I had a somewhat similar experience several years ago. I had a new .223 rifle that I wanted to work up a load for. I found a load for some powder and bullets that I had on hand and decided to start with the minimum load lised in the reloading manual. At the range a couple of days later the first shot produced a much louder report, muzzle blast and significantly more recoil than expected. I also had gas blowback thru the pressure relief hole. Extraction was difficult and the primer was set back and was pierced. After doing some research my conclusion was the volume of that powder charge did not fill the case enough so that when the cartridge was laying flat as in the chamber of the rifle the powder was laying in the bottom of the case. So when the primer went off the spark ignighted the entire charge instead of a progressive ignition of the charge. In the original post he stated it was a minimum charge.