450

In rreply to your post.

I have seen two rifles as you describe. A WR 450 3 1/4 blow out the right barrel about where you describe. I do not know the load but do know it was R-15.

An Army Navy 470 right in front of the chambers. 3031.

I don't use R-15 so I don't know the loads but isn't 87 grains the Federal Premium load for the 470? What should the 465 be in R-15?

It seems you could get a local testing lab to magna flux barrels for a reasonable price if you took it in.