I'm getting loads together for Zim in my 458 Lott, what is the general observation some of you see with heat and velocity changes, IF ANY in the 458 Lott. Of course I'm going to Zim when the temps probably won't rise above 80F so it's probably a moot question.

Some general info of what I'm experiencing at this time, 75grains of R-15 with Swifts and Woodie solids are producing 2140-2150 (in 50 degree weather) respectively over my chrono, I am using Winchester M70 with 21.5 inch barrel. There are no excessive pressure signs with this load.

You should be fine with that load.
It is sub maximal, room for increased presure with heat, and POI changes should be minimal, but you might want to recheck your sight-in when the temperatures are hot.

I have noted about [1 fps/degree F] changes with RL-15 versus [0.5 fps/degree F] with Varget and H4350. IMR 4350 gave [2 fp/degree F].

RL-15 is O.K.

...using the chart from page 96 of P.O.Ackley's HANDBOOK FOR SHOOTERS & RELOADERS volume ll, I've interpolated the numbers for you.

2150fps @ 50F=2185fps, and 80F=2127fps. As you can see, the higher the temp, the lower the velocity. These data are assuming the temp of the powder in the loaded ammo is the same as ambient.

FWIW: Across the screens @ 60F the following numbers are for my M-70 .458WM with a 24"bbl.

72.5grs AA-2230, Fed 215primers, WW brass:
500gr Woodleigh=2125fps
500gr Hornady =2145fps
500gr TBSH =2185fps
500gr AGS =2172fps

Screens @ 15' Vel corrected to muzzle. Keeping the velocity at about 2150-2185fps gives the best accuracy in my rifle. I've shot no less than 600rounds thru this rifle for load developement.

Used the AGS/TBSH bullets in Zim with temps in the high 90'sF. Never had any pressure problems...feed, extraction, etc. Works great on ele's.

Shoot straight.

I have never really worried about it. The only time I kept track of it was in the 9.3x62 with RL-15. A change from 50 F to 80 F increased the velocity of a 286 gr. Partition about 150 fps.

But at 25 yards what do I care?

quote:

Originally posted by tsibindi:
...using the chart from page 96 of P.O.Ackley's HANDBOOK FOR SHOOTERS & RELOADERS volume ll, I've interpolated the numbers for you.

2150fps @ 50F=2185fps, and 80F=2127fps. As you can see, the higher the temp, the lower the velocity. These data are assuming the temp of the powder in the loaded ammo is the same as ambient.

Oh really?

I think you got something bassackward there.
They are talking about correcting your chronograph reading to 70 degree F reading.
The table shows what to subtract from the higher reading at higher ambient temp, to predict what lower velocity you will have at lower temp.

Likewise the table shows what to add when chronographing at lower temperature to predict the 70 F velocity.

That page also says Dupont found only a 60 psi pressure increase per degree F increase, or a 60 psi decrease per degree F decrease. That does not imply lower velocity at higher temperature.

Also noted the changes are greater per degree for 72 F ("normal") to 100 F, than for below that range, i.e., 70 F to 0 F.

They do not even go into differences in powders or cartridge cases.

The table is pretty worthless for application to a particular case such as the .458 Lott with RL-15 and 500-grain bullets.

quote:

Originally posted by RIP:

I have noted about [1 fps/degree F] changes with RL-15 versus [0.5 fps/degree F] with Varget and H4350. IMR 4350 gave [2 fp/degree F].

RL-15 is O.K.

The above was noted in the limited case of .375 Weatherby with those powders and 300-grain Sierra GameKing "generic bullets" over the limited temperature range of 30F to 50F.

Higher temperatures do produce higher pressures and velocities, not lower!!!

...Rip, You are 100% correct. I stand corrected.

That's what happens when the fingers do the typing prior to engaging the brain.

My appoligies to all for my error, and especially to Dirklawyer.

At any rate, 2150fps in 80F conditions is certainly not a problem with the Lott.

I think you are good to go.
Bill

This heat thing is a bunch of hooey by some folks that either don't know how to reload or they are just wanting to be gun editors...

I have hunted Africa for many years and more than most Africans and I have never had such problems in Africa, West Texas or in Arizona.

What causes thoes problems is compaction for one thing and poor loading practices meaning to much powder in a case.

I have let ammo and rifles cook on the truck hood in W. Texas at 110 to 120 degrees for several hours and then shot my .458 Lott, 416 Rem, .375 and 404 with factory and near max but not max handloads and couldn't even manufacture a problem...I did this after an African rag made claims about the .416 Rem...

I will agree with Ray. The powders of today are much better than 50 years ago. I have shot in Texas when it was warm enough to cause the alleged African heat problem and didn’t have a problem.
Bill