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Thread: IMR7383 with the .30 HBC in the 7.5x55 Swiss Schmidt Ruben

  1. #1
    Boolit Master Linstrum's Avatar
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    IMR7383 with the .30 HBC in the 7.5x55 Swiss Schmidt Ruben

    I finally had time to work up some experimental loads for my recently obtained Swiss K-31 using IMR7383 triple base propellant in conjunction with the .30 caliber High Ballistic Coefficient projectile cast in wheel weight alloy and sized to 0.309” with .30 cal gas checks installed. This particular session I wasn't shooting for accuracy, I was just getting pressure and velocity worked out for projectiles in the 200-grain weight range using the inexpensive IMR7383 surplus powder, known for being a bit temperamental in the high pressure department.

    I started by running the HBC through a Lee 0.309 sizing die to install gas checks plus get the size down to where it would be a thousandth of an inch over groove diameter. I used Lee Alox applied by tumbling the boolits in a sandwich baggy and then letting them dry on a paper plate.
    The High Ballistic Coefficient boolits were aged approximately two months after sizing and loading the cartridges, important only if worrying about inadequate hardness. I did not test my boolits for hardness but after running two dozen through my rifle there is no sign of leading.

    I began by loading up a series of 17 cartridges with my starting load at 35 grains IMR7383, increasing them by half-grain increments to 43 grains. From past experience with IMR7383 in other cartridges like the 7.5x54 French, .308 Winchester, and 8x57JS, I knew that with a 200 grain projectile the best cast boolit performance would be with about 37 grains of the IMR7383 since the 7.5x54 French cartridge when loaded with a 139-grain projectile does well with 39 grains of IMR7383. This powder is temperamental because it has a variable burn rate that depends on the shape and volume of the cartridge along with how much powder is present. The more powder used, the faster the burn rate, and it suddenly changes from a burn rate about like IMR4064 to Unique without any warning signs other than it starts to burn clean without leaving mummies behind in the barrel and spent cartridge. By reading the condition of the primers along with looking down the barrel for powder mummies I don't worry about working up a load with IMR7383. The trick with using it with cast boolits is to find a happy medium where it burns well while also providing right around 1800 to 2000 feet per second to help prevent barrel leading problems. The critical transition pressure for the powder where it suddenly changes from a medium burn rate rifle powder to behaving like a fast shot shell powder seems to be around 43,000 psi C.U.P. With pressures well below this danger threshold, but still high enough to burn completely so as not to leave mummies behind, it usually does well for the “fat .30 calibers”, like the British .303, 7.62x54R Russian, 7.65 Mauser, 7.7 Japanese, etc, and will automatically provide velocities in this best-for-cast-boolit-performance-range when used with projectiles of about180 grains to about 210 grains. This is just a generalization and “your mileage may vary”. Every cartridge and projectile combination MUST be tested individually to determine its optimum loading with IMR7383.

    When I did my test the outdoor air temperature was 78°F and the prepared cartridges had been sitting on the front seat of my truck where the inside temperature measured 85°F. I fired about one cartridge per minute and did not leave a cartridge chambered in the warm rifle any longer than it took to sight the target and fire. When I got to 37 grains the powder began to burn cleanly and when I got to 40 grains the primers were getting flattened with the edges around the firing pin indentation becoming noticeably sharp, although still resembling what a safe charge of IMR4064 produces and therefor indicating that pressure was still okay. I dropped the powder charge back to 38 grains since that seems to be the optimum charge for my particular lot of IMR7383 in my particular Swiss K-31 Schmidt-Ruben rifle. Since it was a warm day the loads will be okay for most days, especially since it was dropped back 5% from 40 grains to 38.

    This is my first experience with the beautifully built K-31 and I like the straight pull action. The only rifle action I like better is the Garand/M1A. I did notice that it shot accurately even though I was not using targets where the accuracy could be measured directly. At 100 yards I was hitting rocks the size of a loaf of bread, though, pretty good for the HBC boolit that I have heard some complaints about.


    rl394
    ~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+
    There is no such thing as too many tools, especially when it comes to casting and reloading.
    Howard Hughes said: "He who has the tools rules".

