Max Velocity on 5" 357 mag

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Dec 4, 2018
Rhode Island
If you ventured a guess, what do you think max velocity on a 5" 357 mag revolver will be with published max loads?

I chrono'd 1300fps with Alliant 300MP powder, but was 1.1 grains lower than max published data from Alliant's site. I'm going to be carefully working my way up to max published and am curious if I will see more velocity over chrono. It's going through a GP 100 so I am confident weapon is strong enough.
I believe only so much powder can burnt in a given pistol barrel length with a given diameter, and adding powder will only add to the fireball after the end of the barrel because you have exceeded the barrels capacity. But this is just my speculation.
You could probably get 1450 fps , depending on powder without exceeding max.
I like a 158 cast swc @ 1300 as a nice 357 do-all. You wont gain much in the top end with 357 in my experience, YMMV.
1450 is what I'm hoping for. I think 1450 is optimistic from 5" and it would be a gift to get those results.
I'll post what velocities I get.
1450 is what I'm hoping for. I think 1450 is optimistic from 5" and it would be a gift to get those results.
I'll post what velocities I get.
You can get there, may need to use lil gun or h110 . I have never used 300mp so i can't guess how it does at the top. Does your published loading guide give you a velocity at max or a pressure guesstimate, if so does it give barrel test length?
14-1500 is doable with 158, but it will be a max load. 125gr bullets can go faster, and depending upon what your bore actually measures you could potentially run a .355 115gr at a max load and get some real screamers. The issue at hand in this realm is not as much about velocity as it is about destruction of the gun and subsequently your body if the gun blows. The GP100 that you are using is probably one of the best revolvers on the market to do this with. Be careful. Don’t do anything dumb.
I'm going to stick with the 300MP either way, I really like it so far. Recoil is nice, doesn't burn dirty, no noticeable amounts of unburnt powder in the barrel or cylinder bores like other powders I've used.

1690 fps out of a 10" barrel

From what I read online the general results people report are "not as high pressure" as other powders.....not very scientific and not a good source to base reloading on. I have not seen ANY pressure signs on primers so far. Unlike LOTS of other brands of factory 357 ammo I HAVE seen that flatten the primers out on the regular.
I don't have any interest in the light weight bullets, primarily from reading how these loads burn the forcing cones up.

I'm after the heavier stuff. The 158gr gold dot fits my need. I'm developing the load to get the most I can from my 357 for a woods load. For the highly unlikely event of needing to shoot a black bear with it in defense (I carry it during bird season where we have lots of bears) like I said though UNLIKELY. And any other critters, if a deer was close enough I would take a neck shot with it too.

After hitting steel with a standard 45 auto load, then a 1300 fps 158 grain I now leave the 45 at home. The energy difference is ASTONISHING, I would highly recommend hitting some swinging steel with both if you debate what you should carry in the woods. I can't shoot 44 mag well, and I don't want the extra bulk of a 44 either. I would really like a 5 shot 41 mag GP 100, but can't afford it. Maybe someday ruger will finally make a 5" 41 5 shot to compete with the Taurus tracker, or Smith 69. I'm surprised they haven't, they could sell a ton of them. And ruger likes to only make things that sell. They could call it the Trailhawk.

Until then, I will optimize the 357 I have to be all it can be. Safely.
300-MP appears to favor two things; longer barrels; and heavier bullets. It's supposedly a slightly slower burning powder than H110, and needs a little more time to burn completely.

I got a pretty slow 1190fps from a 4" GP100 with the 158gr Gold Dot/Deep Curl and 18.5grs. From a 20" barrel more like 1750fps. I haven't tried in my 6" GP yet.

300-MP may also favor larger calibers, as it also gets me 1190fps with a .44 mag 300gr WFN out of a 4-5/8" barrel.
I read about LilGun powder burning the forceing cone. I bought a pound jug of it and will not use it after a lot of negitive feed back on this powder.
When I got back to reloading I bought a butt load of one pound containers of powder, lilGun being one of them. Then I read about it and decided to not use it.
I have been making reduced loads in 38spl, 357mag, 44spl, 44mag for my eightvyear old grandson to shoot. He is shooting stouter loads now and maybe next year be up to full power ammo.

I have close to forty handguns for him to choose between.
Lil Gun has given me the highest velocity of any powder I've used in 357. Many variables including bullet hardness, bearing length and b/c gap come into play. I have a load using 16 grains of Lil Gun and a cast 170 grain SWC GC that clocked 1484 fps out of my 6" Ruger Security Six and did so with no signs of excess pressure. That was notably higher than loads worked up with H110.

Lil Gun also gave me the highest velocity in my .454 Casull, with 29 grains pushing a 300 grain Hornandy XTP Mag to 1663 fps and the cases fall out of the cylinder.

Each gun is different so working up is important. My Ruger Security Six can take slightly higher charge weights without signs of excess pressure than my S&W 686 can. I don't shoot these type loads in high volume but they are a lot of fun to touch off.
I'm not sure you can get there in a 5" barrel. I have a 4" Security Six. I worked up to 20.5 grs of 300MP under a 125 XTP, and decided that was as high I would go...not because of pressure, but because that was all the recoil and blast/concussion I cared to take!
I only got about 1310 fps from that load.
These results make me agree with Wronghanded, I think more barrel is needed for 300MP.
Here's what I recorded with Alliant's max load of 18.6 grains of 300MP and Speer JHP bullets in a Ruger Blackhawk 6.5" barrel: 1373 fps.

That falls short of what I got with Remington's 158 grain SJHP in the same gun: 1422 fps.

Or Federal's 158 JHP (cat # C357E) : 1415 fps.

And just for comparison, the Ruger shot Remington's 125 grain SJHP at 1739 fps. So, this Ruger does not have a slow barrel.
One of the only things I don't like about the 357 is I have to bring ear plugs with me into the woods. It's way too loud to shoot with no protection.
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