OAL and Rifling - Choosing OAL

[Dannix]

I'm trying to determine a good OAL for my Miha 140gr 9mm boolit. I took a few unprimed cases, belled and deburred the mouths a bit, and seated the boolits with little concern of any swaging as I simply wanted to see how they would chamber.

I noticed at the longer lengths I like to load to, the top driving band engaged the rifling...I think the technical term is the lead. I could easily push the round all the way into the chamber (these boolits are aircooled and relatively soft), but to extract the round I had to use a tool i.e. a fingernail did not seem to suffice. With the boolit seated deeper, it readily extracted, of course.

Is there any danger in engaging the ... I think the correct term is the lead, or would that first driving band need to be a bore-rider to be a safe, full house load with the longer OAL? I'd like to utilize as much of the limited 9x19mm case capacity as I can.




Additional info:
Shorter OAL: 1.129, determined to roughly be the longest OAL with minimal extraction resistance
Longer OAL: 1.152, determined to roughly be the longest OAL without exposing the lube groove

[uscra112]

Rifle guys like to engage the leade with a cast boolit, so that the round is well centered before it fires. The 9mm is so short, and the chambers it's fired in tend to be so loose, that it hardly matters; it's unlikely that the initial alignment will be significantly better one way or the other.

I'd think it more important that you seat so that the rounds always feed properly, and so you can always extract an unfired round simply by working the slide. The 9mm is a personal defense round, (not much else it's good for), and in a PD situation a misfire has to clear absotively posilutely every single time.

In a tiny case like that, seating depth will have a large effect on pressure. Any seating depth must not be a so long that the boolit might be pushed deeper in the case by the force of chambering it. Engaging the leade may introduce a variable in the starting pressure, which won't do your muzzle velocity standard deviations any good.

[MtGun44]

As long as the round chambers freely, slightly touching the rifling is fine. It may even
help accuracy a bit. One thing that can happen is extracting the boolit from the case
when ejecting a round from the chamber unfired. A proper neck tension setup and taper
crimp should totally eliminate this issue.

Make sure that the boolit cannot telescope into the case, as mentioned, this will increase
pressures, and with the tiny case on the 9mm, small differences can be serious.

Use the dismounted barrel as your gage, it sounds like you are already doing this.

Start low, work up, let the case ejection distance be a useful guide for setting the power
level. Try for around 3-6 ft ejection for just plinking ammo, if the gun seems reliable
with this level of power. Some guns will not be reliable until the cases are flying into
next week.

Bill

[Dannix]

Thanks for the thoughts! The boolit is soft (air cooled 95/2.5/2.5 with some range lead as well) so no worries on it getting pushed back into the case.

I don't crimp 9mm rounds, I simply de-flare if need be. This time out I expanded the cases using the 38AP Lyman M plug. The 38AP plug is too small for .358 though, so I had to run the punch deep to add sufficient flare to seat the .358 boolits. I need to pick up a 38P plug or have a custom one made up. I did check the last round I tried out below to check for any telescoping, thinking perhaps a soft lead boolit would be more inclined to telescope. I did not see any present.

In my evening session below, I found two cases. It was dusk at the time, but let me assure you cases were getting flung.


I just tested a dozen or so 1.152" OAL rounds earlier this evening.

Lee has a following top-end 147 Grain Jacketed Bullet load:
Accur#7, 7.2, 1047, 31900 CUP, 1.095 OAL

So I load got my powder measure in the ballpark and tried the below. I'm unsure on the velocity as I do not have a chronograph.
AA#7, 7.21gr*, 1.152 OAL, Win case, WolfSP
*measured by weighing 10 charges and dividing by 10.

That's with air cooled 95/2.5/2.5 alloy, sized to .358 in my Star with randyrat's TAC #1 lube.


The Results

I haven't shot my alloy Hi-Power* that much at all, but I found I wasn't particularly accurate with it, even with my old j-word tack driver load that is quite accurate in my Glock. The hi-power has a truly awful trigger, but I'm sure its the shooter too. The load has some real umph behind it, but then I may simply be used to snappy 115grn loads.

Below is the my attempt at slow, accurate fire at 10 yards. I also tried some 7.3gr loads and those faired no better. I did noticed a small cluster in the top, right hand corner though.


I had one failure to feed, perhaps due to the HiPower's humped feedramp, or perhaps I was limp wristing it a bit trying to squeeze off some slow, precise shots. (I've had no problems in the past with the slow fire technique with my Glock though, with good results). I set that round aside due to the now somewhat gnarled nose of the boolit. When I had only one 7.21gr round left, I decided to take it and the gnarled nose round and shot a quick pair.

Here's the result of the quick pair at 7 yards. Interesting.



I'm going to have to revisit this load. I did find out though that AA#7 is not, as stated by some I've read, a low flash powder. I may just content myself with a silhouette based subsonic load.

*The alloy Hi Power is not exactly the kind of gun suitable for +P+ loads. I figured this load is well within the manual though, and it's fitted with a 18.5-lb recoil spring (stock is 17-lb) and a buffer.