shoerjbh , your cases stretched, sure they fire formed beautifully but they stretched. Strong neck tension and a jammed bullet will not stop the firing pin from driving the case forwards. Kill a primer, set up the case with NO powder and a bullet seated for a hard jam, a ton of neck tension and chamber it. Remove and measure the OAL. Re-chamber it and pull the trigger on it then re-measure the OAL, it will be shorter. I tried this with 108's as well, not even remotely close to stopping the case.
I hydraulically formed mine with oil instead of water. After a lengthy, vigorous, extremely hot and soapy washing, oil film was still effecting primers until they were fired the first time. I kept them bullet down until firing and all went off unlike bullet up the first firing. When mine were H formed, a very nice shoulder to neck junction was present, the oil worked much better than water.