The Micro-groove rifling was 12 grooves with shallow lands. It was originally offered in the 22 lr and was later added to many of the larger bore rifles marlin offered, including the 30-30, 35s, 444s and 45-70s.
The complaints from cast bullet shooters eventually caused Marlin to offer Ballard rifling, which carried only 6 lands and grooves at about .004 deep. The complaints about cast bullets being inaccurate or causing leading in the 12 groove (Micro groove) barrels was not due to the number of lands and grooves. It was simply due to rough barrels, undersized cast slugs and a lack of awareness of how to make those Micro-groove barrels shoot accurately.
There was a lot of research completed by Veral Smith, one of our Moderators here at GBO and one of his followers, Marshall Stanton of Beartooth Bullets. Veral found that fire-lapping or bore-lapping the micro groove barrels removed the roughness and obdurations often found in mass produced barrels and allowed them to shoot cast bullets more accurately without the leading problem when sized to properly fit the bore (.001 over bore diameter). I believe that both Smith and Stanton found that if you gas-checked the cast bullets you were able to attain jacketed bullet accuracy and velocity with cast bullets. Both Smith and Stanton went on to design heavy for the bore cast gas checked bullets for both the 444 and the 45-70, as well as lots of other calibers and lots of shooters started making the switch to cast gas checked bullets for those big bores.
Stanton in particular found his gas checked cast slugs to shoot incredibly accurately from his bore lapped rifles and used two specific Marlins in 444 to test his bullets and loads. The results are pretty close to phenomenal and raise the lever action to near the same level of accuracy as fine bolt rifles. One important note here is that the bores on Marlin rifles are often larger than some think. All the bores on my 444s measured out to .431 or .432. I bore-lapped all three rifles to .432" and shoot .432 diameter Beartooth bullets through mine. .429" diameter jacketed 44 magnum or 444 Marlin slugs give very good accuracy but the cast slugs shoot better. .429 diameter cast slugs, even gas checked, don't shoot fer squat outta my .432 barrels.
My 444s will group under 1" at 100 m, often with one hole groups, as long as I am benched, steady, unflinching and of calm peace of mind. I scored one 3 hole group at near 1/2" from my shorty Timber Carbine with 335 gn Beartooth Bullets. I will not lie and say it didn't pound the stuffin' outta me but man, that group was worth it.....
Marlin has learned from shooter experiences and offers Ballard rifling in many of its centerfire calibers. However, these are still mass produced barrels and although the rifling is much more conducive to cast bullet shooting i would not hesitate to fire lap any Ballard rifled barrel for better accuracy. JMTCW. Mikey.
