Freebore diameter and accuracy

[Hannibal]

I've been thinking lately about shot out barrels. I know many on this forum recommend sizing to match freebore diameter and not barrel diameter. I've seen some rather heated arguments about which method is correct and I'd rather not rehash it here. My question is, for any of you who spec your own chamber reamers, would you mind sharing your experience as to what you have determined to be the best freebore diameter for accuracy. I've seen recommendations from same as bullet diameter to .003" over bullet diameter. Just like to hear some more opinions if anyone would be kind enough to share.

Thank you in advance.

[B R Shooter]

I spec mine either .0005" or .0007" over bullet diameter.

[DougGuy]

There are a few things about freebore that almost bend the rules of physics. This applies to revolvers, autos, and centerfire rifles equally.

For a revolver, the most optimal arrangement is a boolit that is .001" to .002" greater than groove diameter, and cylinder throats that are .0005" to .001" greater than boolit diameter. This works well with any centerfire revolver, any caliber. Cylinder throats are freebore, it matters not that there is a b/c gap between the cylinder and the barrel.

The same math can be applied to an autoloader as well. It just works.

Now for the "rule bending" part. Unless the boolit is a hard enough alloy that it resists the pressure generated on firing, most boolits will obturate or "bump up" to some degree and in many cases this obturation is only contained by the freebore, and the boolit will exit the throat and enter the rifling leade-ins at the diameter of the freebore. If you have a 44 magnum with boolits cast of 50/50+2% over a full house of slow burning magnum powder, and that revolver has .432" throats in the cylinder for example, you can load ammo with a .430" boolit, which will be presented to the forcing cone at throat diameter, .432" (which can be a BIG help in the case of the 44-40 with it's smaller than desirable cylinder throats getting it to fill and seal in a modern 44 barrel) which will provide the necessary swaging into the rifling to form a good seal in the bore.

Freebore allows the boolit or the bullet to exit the case mouth, it holds it concentric with the center of the bore (provided the throat is concentric) it also squares it up with the centerline of the throat and it is presented to the leade-ins with precise alignment. Taylor throating a revolver usually is an improvement because it puts freebore in the barrel, which is in alignment with the bore, unlike the cylinder throats which are at the mercy of how loose the cylinder latch is and how much misalignment there is between the cylinder and the barrel.

Freebore is critical in getting the boolit from the crimp to the rifling. SAAMI specs call for it, it is laid out in their specs precisely, but for the most part is totally overlooked by the majority of manufacturers.

This is my own knowledge of freebore, as it is applied to the work I do daily to help shooters achieve that perfect fitment of the boolit in their guns.

I am certain there are some opinions on a centerfire rifle that are outside my explanations of freebore which I would enjoy reading. I do not have a lot of experience throating rifle barrels, but it would not surprise me to learn that the same math above, holds true here as well.

[Hannibal]

Thank you for the replies thus far. I'm hoping to get a few more.

[B R Shooter]

I assumed this was referring to a rifle reamer.

[Hannibal]

I assumed this was referring to a rifle reamer.

A rifle reamer was what I had in mind when I posted, but I found DougGuy's post quite interesting and informative. I appreciate and value the benefit of experience.