I loaded some 600 grain Barnes soft points for my .458 Lott. I used the data I could find for those bullets and backed off a couple of grains.
Hornady brass, IMR 4320-75 grains, win mag rifle primers
First shot blew the primer.
Chrono at 1910fps. (expected 2000+)
Too much pressure and below expected velocity.

The 550 grain woodleigh's I loaded with 78 grains worked fine. But still were a little slower than expected, 2050fps

When I pulled the rest of the 600gr loads I noticed that they were A LOT harder to get out than any other bullet I have ever pulled. Micro'd at what appears to be the correct diameter. I would bet they don't even need a crimp to stay in place.
I have two questions;
1. Anyone had bullets this tight and is this the cause of the pressure problem?
2. I want to develop a load with these bullets so what is the best powder to try next, I'm thinking a much slower powder would work best.

.44, I don't have any loading data and my manuals are at the house, but I just thought I'd throw this out here. Are you sure the data called for IMR-4320? 4320 seems like it would be way too fast. I would think you would definitely need something slower than that, but admittedly this is just conjecture. I just thought I'd ask. I'm at work and I can't pull up IMR or Hodgdon's website at the moment to double check/correlate.

IMR-4320 is what was called for.
Here is the copy and paste;

Barnes Original 600gr H335 68gr 1,940
H335 73gr 2,090
H4895 69gr 2,000
H4895 74gr 2,150
IMR-4064 68gr 1,930
IMR-4064 73gr 2,070
IMR-4320 71gr 1,970
IMR-4320 76gr 2,110


I should have gone with the 71 grains to begin with. However, I cross referenced the data with other I found and they agreed. However, Hornady had a much lower amount listed and I brought this point up. After consideration
I felt that the 75 grains should be ok and at the most a little hot, hot it was. The low fps is what's really perplexing.
This is my first loss of a primer, no other marks on the brass and I didn't have to tap the bolt to move it, just apply some pressure. Think any harm done??

I think 4320 is as slow as you can get for this cartridge. The Lott rec. load was 85 gr 4320 w/500 gr bullet in the past. Since the Lott case is only a slight improvement over the 458 Win case, I think you might consider using Re-7 with your 600 gr Barnes. Another powder to consider is Re-15. You are in uncharted waters as to how this might come out. Straight sloping cases do not do well with slow powders anyways. Good luck with finding the right combo.

Quickload says your 4320 load is around 71.5k
It shows 77gn H414 might give 1990 at 62ksi and 100% load density when loaded to 3.6"
FWIW

Lar45
Thanks for the info about H414. I was thinking about H4350, the data from Hodgdon says it is very similar to H414 but is easier to meter and resistant to temp fluctuations. Does your quickload program have stats on this powder application?
Also, I have heard of "Quickload" is it worth the money?

I'm guessing on this but I think 475Guy is right, pretty much all the way around. Straight walls and slow powders aren't normally a good combo. Too, Barnes has a habit of posting load data that is...refreshing. I've yet to make a Max load using their data, and sometimes I don't even get close. Sorry I don't have any suggestions, but I'll bet slower ain't better.

IMR-4320 From 71.0 grains to 76.0 grains
Federal 215 Primer

Read the first post.....I used that information and blew the primer at 75 grains.
I am headed out now to see what 70 grains will do.