Interesting discussion. I've been reloading the .327 FM for a while now (+/- prob close to 8K rounds in total) and have occasionally noticed slight bulging as well. This thread compelled me to go to the reloading room and look a little closer at some of the cases that have already been shot and are waiting to be processed. I have 7-8 different .327's so I'm not sure what gun the cases were run through. Of note to the OP is that one of the guns is an SP 101 like the one he's concerned about and except for the Henry Rifle, all are Ruger revolvers (GP100/Single Sevens).
Went out and sorted through this batch of brass looking for some that might have bulges. All of the brass is Federal or Starline. Either bought as shown or from American Eagle brand store bought ammo and directly from Starline.
I didn't have to look long to find some. The top 3 cases had no bulge. The bottom four did. Getting them to show on an iPhone pic is difficult but the bulges, however slight they are, are there and noticeable when running a thumbnail back/forth across them. 3 of the 4 cases (Starline) are reloads and the case all the way to the right is store bought. What is noteworthy is that all four bulged cases had flattened primers. More discernably flattened at least than the 3 cases that showed no bulges. Mind you, I've seen many AE store bought ammo .327 primers look like this on extraction.
In truth, I never paid much attention to the bulging, slight as it was. The reloads were based on published data, the chrono verified that, and they extracted without drama. However, my experience with this particular caliber of Federal Brass has shown a lot of loose primer pockets on the second reload, to the point I've thrown a fair amount (1/5th?) of all cases away. To be fair to Federal, this round does produce some exceptional PSI numbers and maybe that's just par for the course/maybe not with this particular round. Once I get through all of the stockpiled Federal brass I plan to start with the Starline (shown below) in earnest. I went with Starline because I've had outstanding results (longevity) in a number of larger calibers that I routinely load at the high end .
Again, because of this thread I wanted to double check some things. Having in the past read some threads on the internet were folks complained about weak brass from Federal I just assumed that from the git go (untrimmed/unchamfered) the Starline brass would be more robust. Read that to mean heavier, as in more brass per case. Soooo, on that premise I went ahead and weighed three unprimed cases of each brand.
First pic is a Starline Case..and all three weighed almost exactly the same, off only by 1-2 tenths of a grain.
Second pic is the Federal case...again, all 3 of those within 1-2 tenths of a grain.
As you can see the Federal cases are on average almost a grain heavier. Numerically that's not a big deal but I always thought that Starline was noted for being somewhat of a 'thicker case'.
As a result of this thread I think I'll see about chasing down the cause of the bulge and what gun (if not all) it occurs in and at what powder/weight/FPS it first starts to show. I'll post an update when I have one.
I'm planning to get a press set up for .327FM over the next couple of weeks and start pushing some of these 120 grain .314's through for the Henry rifle so that bulge goose chase will play right into it.