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So much has been written over the years about the accuracy, but mostly about the inaccuracy, of
lead alloy projectiles in these barrels, that I had to find out for myself. To start with, I consulted the Marlin Firearms book by Brophy and found out that there have been three(!) types of .30-30 micro-groove rifling. We’ll call them a., b., & c. These are the Marlin specifications but obviously, could vary slightly on individual rifles due to tolerances:
a.) 1955 to 1957 - 16 grooves / .304” bore /.308” groove dia.
b.) 1958 to 1968 - 22 grooves /.304” bore /.308” groove dia.
c.) 1969 to present -12 grooves/.302” bore /.308” groove dia.
Several years ago I purchased a used Marlin 336A rifle with c.) rifling, and just recently, I have been able to test two earlier Marlins with the a.) & b.) rifling. Fortunately, I have found that by applying the same principles used for successful cast bullet shooting in conventional rifled barrels ( bullet hardness / design / bullet diameters correct to fit the barrel ), one can attain equal results in micro-groove barrels to over 2,400 f.p.s.!!
Let’s dispell one myth which I have seen in print many times, and which goes something like this - ”The shallow micro-groove rifling does not grip the bullet as well as ballard or conventional rifling and can strip.” A typical .30 caliber conventional barrel has 6 grooves with a bore dia. of .300” and a groove diameter of .308”. So doing a little calculation, we find that the rifling is .004“ deep x 6 grooves = .024“ of bearing surface depth (b.s.d.). By comparison we find that, in fact, all three forms of micro-groove rifling have more b.s.d. …
a.) .002” deep x 16 grooves = .032” b.s.d. or 33% more than ballard rifling
b.) .002” deep x 22 grooves = .044” b.s.d. or 83% more than ballard rifling
c.) .003” deep x 12 grooves = .036” b.s.d. or 50% more than ballard rifling
As we can see, bearing surface on the bullet is not a problem. A problem does exist in that on a.)& b.) type rifling, if you use a .30 caliber cast bullet designed with the standard .300“- .301 “ bore ride portion that is longer than ½ the bullet length, the .304” bore diameter is too big to provide support to the front of the bullet. In this situation, when pushed too fast, the bullet will yaw in the barrel, destroying accuracy. The bullet must fit the barrel.
In all three types of micro-groove barrels, Lyman’s 311041 g.c. bullet gave fine accuracy to 2,000 f.p.s. using 36 grs. of H414 powder. Over that velocity, the c.) rifling gave better results since the smaller bore diameter provided better support. Lyman’s 150 gr. Loverin style bullet works well in all three types right up to over 2,400 f.p.s. since it is pretty much bore diameter for most of its length. Once again, bullet hardness, bullet design and a bullet that fits the barrel are the keys.
Another myth: “Micro-Groove barrels are more prone to leading.” That is also false. Contributors to leading are rough barrel finishes and incorrect alloy hardness for the load being used, not the type of rifling. They also shoot plain based bullets extremely well.
What have your xperiences been?
Sincerely,
Jack
lead alloy projectiles in these barrels, that I had to find out for myself. To start with, I consulted the Marlin Firearms book by Brophy and found out that there have been three(!) types of .30-30 micro-groove rifling. We’ll call them a., b., & c. These are the Marlin specifications but obviously, could vary slightly on individual rifles due to tolerances:
a.) 1955 to 1957 - 16 grooves / .304” bore /.308” groove dia.
b.) 1958 to 1968 - 22 grooves /.304” bore /.308” groove dia.
c.) 1969 to present -12 grooves/.302” bore /.308” groove dia.
Several years ago I purchased a used Marlin 336A rifle with c.) rifling, and just recently, I have been able to test two earlier Marlins with the a.) & b.) rifling. Fortunately, I have found that by applying the same principles used for successful cast bullet shooting in conventional rifled barrels ( bullet hardness / design / bullet diameters correct to fit the barrel ), one can attain equal results in micro-groove barrels to over 2,400 f.p.s.!!
Let’s dispell one myth which I have seen in print many times, and which goes something like this - ”The shallow micro-groove rifling does not grip the bullet as well as ballard or conventional rifling and can strip.” A typical .30 caliber conventional barrel has 6 grooves with a bore dia. of .300” and a groove diameter of .308”. So doing a little calculation, we find that the rifling is .004“ deep x 6 grooves = .024“ of bearing surface depth (b.s.d.). By comparison we find that, in fact, all three forms of micro-groove rifling have more b.s.d. …
a.) .002” deep x 16 grooves = .032” b.s.d. or 33% more than ballard rifling
b.) .002” deep x 22 grooves = .044” b.s.d. or 83% more than ballard rifling
c.) .003” deep x 12 grooves = .036” b.s.d. or 50% more than ballard rifling
As we can see, bearing surface on the bullet is not a problem. A problem does exist in that on a.)& b.) type rifling, if you use a .30 caliber cast bullet designed with the standard .300“- .301 “ bore ride portion that is longer than ½ the bullet length, the .304” bore diameter is too big to provide support to the front of the bullet. In this situation, when pushed too fast, the bullet will yaw in the barrel, destroying accuracy. The bullet must fit the barrel.
In all three types of micro-groove barrels, Lyman’s 311041 g.c. bullet gave fine accuracy to 2,000 f.p.s. using 36 grs. of H414 powder. Over that velocity, the c.) rifling gave better results since the smaller bore diameter provided better support. Lyman’s 150 gr. Loverin style bullet works well in all three types right up to over 2,400 f.p.s. since it is pretty much bore diameter for most of its length. Once again, bullet hardness, bullet design and a bullet that fits the barrel are the keys.
Another myth: “Micro-Groove barrels are more prone to leading.” That is also false. Contributors to leading are rough barrel finishes and incorrect alloy hardness for the load being used, not the type of rifling. They also shoot plain based bullets extremely well.
What have your xperiences been?
Sincerely,
Jack
