Beyond the usual bore size.

[OverMax]

Have always wonder what the consequences are for using a oversize boolit having a listed upper side powder charge under it.
For instance: Marlin's shoot well with a .310 cast. Winchesters do well with a .311 cast. What if I were to shoot a .311 G/C cast thru my Marlin or a .312 G/C thru my Winchester.
What if anything could/would happen?
Accuracy improvement or not?

[Tatume]

It is doubtful that cast bullets larger than 0.310" will provide an accuracy improvement in your rifles, unless the barrel or throat are severely worn. However, it is safe to say that you will not encounter any problems shooting cast bullets that are a few thousandths oversized, provided the loaded ammunition will chamber in your gun.

[44man]

I shoot .311" in the Marlin. It needs larger then the WW anyway. Micro groove. No need to go larger in the WW.

[DougGuy]

It is doubtful that cast bullets larger than 0.310" will provide an accuracy improvement in your rifles, unless the barrel or throat are severely worn. However, it is safe to say that you will not encounter any problems shooting cast bullets that are a few thousandths oversized, provided the loaded ammunition will chamber in your gun.

Partially true. If your groove diameter is .310" and your alloy is quite hard you would likely see an improvement sizing .311" or even .312" since no bore is consistent from one end to the other and shooting a boolit the same size as groove diameter in a hard alloy will make it very easy for powder gases to escape along the sides of the boolit which causes leading and a loss of velocity and a loss of accuracy.

Pressure from firing a round larger than the bore is exponential to hardness. The softer the alloy, the less resistance to obturation and pressure would also be less than shooting the same sized boolit in a harder alloy. Generally speaking, if a loaded round will chamber in your rifle, it is not sized big enough to cause issues with pressure and load data should not be altered or compromised to take this into account. Obviously there is an upper limit to everything, and I would speculate that until you get several thousandths above groove diameter, you wouldn't notice problems from firing larger boolits. Personally I try to stick within .002" of groove diameter but this is not practical with all firearms.

I have a Uberti Old West model with .451" groove diameter, and .4565" cylinder throats. I size .456" so the boolits fill the cylinder throats and they will handily swage down to enter the bore BUT these are cast soft enough that you can dig a thumbnail into the alloy. I don't think pressure of a standard "Cowboy" load will rise dramatically if the same boolit were cast in a much harder alloy, but you would definitely notice the difference in recoil which usually translates as a reaction from the pressure of firing so yes harder alloy swaging down as it travels from cartridge to muzzle will raise pressures.

As far as accuracy improvements in your own rifles, you really would have to try some at the dimensions you listed to see one way or the other. If you are at the upper limit of your load data, and you are using super hard alloy, pressure could become a factor the farther you go over groove diameter. How much of a factor? Well, how hard is the alloy and how far over groove diameter do you want to size? Lyman #2, WW, or 50/50+2% sized .312" with a GC, I wouldn't worry about it at all.

[Tatume]

Completely true. The rifles do not have 0.310" groove diameters, unless they are worn.