I fail to see why a cartridge fired in a chamber with the top portion cut away would not "blow". How can someone expect a brass cartridge case to resist 50,000 pounds per square inch exerted by firing the cartridge. The principle on which the copper crusher system of chamber pressure measurement is based assumes that the cartridge case will give way and propel the piston which crushes the copper cylinder. The diameter of the piston is only a fraction of an inch, but the cartridge case ends up with a hole blown in it corresponding to the size of the hole bored in the chamber to accommodate it.

If you insist on firing this mutilated rifle, I suggest you use a long lanyard and tie the rifle down to a tire carcass.