Low pressure problem?

[Sure Shot Mc Gee]

I've got and old Dupont /IMR reloaders manully here (1986) that states:
Sr 4756 its Max charge rate is 6.3 grs. (compressed) under 115 gr REM JHPs.

So at 4.5 your charge is somewhat on the low side. Consider bumping up the powder charge by a full grain to start with and then work around that charge up or down to achieve best accuracy.

Btw: a 6.3 compressed charge has a speed of 1175 and 30600 cup.

Hope the info helps.

[SL1]

Eomfd,

I looked at the Hodgdon website and saw the data you referenced. (It says the data is for a lead round-nosed bullet.)

But, I also looked at old DuPont reloading pamphlets and a Hodgdon pamphlet from 2008 that show only a max load of 6.3 grains (which is compressed) with 115 grain Remington JHP bullets and Rem cases. The Dupont pamphlet used Rem 1½ primers and the old Hodgdon pamphlet used CCI-500 primers (both standard strength). COL for both is 1.110 In addition, my Hornady 8th addition manual lists 5.2 to 6.4 grains for their 115 XTP at a COL of 1.075" or their FMJ-RN at 1.100".

ALL of that other data, plus your experience with the Ranier bullet at longer COL indicates to me that your load must be underpressure. Considering these data coupled with your load performance, I think you will be able to work-up a load of SR-4756 to cycle your pistol without encountering any over-pressure problems, even if you have to go over the 5.2 grain max load in the Hodgdon on-line data.

COL is a very important pressure-determining factor for the short 9mm cartridge, so EITHER increase the charge weight OR reduce the COL by a small step in any test load. Doing both at once might get you there faster than you suspect. Another point is that SR-4756 has a reputation for varying pressure by unusually large amounts from lot to lot. So, don't automatically assume that you can't get enough in the case to cause problems just because there is some data that showes compressed loads with it were OK with somebody else's lot #.

SL1

[Unclenick]

SL1 beat me to it.

Since you have such low pressure signs, I also decided to look at some data other than from Hodgdon's nice, tight SAAMI minimum dimension test barrel chamber. Most real pistol chambers aren't so tight. I found no other lead or plated bullet loads, but a rule of thumb is you can load plated bullets to the middle of jacketed bullet ranges. Here are three that I found:

Nosler, 115 gr. JHP
IMR 4756
Start: 5.5 grains
Max: 6.5 grains

Lee's Modern Reloading. 115 gr. JHP
Start: 6.0 grains
Max: 6.3 grains

Hornady, 115 gr. JHP and TC
Start: 5.2 grains
Max: 6.4 grains

Note that the Nosler manual said this was the most accurate powder they tested with this bullet weight.

So, before you give up, it seems clear to me that from the middle load theory and your convincing low pressure signs, that you could work up to 6.0 grains without worrying about stripping the bullet plating, and I think that's what I would do. I would probably just go up in 0.2 grain steps, loading maybe 5 of each and watching the primer and looking for function. I'm doubting from this other data that you'll see any serious pressure sign, so just fire each 5 for accuracy and see where the smallest groups start to appear and figure you don't need to go any higher.

[SL1]

Another thing to look for is clean burning. SR-4756 is single-based and, at least in my experience, will burn relatively cleanly at allowable pressures in the 9mm cartridge. If accuracy is not different enough to choose among charge weights, then the lowest charge weight that burns clean is another way to choose.

SL1

[Eomfd]

Thanks again everyone! Didn't get to the range today but glad I didn't. I will take the advice and up my grains and possibly shorten my OAL. I will make 5 ea with 5.5 gr and 5 ea with 5.5 gr with a reduced OAL. Pictures to come!

[Unclenick]

Eomfd,

Interestingly, shortening COL is not guaranteed to help. Pressures are funny, and in a lot of pistols with short powder columns you get the primers unseating bullets before the burn gets well underway. Hodgdon's site has an excellent example with the same 148 grain LWC in .38 Special and .357 cases. They show more pressure from a lower charge of HP38/231 in the larger .357 case. I wrote them about it and verified there was no typo in this. It's just that the primer is starting the bullet forward in the smaller .38 Special powder space, so the pressure peaks with it further forward.

The other phenomenon is the case expanding and gas bypassing the bullet before it gets to the throat, which can drop pressure, too. After a lot of playing with target loads, I found better accuracy and less bullet metal fouling occurred when the bullet was seated out to kiss the lands. As with a rifle, this also raises pressure because of the gas bypass cutoff. Curves of pressure vs. seating depth drop as the bullet sticks further out up to a point, then rise again as it nears the throat.

If your magazine has room for them and the long rounds still feed ok, you'll get the most accuracy and least fouling loading as shown third from the left, below.

[Nick_C_S]

^^ I consider this illustration the "bible" of OAL. ^^ Thanks (yet again) Unclenick.

[Eomfd]

It worked!!! Both batches of 5.5 grains fired all 5 rounds with no issues. The casings are still a little scorched but it feels good to finally string a volley together. I did have a stove pipe when I shortened my OAL and reduced my grains but now that I am in the ballpark I can tweak it some. Here is a picture of my first two volleys. The one on the left is 5.5 gr with 1.140 OAL and the right is 5.5 gr with 1.125 OAL. The last picture is my 5.2 gr with 1.115 OAL.


Unclenick, I think my results could verify your exact statement. Glad to know they have the smart people in charge. ;-)

[Unclenick]

The primers are so very round that it's clear you can go up further, now that you've established the minimum for function. You may find precision (group size) improving when you do.

[Nick_C_S]

The primers are so very round that it's clear you can go up further.

^^ I was thinking the exact same thing. ^^