Ive been looking at the Olympic UltraMatch SUM 20inch barrel for my first build.Ill be shooting 223 rem,and 5.56NATO.At least I was hoping to shoot 5.56NATO.This barrel has a 223rem chamber.Heres the description from their websight. "These barrels are our most accurate barrels, and quite frankly, these are the most accurate AR-15 barrels on the market. These barrels have Broach-cut Rifling. What is that you say? Well, broaching is the method used to create the lands and grooves.  For these barrels, we use the stamping "SUM" which means Stainless Ultra Match. SUM barrels are all chambered with minimum SAAMI 223 Remington reamers. These barrels can also be identified by looking down the bore. All of our broach-cut or Ultramatch barrels have only 4 lands and 4 grooves." Can I shoot both 223 rem and also shoot the 5.56NATOsurplus stuff with this barrel?

I would expect that you will have chambering problems with the 5.56 NATO ammo. The throat on the 223 is shorter than on the 5.56 and the diameter of the chamber immediately behind the bottleneck is approximately 0.0014 tighter on the 223 chamberwhen compared to the 5.56 chamber. After it is broken in it will probably be fine. Good luck.

You CAN NOT shoot 5.56 in an Oly SUM bbl. If you go SUM it will have to be all .223. The Oly is an excellent bbl though and can't be touched in accuracy for the price.

IBTM!

I wish I knew what that meant..........

In before the move.

IBTM = in before the move

meaning that this is likely in the wrong forum and will be moved by a concerned moderator.


Other useful acronyms

IBTM = IN BEFORE THE MARKUM!

Ive been looking for this thread all over the place.I thought I posted in the AR-15 general discussion.