5mm FBI is what is labeled on the reamer. I got the reamer, dies and brass with two used rifles. This cartridge is pretty much like a 20SCC or 20-221AI 40 degree shoulder but is a turn neck version. I'm turning necks to 11 thou thick. The rifle is a CZ527 American that's been bedded and has a Pacnor 11 twist barrel at 21.5 ". The rifle came with 1" rings but most of my scopes are 30 or 34mm so I borrowed an old B&L 6-24 for this test. I didn't want to attempt the ladder with 9x and no parallax adjustment which is my highest mag 1" tubed scope. I'm still trying to decide which scope to buy for this rifle??? My test was shot on the ground with a triad tactical bag and Harris bipod on a horrible morning, wind was gusting at a whopping 1.5mph from 7 o'clock. Life's rough and you just have to roll with it sometimes right, lol. I started the ladder with 17.5 grains of H4198 and ended with 19 grains, all loaded in .1 grain increments, I chose it because it's not so temp sensitive and it has great fill density. I had all the info I needed by shot 15 so I fired the 19 grain load at a rock 400Y away and it caused a sticky bolt so that was a good call. Primer was CCI450. Bullet is the 32gr vmax and throating is such that the bottom of the bullet is well above the NS junction when just off the lands. I didn't use chrono because I don't have a mechanism to hold the 35P sky screens close to the ground. The short of it is, all 15 shots went into 1.7" CTC at ""250 yards"". I would have put the target out farther but the scope was topped out on elevation and I was worried the wind would kick up during the test and these bullets with their .210 BC could blow off the target with some wind. The box as shown is upright like it was when I did the test, it looks funny because I bent over and wrote the numbers sideways. I assumed by shot 10 that that was the scatter node because I'm pretty sure I didn't muff the shot. It's also pretty rough having a scatter node of 1.2" at 250Y, i'll tell yuh. Shots 1 through 5 went into 3/4" or so and shots 3-4 went into a 1/4" and if I include shot five it's .35"CTC. I chose 17.7 grains as middle node which is shot number 3 but I will shoot 3 and 4/17.8 for groups at some point. I'm guessing fps around 3700 fps, a bit slow but the barrel is short and I don't want to go through the brass quickly, they should last many years I think. Well truthfully I didn't think this rifle was that accurate and I also assumed the total vertical at 250Y would be more like 3". There's so many bullet holes going into others I found it hard to see which one was which but for the most part I think I found which one went where. No matter it was the scatter node that spread the shots into the lower part of the test. I'm not even going to bother with a seating test! Love this rifle and cartridge!