Accuracy Epiphany

[edgehit]

I have found that a Brownells 45 degree muzzle chamfer cutter will cure acutely inaccurate rifles. Case in point, a Marlin 39A of mine consistently shot 2-inch groups at 50-yds with any ammo I ran in it. It was so consistently bad that I switched scopes, thinking I had a bad one and got the same results. I decided the only defect I could cure was the crown, so I ordered the 45 degree cutter. Immediately, upon inserting the pilot and contacting steel, I could tell the crown wasn’t straight. About 5 light turns with the cutter and the job was done. Next day at the range it shot a sub-1/2 inch, 10-shot group. I could tell by the 3rd shot it was cured.

Being excited by the results, I turned my attention to a new miroku Winchester. So I made a pilot to custom fit its bore. Same results, it went to shooting tight groups.

This was less than a $100 order and the results were immediate.

[TheBugFather]

Good information Joe

[TheBugFather]

Good information Joe

Dennis

[No1_49er]

Curious about a couple of things here.
Why did you choose the 45 deg cutter rather than 11 (79) deg, and,
what is different about the Miroku / Winchester bore that required a different pilot?

49er

[edgehit]

45 degree is most applicable to use on round crowns, like those on early Marlin 39A’s. It’s also useful to touch up flat muzzles like those on Winchesters. Very little material is cut. Something like 0.010” is fine.

My Miroku Winchester is a 357 magnum and the Marlin 39A pilot would wobble in its bore.

A 12-degree crown is best used on high power rifles. I’m not an expert on the reasons why.

Cosmetically, lever guns look the best with 90-deg and round crowns. You could cut a rounded crown with a 12-deg cutter but a lot of material would have to be removed before kissing the rifling.

Every bore caliber has a different land diameter contacting the pilot. 22lr bore specs are all over the place! Plus rifling methods result in potential for 2-3 different diameter pilots to address nuances between rifling styles and manufacturing methods. For instance, microgroove rifling is pretty dang skinny, say 001”, and will require a larger diameter pilot than the same caliber barrel rifled in a conventional manner. Then you have hammer forged and Ballard cut rifle barrels also common on lever guns. You just don’t know until you slug the bore and measure the slug’s minor diameter.

A snug fitting pilot cuts the most precise crown with this tool. Obviously the most precisely cut crown is one turned on a lathe but then again, a pilot has to be inserted in the bore to indicate center. Failure to do this step will get you a crown that’s concentric to outside surface instead of the bore.

Brownells sells brass and steel pilots. I’ve seen steel used and I’m sure it’s because of its durability. I bought brass from Brownells, then turned my extras from aluminum. I’m certain a dropping the tool will ruin a brass or aluminum pilots’ precision, hence the benefit of more durable steel. Shipping damage may be another reason to select steel. These parts are shipped in a padded envelope so damage is possible.

[No1_49er]

Thanks for the response.
It hadn't been clear, in the original post, that the Miroku/Winchester was a different calibre.
I guess it's personal preference as to the end of barrel profile - all my 39s, BLR22, Win 9422 have a "square" end.
Agreed that the pilot has to fit the rifling profile as closely as possible for correct alignment.

49er