40X original barrel has extremely shallow rifling

[Tim K]

I've owned this rifle for a few months and somehow never looked down the bore from the muzzle end. The rifling is so shallow that it's invisible looking from the muzzle unless a light is shining down the barrel from the breech. It looks like a tiny shotgun bore.

The rest of the gun looks like it had very little use considering it was made in 1957. It a commercial model.

Did these barrels come this way or has mine been shot out or lapped out?

Up until a couple weeks ago it shot great. I'm chasing an accuracy problem that is most likely me, but it caused me to look down the bore finally.

[TFOGGER]

Remington has used "Microgroove" rifling in their .22s for a lot of years, dunno if the 40x was one of them, however.

MicroGroove Rifling[edit]

Micro-Groove rifling

In 1953 Marlin Firearms was issued U.S. Patent 3,100,358 for what was named MicroGroove Rifling, which was a departure from the standard "Ballard," or cut rifling. One purpose of Microgroove rifling was to increase the speed of producing rifle barrels. Microgroove rifling is described in the patent as having 5 grooves for every 1/10 of an inch bore diameter, and that the driving side of each land would be "tangentially disposed" to prevent accumulating fouling in use.
Marlin introduced Microgroove rifling in their .22 rimfire barrels in July 1953, with 16 grooves that were .014" wide, and nominally .0015" deep. Ballard rifled barrels have grooves generally in the range of .069-.090" wide, and .0015-.003" deep. This change was marketed in the 1954 Marlin catalog, as having numerous advantages that this new form of rifling had, including better accuracy, ease of cleaning, elimination of gas leakage, higher velocities and lower chamber pressures. The catalog also claimed that Microgroove rifling did not distort the bullet jacket as deeply as Ballard rifling hence improving accuracy with jacketed bullets at standard velocity.
Designed for factory loaded ammunition, Microgroove barrels have a reputation for accuracy problems with centerfire ammunition handloaded with cast lead bullets due to the increased bore diameter generated by the shallow grooves. The use of oversized cast bullets greatly solves this problem, restoring accuracy with cast bullet handloads to levels seen from Ballard rifled barrels.^[4] Early Marlin .30-30 microgroove barrels had a twist rate of 1 turn in 10 inches optimized for factory ammunition with jacketed bullets; later Marlin .30-30 microgroove barrels show a twist rate of 1 turn in 10.5 inches which improves accuracy with cartridges loaded to lower velocity than standard.

[Tim K]

I took a pic. Scaling off the image, I'd estimate the rifling at 0.004" deep. It's about ten times easier to see in this pic than it is in person.

[Great-Kazoo]

I took a pic. Scaling off the image, I'd estimate the rifling at 0.004" deep. It's about ten times easier to see in this pic than it is in person.

Looks deeper than my marlins.

[Tim K]

I think it was just hard to see with the naked eye. When you see it in person it looks like a shotgun barrel, but it's clearly ok under some magnification.