Range report: 300-MP in 357 and 44 Mag

[Nick_C_S]

For quite a few years, I’ve been curious about 300-MP. I’ve been using – and am quite satisfied with – W296 since I first discovered it back in 1985. As we all know, W296 works great for its intended purpose; and that is full-throttle magnum ammunition.

Well this past April, some 300-MP showed up at my LGS; so, I figured what the heck, let’s give ‘er a whirl.

I’ve spent this spring doing load workups using 300-MP for both 357 and 44 Magnum – 158 and 240 grain bullets, respectively. My intended use is through my Henry (357) and Marlin (44) rifles, but I’ll occasionally shoot them through my revolvers for the novelty of it.

For consistency, I chronographed the 357 Magnum workups through my 686 4” bbl; as, it is my “test gun,” so to speak. This gives me an “apples to apples” comparison with other loadings. For 44 Magnum, I did the workups with the Marlin; as chronographing lots of full throttle 44 Mag ammo in a revolver is a bit much recoil for me these days. Also, the bullet is a .431” slug (JSP) that I use exclusively for my over-bored Marlin. .429” slugs work just fine in my revolvers.

CAUTION: The following post (or a page linked to) includes or discusses loading data not covered by currently published sources of tested data for this cartridge (QuickLOAD or Gordon's Reloading Tool data is not professionally tested). USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.

Some of the load charge weights posted here exceed published data. This post is intended to simply share my experience, and should in no way be construed as load advice. As always, do your own load workups, following all safe practices.

There isn’t much load data for 300-MP; but the Hornady manual has it for both 158 and 240 grain bullets. Alliant’s reloader’s guide has both as well.

I started with 357 Magnum, using Everglade’s 158gn JSP. This is my preferred bullet for use with my Henry (16” bbl) lever action. It is a great performing bullet using 2400, W296, and others. Seemed logical to use it with the 300-MP. In the first session, I loaded charge weights of 16.1, 16.4, 16.7, and 17.0 grains. 17.0 is Hornady’s max. I used a CCI 550 primer; Starline brass with a trim of 1.280,” with a generous roll crimp. As mentioned, the gun is a Smith 686 with a 4” bbl. 10-round samples, all. Here are the results:

16.1 – 1017 f/s; 26.73 SD
16.4 – 1025 f/s; 24.36 SD
16.7 – 1054 f/s; 15.82 SD
17.0 – 1087 f/s; 32.86 SD

As can be the case with opening load workup charge weights, these were a joke. They sounded like I was shooting a black powder gun. Pressures were clearly very low. It astounds me that Hornady’s manual maxes out at 17.0 grains when this load wasn’t even beginning to act like a “normal” revolver round; much less, a magnum round. Hornady claims to have achieved 1400 f/s through their 8” Python. I’d have to see it to believe it. Back to the load bench . . .

The next load session, I used charge weights of 17.4, 17.7, 18.0, 18.3, 18.6, & 18.9 grains. Noteworthy here is that Alliant’s load data maxes at 18.6 (their bullet was a Speer 158 GDHP – a bonded bullet). Here’s the results:

17.4 – 1131 f/s; 16.36 SD
17.7 – 1149 f/s; 17.50 SD
18.0 – 1170 f/s; 10.66 SD – these were finally starting to behave like a magnum.
18.3 – 1172 f/s; 13.13 SD
18.6 – 1212 f/s; 29.94 SD – this is Alliant’s max
18.9 – 1244 f/s; 11.78 SD

At this point, there were still no signs of pressure. Primers looked like I had just installed them, and the brass practically fell out of the charge holes. And I still haven’t achieved W296 performance. Time to go back to the bench and keep going. Next, I loaded charge weights of 19.2; 19.5; and 19.8 grains. Here’s the results:

19.2 – 1269 f/s; 21.60 SD – these finally met W296 performance.
19.5 – 1309 f/s; 30.09 SD
19.8 – 1325 f/s; 27.40 SD – I recorded these as having some primer flattening and slightly sticky extraction. I chose to not continue increasing the charge weight.

Noteworthy here is that for some reason, my Smith 686 4” bbl gun exhibits sticky case extraction much earlier than any of my other 357’s (686 3”; 686 8-3/8”; Python 6”). I can have sticky extraction in my 4” when the cases of the same loading just fall out of all my other 357’s. I have no idea why but it’s always been that way. I rather like it because it’s a “canary in a coal mine,” so to speak.

My next test was to settle on the charge weight of 19.6 grains and test this load against W296 and 2400, using my Henry rifle, my 686 8-3/8” bbl, and my 686 4” bbl. Here’s how that went:

19.6gn 300-MP – Henry – 1887 f/s, 48.08 SD (odd)
19.6gn 300-MP – 686 8” – 1381 f/s, 38.67 SD
19.6gn 300-MP – 686 4” – 1299 f/s, 20.45 SD

17.0gn W296 – Henry – 1767 f/s, 7.93 SD
17.0gn W296 – 686 8” – 1341 f/s, 30.37 SD
17.0gn W296 – 686 4” – 1250 f/s, 28.35 SD

14.6gn 2400 – Henry – 1757 f/s, 42.75 SD (odd)
14.6gn 2400 – 686 8” – 1305 f/s, 20.00 SD
14.6gn 2400 – 686 4” – 1268 f/s, 14.91 SD

A note here: It’s been so long since I’ve done a load workup on W296, I don’t actually know where the charge weight of 17.0 grains lands, compared to whatever the highest charge weight I tested was. I did this workup decades ago and the data was recorded pre-computer and lost to time. It’s just the charge weight that has worked for me since the ‘80’s, and so I still use it.

