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The following is common knowledge among experienced shooters, but I am amazed how many new shooters are ignorant of the effect of chamber size, and they still try to fit bullets by the old folklore and mythology in old Ideal manuals about slugging the groove diameter of their barrel.

To determine the correct bullet diameter for a rifle, the groove diameter of the barrel is NOT the determinant.

INSTEAD you want to measure the throat, or the unrifled portion of the barrel forcing cone or "ball seat" ahead of the case mouth, before the rifling starts. The best way to do this is from a chamber cast or upset throat slug.

If you forget EVERYTHING you ever read about slugging barrels and simply cast chambers from now on, and get bullets to FIT THE THROAT you will be far happier in the long run.

The limiting factor in bullet diameter is neck clearance. So you must ALSO measure the neck diameter of the chamber on the cast, then the loaded cartridge neck diameter must not be larger than 0.0015" SMALLER than the chamber cast at that point, to ensure safe expansion for bullet release.

In a min. .44-40 SAAMI chamber the neck diameter is 0.445" where it joins the shoulder and 0.4435" at the mouth. Typical case mouth wall thickness of Starline brass is 0.007," so .445" minus twice neck thickness (0.014") = .431", minus another 0.0015 to allow for safe neck expansion and fouling = .4295" max. bullet IF you have a minimum chamber. The normal allowed tolerance for a sporting chamber is +0.002" on diameters, so IF your chamber neck casts as large as .447", which My Marlin 1894S does, you can load a .431" bullet in that Starline brass!

Try a dummy round for fit first!!!!

Any questions or discussion? Preaching to the choir here.
 

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Well noted! Thanks for the insight. There is always one more way to skin a cat. I've never had to go to this extreme to get good accuracy, but if I run into something that doesn't work with slugging, I will give it a try.

Keith
 

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I find for my vintage Marlins the spec is out the window! I go with groove size on my Vintage Marlins. Undersize bullets leave lead in my barrel and some brass must be shortened for rounds to chamber fat bullets.

So far it works for me.
 
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