Do the new Marlin Microgrooves have tighter bores?

[Cast of Thousands]

I just slugged my Remlin 30 30 with microgroove rifling. It measures .308! This is not what I expected. All the Marlin owners of older guns state they are larger. I am now sizing boolits for this rifle to 309 with good results. Have any of you had similar results?

[Tatume]

Absolutely! My pre-safety Model 336 also has 0.308" groove diameter, and 0.309" cast bullets shoot with wonderful accuracy.

[3leggedturtle]

The 44 Mag 1894’s were .432 or so. Never heard if any others were larger, my 1895 isn’t either. YMMV. Todd/3leg

[Tatume]

The SAAMI specification for 44 Magnum is different for rifle and handgun. Since Marlin is a SAAMI member, they are obligated to use the larger specification for rifle. Nobody knows why SAAMI did this.

[yeahbub]

The bores are generally larger due to microgroove barrels having 12 grooves and not needing to be as deep as just 6. Reducing boolit distortion and all that. Groove diameters that I have measured are generally where they should be, so I'm not surprised a .30 cal is .308 in the groove. It might be .302 or so in the bore, though, YMMV. Folks seem to prefer a cast boolit be .002 or so over groove dia to get good results. If you have access to a pin gage set, you can easily see what the bore diameter is. I messed with one Marlin '94 in .41 Mag that was a tight .409 in the groove. MG barrels may be a challenge for accurate cast shooting in some instances, but I haven't met one yet that doesn't do well with paper patched boolits.