I just traded for another AR upper. I was told that this is a factory Bushmaster target upper. 20 in. heavy stainless barrel, flat-top reciever, free-float tube etc. However, my barrel is marked "SUM 5.56", which leads me to believe this is actually an Olympic barrel, the whole thing has a really slick black finish, almost like teflon moly, but harder. Any idea what the hell it actually is? Even as an Oly, it's easily worth more than I have in it. I'm also trying to determine the twist rate, I tried the tight fitting patch on the cleaning rod trick, but my jag keeps unscrewing. Of course the barrels not marked with this information. I think this must be an oldie.

Bob

Try pulling your cleaning rod through the barrel rather than pushing.  This will keep the rod from unthreading.

Olympic SUM 5.56 barrel? The common twist rate is 1:10...

Quoted: But if he pulls the cleaning rod instead of pushing then wouldn't he have to count backwards?

Only South of the Equator.





WIZZO

The barrel is an Oly barrel.  Definitely not Bushmaster.