In the past I've either shot cheap store bought ammo in my 4" GP100, or some low power plinking lead bullet rounds I loaded up (around 900fps).  

I decided to try my hand at loading up some heavy magnum loads with W296 and 158gr hard cast coated bullets.  The reloading data for H100/W296 is all over the place.  I decided to use 15.0gr as my starting load, and work my way up to 16.7 (Hodgsons listed max).  I even loaded up a few 17.0 and 17.2 gr loads to shoot if I didn't see any pressure signs with the 16.7.

I started off with the 15gr loads (around 1230 fps), and worked my way up.  Around 16.5gr the velocities stopped climbing and stayed right around 1380fps, all the way up to the 17.2 gr loads...still around 1380-ish.  

Is that normal for the velocities to stop climbing even when you are going larger on the powder charge?  Also, is that an acceptable speed from a 158gr hard cast bullet coming out of a 4" barrel?

There is a point of diminishing returns.  Limits of the brass and powder.


Also...........lead bullets will get sticky at high speeds.    If you want more speed.......load copper jacketed bullets.

My bet is you ran out of barrel length.   You are burning the extra powder beyond the end of the barrel.   You might have to switch to a faster powder if you want more velocity.

They were powder coated hard cast bullets....supposedly you can push them at copper jacketed speeds.

Gas checked hard cast lead bullets can be pushed harder and faster than any copper jacketed bullet due to the friction of lead vs. copper.

You're probably running out of barrel length for the slower burning powder. Try a bit quicker powder. You ought to be able to get the 357 out of your GP100 up to 1550-1600 fps max.

You need a longer barrel to get max velocity out of a 357 Magnum.  Look at the velocity gain shooting Winchester 296 out of an 18" lever action carbine compared to a 4" or 6" barrel revolver.  The longer barrel contains the pressure longer even if all the powder is consumed.  To get higher velocity than your seeing your going to need a 6" or 8 3/8" barrel.  The lighter  140 grain or 125 grains can be pushed a little faster and is part of the reason they were released for handgun use.

A 158gr Hard Cast running 1380fps out of a 4" barrel is moving! How's the accuracy at this speed compared to say around 1200fps???

I agree with others that said you are running out of barrel length if your goal is to push them to around 1500fps... Are you trying to develop a hard hitting woodsman or hunting load? I can understand and appreciate what you are doing... Maybe try a faster burning powder as another poster suggested. However, I would be very happy with 1380fps for a 158Hard Cast as long as it was accurate out of my pistol. That makes for a pretty hard hitter IMO...

Maybe this will help and a S&W Registered Magnum with a 8 3/8" barrel.

http://www.ballisticsbytheinch.com/357mag.html

Your chronograph did its job and told you you're load is maxed out.

Could you feel the heat off the muzzle flash?  

H110 is fun in the 357 and needs to be loaded hot.

My main goal was to come up with a strong load that I could use in my hiking pistol...my GP100.  I didn't want to spend the money on Buffalo Bore ammo, but I think I've came up with a load that is pretty close to what they offer. I'm pretty happy with 1380fps with a 158gr hard cast slug coming out of my pistol.  Should be plenty for most 4 legged critters, and surely enough for any 2 legged ones that would ever try to do me of my family any harm.

As far as accuracy...I didn't notice much change between the starting load of 15.0gr at 1200fps and 16.7gr at 1380fps......so I'm sticking with 16.7 grains, with is what Hodgson's manual lists as max load charge.

You can't definitely feel them when you shoot them....much more snappy than the 900fps plinking rounds I load

Somewhere is the 1350fps range is about the top end speed for 158gr out of 4" barrel.
I load and sometimes carry 185gr FN-GC @ 1,370 fps out of my 4 5/8" Ruger Blackhawk.

Shot mine at the range today. 18.0 grains H110 under a 140 JHP.  An attention getter. Fireball like a basketball.  

Can't wait to shoot it at night.