    Safe casting and shooting!

    Linstrum, member F.O.B.C. (Fraternal Order of Boolit Casters), Shooters.com alumnus, and original alloutdoors.com survivor.

  2. #2
    Boolit Master Ricochet's Avatar
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    Smile

    BTW, those HBCs were of oven treated WW, previously aged a couple of years.

    And I went past the transition point and got sticky extraction, flat primers and excessive obturation with those boolits at 3.7cc of 7383. I wasn't expecting that.
    "A cheerful heart is good medicine."

  3. #3
    Boolit Master Maven's Avatar
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    Thumbs up

    Linstrum, Coincidentally I decided to try IMR 7383 in my 8 x 57mm [Shaw rebarreled] Mauser as well and loaded a small handful last week. I decided to start with 37grs. with a 195gr. LBT spire point (.3245" and 2.96" OAL). Tonight I added 10 more, but with 38grs. 7383. Unfortunately, I won't be able to test and chronograph them until next week. Btw, I've also had good results with 36grs. in my K-31 with ~180gr. CB's.

  4. #4
    Boolit Master Linstrum's Avatar
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    Hi, Maven, nice to hear from you.

    I loaded up forty 8x57JS a few weeks ago using the Lee 175 grain sized to 0.3245" for a Yugo 24/47 with 0.323 bore. Twenty I put in 36.7 grains and for the other twenty I used 37 grains. I got around to test firing them a few days ago and they work great but leave a few mummies. Since I am using the 175 grain Lee and the fact that they ALMOST burn clean I would say that what you loaded up will do fine. I still don't have my chronometer set up but judging from the consistency of shot placement I'd say that the velocity is adequately uniform and you should get results that are pretty good as far as uniformity of velocity from one shot to the next.

    One of the things I like about IMR7383 is its consistency from one shot to the next when loaded up so that it does not exceed its pressure limitations. I suspect that 7383's performance is no accident since the spotting rounds the stuff was designed to be used in had to be pretty accurate, the equivalent of a cannoneer's bench rest round.

    Let me know how the stuff does with your 8mm Mauser, I bought a lot IMR7383 with the intention of using it as my main powder in the calibers where it works well, cast and otherwise, and I am still discovering which ones those are. So far it does well in all WW2-era bolt action milsurp calibers. I remember your work in the .243 Win and others some years back and refer to that once in awhile.


    rl396
    ~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+~+:/&\:+
    There is no such thing as too many tools, especially when it comes to casting and reloading.
    Howard Hughes said: "He who has the tools rules".

    Safe casting and shooting!

    Linstrum, member F.O.B.C. (Fraternal Order of Boolit Casters), Shooters.com alumnus, and original alloutdoors.com survivor.

  5. #5
    Boolit Master Maven's Avatar
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    Linstrum, I guess I "discovered" 7383 because at that time it was dirt cheap and because locally the selection of smokeless-, black-, and Pyrodex powders is almost nonexistent and very expensive when you can find it. Be that as it may, I'm very pleased with its performance with jacketed bullets in the .30-06 and the .243Win., as you mentioned. In those two cartridges, it mirrors the results I've had with IMR 4350. Moreover, it also produced great accuracy with the .30HBC with a +40gr. charge. I had hoped to try it in the 8mm Mau. with jacketed bullets, but couldn't get enough in the case to even approach the velocity I get with 48grs. IMR 4064 (~2,500fps). On the other hand, 37 & 38grs. may be the perfect charge in my rifle with the 195gr. CB's. (I also have the Lee 175gr. CB, which is accurate @ 50 yds., but haven't yet tried it @ 100 yds. Once I do, I'll try it with 7383.) I'll chronograph my results and post them on the CB forum.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check