How much to size down for 45 Colt

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I ordered a 454-260 Gas Check along with some gas checks for my 45 Colt chambered rifles and revolvers. I plan to size these through a lyman 450 lube sizer. I slugged all my barrels and they all measure at exactly .442 and .450 for the bore and grooves.
The question is, what would be the best size die to use for these? .452? .454?,  .453?  , .451???

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[rickyhansenParticipant]

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Usually  the sizing die should  0.001″ to 0.002″ larger than the bore. In your case 0.451″ to 0.452″.  I would recommend 0.451″.

[leonhart]

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You may find better accuracy in your rifles by checking the throat size, then size to fit there.
On your revolvers check the chamber throat and size to fit there.
You may find that your rifles my have a larger throat than your revolvers.
On the rifles measure the ID of the mouth of a fired case. Try sizing to that diameter. This is about the largest that will chamber in that rifle. You may find each rifle likes a different size.
On revolvers the chamber mouth should be the same as the bore diameter or a bit bigger. By sizing to match the chamber mouths the bullet is straight in the chamber and is straighter as it passes through the gap and into the forcing cone and rifling.
On my 44Mag Ruger SBH And Red Hawk they shot well at about .430. My Marlin 1894 likes larger bullets. At least .432. I don’t use gas checks and I shoot a lot as cast and lubed with LLA. They shoot well and very little to no leading.
The SBH at 25 yards, if I do my job is one ragged hole.
The Marlin isn’t quite as accurate but still puts them in small groups.
Leo

[Author]

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