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Discussion Starter #1
Tried "search"; it doesn't like me, so it ignores me, you'd think I was married to it; OH WELL.
Was 32 Special ever chambered in a Micro Groove barrel on the 336??
 

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YES
In 1953 Marlin applied for a patent on Microgroove rifling (US Patent #3,100,358 was granted on Aug. 13, 1953). In this patent, Microgroove rifling was described as having 5 grooves for every 1/10th of an inch bore diameter, and that the driving side of each land would be "tangentially disposed" (i.e. beveled, presumably to prevent the accumulation of fouling). On July 29, 1953, Marlin introduced Microgroove rifling in their .22 rim fire barrels, with 16 grooves that were .014" wide, and nominally .0015" deep. In their 1954 catalog, they outlined 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. They also mentioned "...a bore of greater than standard size..." in their discussion of how Microgroove rifling did not engrave (distort) a bullet jacket as deeply as conventional rifling. Hold that thought....

Marlin continued to experiment with Microgroove rifling in their centerfire barrels, primarily in the Model 322 chambered for the .222 Remington (the highest velocity round that Marlin chambered for). The results of this testing convinced the Marlin engineers that Microgroove rifling was indeed suitable for high-velocity centerfire rifles and in early 1956 Marlin announced that all of their high-powered rifles would henceforth be equipped with Microgroove rifling.

For the common centerfire calibers, the initial specifications (circa 1956) for Microgroove barrels were as follows:

.22 centerfire -- 16 grooves, .015" wide, .001" deep
.30 caliber -- 16 grooves, .030" wide and .002" deep (this would be changed in 1958 to 22 grooves .024" wide, and .002" deep; and then again in 1968 to 12 grooves, .040" wide, .0028" deep)
32 caliber -- 16 grooves, .035" wide, and .0015" deep
.35 caliber -- 16 grooves, .040" wide, and .002" deep (this would be changed in 1968 to 12 grooves, .055" wide, .0028" deep)
 

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The Model 336 chambered in 32 Special and fitted Micro-grooved barrels first became available in 1956, as noted above (although the 336SC model chambered in 219 Zipper was available in 1955); and was last available in 32 Special in 1963 when the 32 Special chambering was dropped.
 

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Discussion Starter #4
Thanks to both of the replies; good information.
 
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