I got my Freedom Arms .454 in 1985. At that time, large pistol primers were recommended, then changed to Small Rifle. I am not aware that magnum primers were ever designated and I have used CCI 400 ever since.
Freedom Arms supplied primer pocket inserts to convert the LPP pockets to SRP size. They had to be swaged, but I didn't have a tool, so I just did my best to install the insert and then prime the pocket. It generally worked but at times, deformed the primer to some degree which didn't become a problem until one day at the range I had a squibb. I was totally perplexed because this never happened and I absolutely knew I had not missed a powder charge because I did everything manually, and I found powder had escaped from the case. I was about to fire another round when I asked myself, "Where did that bullet go?" Well, inspection revealed it was stuck in the barrel. The improper primer seating was enough to discharge the primer but not enough to ignite the H-110 charge. The FA bullet was a 240gr JFP; the barrel is 7-1/2 inches.
My thought is the OP's experience was primer failure, or, as big Al Hunter pointed out, media blocking the primer pocket.
Freedom Arms supplied primer pocket inserts to convert the LPP pockets to SRP size. They had to be swaged, but I didn't have a tool, so I just did my best to install the insert and then prime the pocket. It generally worked but at times, deformed the primer to some degree which didn't become a problem until one day at the range I had a squibb. I was totally perplexed because this never happened and I absolutely knew I had not missed a powder charge because I did everything manually, and I found powder had escaped from the case. I was about to fire another round when I asked myself, "Where did that bullet go?" Well, inspection revealed it was stuck in the barrel. The improper primer seating was enough to discharge the primer but not enough to ignite the H-110 charge. The FA bullet was a 240gr JFP; the barrel is 7-1/2 inches.
My thought is the OP's experience was primer failure, or, as big Al Hunter pointed out, media blocking the primer pocket.