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.
I'm not familular with this slugging. Could someone explan it and how do you go about doing it. Are there special tools required too.
Also, Nicknack i love what you've done with that cowboy. Whom did you get that Green mountain barrel from? It's been hard finding one of those 24" cowboy models i thought of getting a barrel and slapping on an action just like you. Could you send info in a PM
Slugging is a procedure wherein you drive a soft lead egg sinker or muzzle loader ball down the bore to obtain a groove diameter measurement. Groove diameter and whether or not there are tight points in the bore (under the roll marks for instance) is important information for cast bullet shooters. Doesn't matter much for jacketed bullets...
ETA: Run a patch with light oil down the bore. Start the slug into the muzzle with a non marring hammer. Then push the slug through the bore with an aluminum or brass rod as close to bore diameter as possible. Short blows with a heavy hammer are better than fast blows with a light hammer. While driving the slug, note areas of tightness along the bore.
Many folks recommend using a hardwood dowel to drive the slug through the bore, but you don't have to look very hard to find horror stories of broken dowels wedged hopelessly in the barrel. If you do use a dowel be careful. If it breaks STOP! You cannot hit a broken dowel a little harder and get it out.
I went over to the steel yard and bought a 3' piece of 3/8 aluminum rod for a couple bucks. Works great for slugging...
As for egg sinkers and why they are better, supposedly the hole down the center gives the lead that gets squeezed a place to expand into which helps eliminate any potential spring back of the slug once it exits the barrel. Whether that is true or not is arguable but they do work great and are easy to find in the fishing section. If you see any new Marlins on the racks as you walk by try not to look!
Here is a link that shows what size sinkers you will need...
Also make sure you can dent the lead with your fingernail before you buy, supposedly there is one brand that has zink or some such in it and wont work at all. http://www.marlinowners.com/forums/index.php/topic,69736.msg687223.html#new
They are listed as 44-40 but the twist rate 1 in 26'' is faster than Marlins 1 in 28'' I don't know why they list it as a 44-40 as the bore size for a 44-40 is usually .427 and they have made there's .430 even .429 would have been better.
These barrels shoot the Hornady .430 240 grain XTP very well. I haven't tried a .429 bullet yet but they will probably squeeze up a thousand of a inch bigger.
Or you can get a .357 octagon barrel but the twist rate is a little slow at 1 in 20'' but i don't think it will worry bullets up to 158 grain
They used to make a 25-20 in octagon but is now only available in round.