Also, I don’t care for the W296 loading through my 686 4”. It’s just a big boomy mess with the shorter barrel. This is why I have and use 2400. 2400 performs much better in the 4” than W296. Less blast. Less recoil. And like velocity (more, in the case of this particular test). This is also why I included it in this test – and it didn’t disappoint.

This test was an excellent showcase of how slower propellants reach more of their potential with longer barrels. This was a great test. Yes, 300-MP was clearly the performance champ in the Henry rifle – by a whopping 120 f/s.

On to 44 Magnum. With the 44 Magnum workup, I only used my Marlin rifle (20” bbl). I have two Smith 629 revolvers – one 8-3/8,” and one 5” Classic. But these days, I find the recoil of full-power 44 Magnum rounds to be too punishing on my hand to do a full load workup. Full power mag rounds through my revolvers are just a cylinder or two, for the novelty of it these days. In fact, I’m working on some “de-tuned” magnum ammo, using lighter bullets and faster propellants; but, that’s for another post.

Load data: Hornady’s data, using their 240 XTP maxes at 25.6 grains. Alliant’s max is 25.0 grains. I chose my first bench session to use charge weights of 23.0, 23.4, & 23.8 grains. I’m using an Everglades 240gn JSP, .431” bullet (a must, because my Marlin’s bore is something larger than .429); CCI 350 primer; Winchester brass, circa 1985, trimmed to 1.275,” with a generous roll crimp. Here’s the results:

23.0 – 1693 f/s, 23.42 SD
23.4 – 1693 f/s, 12.92 SD – same velocity, not a typo.
23.8 – 1705 f/s, 16.28 SD – this matches my W296 loading*.

No signs of pressure. Back to the load bench. The next charge weights were 23.8 (intentional “overlap” charge weight, for comparison), 24.2, 24.6, 25.0, & 25.4 grains. Here’s the results:

23.8 – 1713 f/s, 10.95 SD
24.2 – 1724 f/s, 25.21 SD
24.6 – 1751 f/s, 14.14 SD
25.0 – 1771 f/s, 9.32 SD
25.4 – 1802 f/s, 7.93 SD

I’ll just paste my workup notes: “These all shot well. The highest charge weight seemed to do the best without any extraction problems. Primers got pretty flattened, but that's more or less normal with the Marlin. There is little doubt that the charge weight can be further increased; but there isn't really any point. These are big, potent rounds. The 25.0 grain charge should be loaded in quantity and tested for accuracy.”

*It’s important to note that my W296 loading uses 22.5 grains. That is rather de-tuned. In my younger days, I used to load it at 24.7 grains. So my current W296 loading isn’t really full potential.

And I did load and test the 25.0 grain loading for accuracy. They go nice n straight – at least, as straight as I can shoot them. I shot them back n forth with my W296 loading. The 300-MP may have had ever so slightly more recoil – as one would expect. It was a fun time shooting them both, I’ll say. I have enough 300-MP to load about 70 more of these; and I intend to. Once those are loaded (likely today), I will be out of both 300-MP and W296. At some point I need to decide which one I want to continue using (trying to keep the numbers of different propellants to a minimum). Considering that lever rifle is how I use this stuff these days, the 300-MP clearly outperforms W296. But on the other hand, I’ve been using W296 for decades and I’m truly happy with it. So for me, I rather see 300-MP as a solution in search of a problem. But it is indeed quite a performer.

[Recycled bullet]

Thank you for doing all that work and documenting it and then sharing it. I greatly enjoyed reading it. I am fascinated to compare gunpowders.

With 300Mp I see you have found the maximum amount of gun powder can still have a 357 Magnum subsonic.

How cool is that!!!

[Unclenick]

Interesting work, and it certainly comports with complaints from some folks that they didn't get anything out of 300 MP that they couldn't already get from 296/H110. They probably didn't push past the published loads as you did.

If you look at Alliant's 357 data, it is all for a 10" V&P (velocity and pressure) barrel. The SAAMI standard for the 10" V&P barrel models a single-shot pistol barrel, so it has no vent to mimic the barrel/cylinder gap in a revolver. Only the SAMI 4" V&P barrel spec includes a vent. Given how long these slow powders take to get burning, I suspect that venting through the barrel/cylinder gap on your Smith is lowering the load peak pressure of the book loads, and that is why you can run higher charges. On the other hand, I don't really know what the lot-to-lot variation for that powder looks like, and it and ambient temperature conditions could all be playing roles.

[44 AMP]

Also keep in mind a couple points, one is that your test gun and ammo is not what was used for the published data. Similar is possible, but identical is not.

Next point is that published max loads might be either where they had to stop, or might be just where they chose to stop, and unless they tell you, you just don't know which it is.

Individual guns do individual things. When your gun gets sticky exctraction (no matter what the load level/pressure is, its time to stop for THAT gun.

Long time back I had a top level .357 load, 125gr JHP over a case full of 2400. a couple tenths less than the max load listed in the Speer manual of the day.

Shot out of a 6" Model 19, it clocked 1620fps, and fired cases had to be driven out with a rod and a small hammer. Could not be extracted by hand. Same ammo fired from my 6" Model 28, clocked 1670fps and ejected fine by hand. Same ammo fired through a Desert Eagle, clocked 1720fps and function was flawless. And fired through a Marlin carbine, velocity was 2200fps and function was flawless.

A different model 19 MIGHT not have stuck the cases in the cylinder, I don't know, we only had the one that did for testing. Individual guns, individual results.

Based on your results, I would abandon 300-MP for top end loads, and just stick with 296 (or 2400) as more suitable.
But, that's just me...

[RC20]

I always did well with 2400.