45 Colt Blow back in Cowboy Action Loads

[epiii]

I recently purchased a Uberti rifle in the Winchester 1873 pattern.  I am having a problem with the powder blowing back around the cases like it is not obturating sufficiently to seal the chamber.  I put a slug through the bore and it mics out at 0.451".  I am using cast bullets of 0.452" diameter and I have increased the case mouth crimp to 0.015 to try to improve the pressure.  I was using 200 gn LRNFP and have tried to use 250 gn.  I checked the rifle chamber and it is 0.485.  The sizing die is reducing the case diameter down to 0.473".  I was thinking I should try to not size the cases  using a universal decapper and see if this would help.  The results seem to be the same.  I am using 5.2 grains of Clays powder which gives me about 950 fps from the 24" barrel.  One person suggested that I use a 0.454" bullet to increase the pressure.  I am shooting the rifle in cowboy action shooting and would like to use a light load to keep recoil down.  Any suggestions?

[Veral]

The problem comes from too little pressure to expand the case mouth out against the chamber.
  The solution is higher pressure loads, which possibly can be obtained at the low velocity by using a faster burning powder.  Heavier bullets will help also, but bullet diameter with lead has little effect on chamber pressure.  If you can purchase bullets large enough to fill the chamber to the closest fit that will feed smoothly you may solve the problem, and will certainly tighten accuracy dramatically.  The accuracy improvement being because the bullets cannot lean as much while starting into the rifling.

  Filling the loads with enough shot buffer to get slight compression will probably also solve the problem and allow you to keep velocities down where you want them.  The compressed loads will shoot with higher pressure/velocity, which means you'll need to reduce the powder charge a little to maintain your desired 950 fps speed.

  Shot buffer is available from any gun shop that sells shotgun reloading supplies and will be available in most larger cities, or try www.ballisticproducts.com.