I've been doing some load development these past few weeks with two of my 338 Win Mags and working up a load for a co-worker's 338 Win Mag. In the past, I have had excellent accuracy with the following load in four 338 Win Mags. 225 grain Barnes X/TTSX bullets fueled by 67.5 grains IMR 4350, Winchester cases and CCI 250 or Federal 215M primers. Ironically, the best seating depth in three of these 338s turned out to be the same. I can't recall the seating depth on the fourth one and I sold it several years ago. At any rate, I wanted to try H4350 (because my three 30-06s love it with a 168 TTSX) and the new IMR 4451 to see how they worked.
The rifles I used were a Browning XBolt with 26 inch barrel and a Weatherby Vanguard with a 24 inch barrel. Here's how the velocity stacked between the three powders using Norma cases instead of Winchester and Fed 215M primers: (I had to increase the IMR 4350 charge weight one full grain to get comparable velocities when switching from Winchester to Norma cases)
68.5 grains IMR 4350 average velocity
XBolt 2825 fps
Vanguard 2764 fps
66.5 grains H4350 average velocity
XBolt 2808 fps
68.5 grains IMR 4451 average velocity
XBolt 2786 fps
Vanguard 2739 fps
So, it would appear the new IMR 4451 is slower than IMR 4350 and definitely slower than H4350. H4350 didn't produce decent groups in either rifle but IMR 4451 did in both rifles. I loaded a few rounds up to 69.0 grains using IMR 4451 and didn't see any pressure signs in either rifle. I may bump the charge weight up and shoot a few more til I see pressure signs then back off accordingly.
I emailed Barnes to see if they had any load data for IMR 4451 for the 338 Win Mag and Ty responded saying they don't right now but they are working on a new reloading manual. He suggested I use Hodgdon's load data from their web site as a rough baseline.
IMR 4350 has been very good to me over the years but I'm gonna see what I can get out of this new IMR 4451. Allegedly it's a little more temperature stable than IMR 4350. We'll see...