 5.56 fodder better.  I will be getting a F/A AR-15 soon so I'll be saving the UMC for that.  

I have a Bushy Shorty A1/C7 and a 20A1 HBAR.
both Rifles shoot M855/SS109 better than M193?
My 20HBAR also shoots the 69gr. BTHP very well! My Shorty shoots the PMP very tight! But then agian it also shoots wolf tight?