I bought a "Parts" AR at the Fun Store a short time ago am trying to identify the barrel.

Barrel is 24" & marked SUM 5.56 and is 12 years old. Yes, it has been sitting on a shelf 12 years, the gunsmith went trough it about a year ago when he bought it.

I am curious what the twist rate is, and if it is chrome lined?

Anyone know the make?

sounds like an Olympic arms Ultra Match. I have one on one of my rifles. There not chrome lined and the chambers cut in 223 not 5.56. I have no complaints mine shoots better than I can hold. Mines 1-10 twist.

What kind of groups do you think I could get at 100 yrds?

Do these perfer a certain ammo, grain, bullet weight?

Quoted: Barrel is 24" & marked SUM 5.56 and is AT LEAST 12 years old.

It is 1:10 twist by default.  These are SS broach cut barrels.  They are VERY good barrels - better than most by far.

SUM=Stainless Ultra Match. No chrome. Very good barrels. Best way to determine twist is to take a cleaning rod that has a rotating ball bearing handle. Place a tight fitting oiled patch on a jag and insert it into the barrel a few inches shy of coming out of the muzzle. Then mark your rod with a marker or piece of tape on the top of the shaft at the edge of the barrel. Now gently pull the patch through while watching the mark on the rod. When the mark makes 1 full rotation and is back at the top of the rod mark the rod at the edge of the barrel. Then measure the distance between the marks to determine your twist. It will be 1 in (your measurement)"

What is a good load to shoot from this?

Where are my balistics guys?