I bought my Marlin 24'' Cowboy secondhand and it was shooting terrible groups, slugged the barrel.433 must have been made on a Monday morning at Marlin.(very disappointing that they let them out with such an over bore)
Imported a Green mountain 27'' octagon .430 groove with a 1 in 36'' twist and now it shoots like a benchrest rifle.
My Cowboy now sports a 26'' octagon barrel without the rear sight dovetail cut, as I only like to use peep sights.
The Green Mountain barrel is very well made. I put about a 150 lead bullets through it and about thirty jacketed and no sign of copper fouling.I wish Green Mountain would make a .308 octagon barrel blank.
regards Dallas