S&W 25-2 6-inch

[rcmodel]

Chamber throats that are larger then the bore are not a problem.
That's what the forcing cone is for.

Chamber throats that are smaller the the bore is the big problem.

As the undersize bullet will suffer gas blow by, and bore leading cannot be avoided.

rc

 

[ClemY]

I think what is happening is that the .45 ACP case is so small, and the primer so relatively powerful, that the pressure pulse of the primer pushes the bullet through the oversized throat and into the forcing cone before the pressure curve of the power can rise enough to produce blow-by of the bullet. In a larger case like .45 Colt, those same oversized throats play hell with accuracy because the gas blow-by can damage the base of the bullet and cause it to enter the forcing cone at an angle. The whole thing can easily damage accuracy. That is why the .456” throats of the 25-2 work just as well as the .452” throats of the 625, but in .45 Colt, those same .456” throats with .451”-.452” bullets more typically cause poor enough accuracy that owners of 25-5’s will sometimes pay to have a new cylinder fitted by the factory with .452” throats.

 

[AlfonsDeWolf]

Here's a visual..... http://www.shootingtimes.com/ammo/ammunition_st_lowdownleadfoul_201002/

 

[fireman 9731]

Beautiful!

The model 25 in 45acp is on my short list.

 

[gotboostvr]

ClyemY, that's an interesting hypothesis.
Seems plausible to me.
Lack of roll crimping may help exaggerate the effect of the primer as well.

 

[Walkalong]

think what is happening is that the .45 ACP case is so small, and the primer so relatively powerful, that the pressure pulse of the primer pushes the bullet through the oversized throat and into the forcing cone before the pressure curve of the power can rise enough to produce blow-by of the bullet. In a larger case like .45 Colt, those same oversized throats play hell with accuracy because the gas blow-by can damage the base of the bullet and cause it to enter the forcing cone at an angle.

You can still get gas cutting if the bullet is undersized and doesn't bump up to seal the throats.

5 ACP 21,000

45 ACP +P 23,000 PSI

45 Colt 14,000 PSI

21,000 or 23,000 PSI bumps up a bullet to fit the throat a lot easier than 14,000 does. Standard commercial cast bullets are really harder than needed for .45 ACP, much less .45 Colt.