I am certainly not a National match caliber shooter.
I did decrease my group size by about half at 100 yds by replacing my A4's trigger. The one it came with was terrible at best and thats being kind.
The sighting system on many AR's is less than optimal, They were built to be used in poor light and after being made dirty. Kind of like the old MOA peep site on the M1 Garand. What type of sight system is on the AR and whats on the Winchester?
Twist rate of the barrel need to be matched with the bullet weights you are shooting. Some twist rates lend better to heavier bullets than others. This tends to be more of an issue smaller diameter bullets. Quality of ammo may play a role. If the stuff you were pushing through the AR was surplus junk, it may not group well at all. They sell that stuff by the boat load to the "spray and pray" crowd. Were you shooting the same Cal rifle and the same ammo out barrels of the same twist rate?
As in a previous reply, you can purchase off the shelf accurate AR platforms, but be prepared to pay a fair chunk of change more for it.