9mm brass is everywhere & it's also free and it doesn't have the drawbacks of the IMT brass. The 9mm cartridge is a high pressure cartridge with an extremely small case capacity for powder after the bullet has been seated. The IMT cases make that window even smaller. That step in the IMT cases is actually a double edged sword. Not only does it make the min/max pressure level window smaller, it makes the performance level of the IMT case have an extremely small window to function properly/safely.
Why the IMT cases separate:
It's a combination of work hardening of the brass and the extreme pressure differences the design of the interior of the IMT case creates coupled with the springback of the brass. When a round is fired in these 9mm semi-auto's the brass expands from the pressure of the load. It seals the chamber of the bbl. Everyone's seen soot on the side of the mouth of low pressure rounds that have been fired, that's from the brass not fully expanding/sealing the bbl. So the trigger is pulled, the prime lights the powder, the powder burns and that creates the pressure that expands the case to seal the bbl. As the pressure decreases the case shrinks letting go of the chamber walls and the slide pulls the spent case out of the chamber. The thicker web of the IMT brass is harder to expand than the thin case mouth. The end result is the slide is pulling on the case and the front thin area of the IMT case is still locked to the chamber and the back half of the IMT case is already free from the chamber. The end result is the brass separates at that point in the IMT case.
The OP don't do cup & says it means nothing to him. GOOD!!! I have no idea why he keeps posting hodgdons finding, they deal in cup which he doesn't use unless it suits his needs. Seeing how the OP only deals in psi's let's take a look at some data from the powder mfg's. All data (including the OP's is with a 4" bbl) and from the OP. winchester manual & allaint manual.
Op load/ 4.5gr ww231(4.75gr with 5% added) 124gr fmj 1090fps
ww load/4.5gr ww231 124gr fmj 1060fps 32,700psi
OP load/ 5.2gr ww231 (5.46gr with 5% added) 115gr hp 1178fps
ww load/ 4.8gr ww231 115gr hp 1120fps 32,100psi
OP load/6.4gr power pistol (6.73gr with 5% added) 124gr fmj 1133fps
Alliant load/ 6.6gr power pistol 124gr fmj 1233fps 34,000psi
The Op stated his 115gr load was only 30,300psi (post #5), that leaves the 2 124gr fmj loads to hit the 37,5000psi max he stated he hit in post #4. This tells me 2 things.
1. The case failed with the 115gr bullet/231 load. Here's why.
If you look at any and all published data for ww231 when psi value's are listed you will find that there is a min load listed with a psi # and a max load listed with a psi #. Typically for every 1/10gr you go above the minimum recommended load data with the 9mm/ww231 combo you will find a +/- 1,000psi gain in pressure. Here's a link to a ww reloading manual that has 6 different bullets listed for the ww231/9mm combo. The all have a min/max psi listing along with a min/max ww231 load listed in 1/10gr increments. It clearly show a +/- 1,000psi gain per 1/10gr of ww231 increase.
http://castpics.net/LoadData/Freebies/RM/Winchester/Winchester1997.pdf
The OP's 115gr bullet load was over max with powder but his velocity a wash with the hodgdon velocity using the same bullet.
Hodgdon 5.1gr/1167fps
Op 5.46gr/1178fps
.36gr over max yet no velocity gain.
2. The case failed with the 124gr/pp load. Again over max load and this time a loss in velocity of 100fps.
Difference in hodgdon (same bullet) load vs op 115gr load ='s 9fps difference
Difference in hodgdon 125gr fmj load vs op 124gr fmj load ='s 2fps difference
Difference in ww 124gr fmj load vs op 124gr fmj load ='s 30fps difference
Loosing 100fps is huge.
Without getting into pressures I believe the hodgdon velocity #'s. Their 115gr bullet is the same as the op's and their 124gr fmj data mirrors the op's data.
Hodgdon 124gr fmj 4.8gr ww231 1088fps
op 124gr fmj 4.5gr ww231 (4.5 + 5% ='s .225gr) 4.5gr + .225gr ='s 4.725gr ww321 load 1090fps
The op had .36gr more ww231 for the 115gr load, was spot on for the 124grfmj/ww231 load and was over for the power pistol 124grfmj load. I have no reason not to believe his data. It clearly shows the IMT cases failed with 2 different loads. Namely the over charged 115gr load that didn't mange to gain any velocity and the 124gr power pistol load that was over charged the lost a bunch of velocity.
The IMT cases just aren't worth it. Heck if you look around you can find 1x 9mm brass for $90 for 5,000 pieces shipped. That 1.8 cents apiece delivered to your door.
