I'd bet 100% bad shell. It happens even with quality ammo. Back in 1992 when we were shooting BR-50, a coupla friends and myself went to Camp Perry during small bore week. We were in search of good lots of ammo, so we bought 2 boxes of each lot # of Eley Tenex to test. We had 22 different lots. We tested them with a 54 Anschutz that had been threaded in the receiver. The test was held inside a barn and the range was 33 yds.
Out of 22 lots, we found 2....that's right, 2 that were acceptable. Not exceptional, but acceptable. (Olympic years are always hard to get good ammo)

Anyway, I ended with case of one of the lots, and life was good. Well not for long. It took a while to catch on, but I found that every box had a flier that would drop straight low 1". You would think a dud like that would be easy to find and sort out. I drove myself nuts, weighing, measuring, and gauging rim thickness. I finally gave up and accepted the fact that it would always be there, and when it happened, I could write it off as a bad round, and not a condition change that I missed.
When I was sorting ammo, I placed them in a MTM box, and that's how I shot them. After I throwed the towel in, I just started shooting them out of the box. This is where the miracle occurred. Long story short.....the fliers were always in the same position in the box. I proved it by opening every box and finding the flier. I was triumphant until I realised that I only had 1 brick left.