What are you calling leading? My 32-40 shaves small slivers of lead off the bullet when breach seated. After 20 or 30 shots I will run a dry patch from the breach to the muzzle and the small slivers can be seen in a bright light. I do not think of this as leading the bore. The accuracy of my rifle will shoot with the best of them with these small slivers being left behind. What I call leading the bore usually starts at the muzzle and builds up towards the breach and is caused by HOT gasses  blowing past the bullet and acting like a cutting torch. As the bullet travels down the bore the gas pressures start to drop and cool and lead is deposited in the bore. Another cause of leading is failure of the bullet lube. 
Take a naked lead bullet and rub the nose across a piece of white paper and it will leave a lead smear. Take the same lead bullet and rub it across a piece of wax paper and no mark.

Just my 2 cents worth
A. J. Palik

I wasn't trying to say the thing was going to shoot well, or that it woudn't lead being oversized. I was just pointing out that by the time it's actually breech seated  in the barrel it's like it has been sized to fit that barrel. Thus, no undue pressure sign would occur from the size of a lead slug. If the pressure sign he had (blowing primers) happened it was because of something besides sizing. Too much fast powder? Wrong primer? Something else?

I think what is going on is the misuse of terms. I think he is measuring the base of the bullet and then to the bottom of the groove that the rifling leaves in the seated slug that is groove diameter.

Bore diameter is the measurement to the bottom of the rifling on a seated slug that is the bore of the gun.
The groove diameter is the measurement of the raised portion of the bullet. 

The Base band of your bullet should be .001” to .002” larger than the largest measurement of the seated slug.
Make sure that you can see where the slug has touched the throating all the way around before you measure.

40 Rod