I been trying Sinterfire bullets. For the first test with 75 gr 380 auto I looked at the Sinterfire data and it has one load (Titegroup), which is exactly the same as 100 gr 9mm using 231. So I backed away from that load and looked elsewhere. The only other load I could find was in the Shooters World load data using Clean Shot. I had some Clean Shot but have never used any of it. They show 4.0 to 4.5 gr for velocities of 862 to 1000 fps. So, I created a ladder in 0.1 gr increments, 10 rounds each starting at 4.0. Then I went out to test them with a chronograph (Labradar). Starting with the lowest load, the first round felt and sounded normal but the gun jammed (P3AT). The fired case was half way out of the chamber with the next round pushing into it. So the slide traveled back enough to get the next round but didn't eject the first round. The chronograph showed 1116 fps when I was expecting less than 862 (the barrel is pretty short). The spent case came out reasonably easy, the primer looked fine, but there was an excessive bulge in the brass. I packed up and went back to the bench.
The case is bulged more than normal with a slight "blister" above the base. It is similar to the pictures I have found of overpressured brass but much less severe. The case diameter is 0.014 larger than the unfired case and about 0.008 larger than other fired cases I have. The loaded rounds drop out of the case guage easily and about zero crimp, I wouldn't expect to have cracked a bullet. I don't know if a cracked bullet can cause overpressure like this. I verified the powder in the load data, on the bench, and in the measure, it's correct (Clean Shot). The measure continues to throw 4.5 gr, the last step in the ladder. I recorded the weights of each step in the ladder as I loaded as well as the setting of the micrometer of the Hornady LNL powder measure. I can see from what I wrote that what was in the fired round is much less than the 4.5 I ended on. I load a single powder weight, place them in a box, label the box, and then move to the next weight. The fired round came from box labeled 1, 4.0 gr, micrometer 2.74. The last was labeled load 6, 4.5 gr, micrometer 3.18. I don't think I could have fired the wrong loading. The Clean Shot powder was measuring very consistently.
At 4.5 gr the bullet is sitting on top of the powder, I was going to be watching that closely as I worked up to the higher loads. The 4.0 load had a very small air space and should not have been too much or compressed.
I haven't found any other load data for the 75 gr Sinterfire and I'm reluctant to use this loading again. Any thoughts on why a minimum load that almost fills the case would overpressure like that?