375 ultra mag

Chase723

Well-Known Member
Joined
Nov 22, 2009
Messages
620
No. My name is Chase. Lol. Mine is an 1:8 twist. It’s going to be setup to run the 365gr Bandlands at 20-40 thou off the lands with the base at the neck shoulder junction.

What do you mean when you say mixed results w/ RL26?

…looking like I’ll have to get my hands on some RL16 or H4350 then.
 
Last edited:

Chase723

Well-Known Member
Joined
Nov 22, 2009
Messages
620
To keep this thread going as a source of knowledge, I got my barrel back and did some pressure testing. With the 365gr SB2s at 10 thou off the lands my COAL is 4.446…it’s obviously setup to maximize case capacity etc. so take that for what it’s worth before trying anything that I’m about to post. My starting points were 2gr below what quickload said was max for the 350gr SMK and then I worked up from there.

Tried RL16, N565, N570, and H50bmg.

98gr N565 gave 2730fps and would be absolute max that I would consider loading. Probably real hard on brass. Ejector, little sticky bolt lift, little bolt click. 97gr was 2690fps. Slight ejector mark, slight resistance with bolt lift. Probably a much more reasonable max.

103gr N570 shot 2740 w/o pressure (no ejector, bolt popped right open, easy extraction) but I couldn’t fit anymore in the case by simply pouring it in via the funnel….when I got home, I used an electronic toothbrush to vibrate the case and was able to get 106gr in without affecting my seating depth so I’ll see if I pressure out between 103–>106 tomorrow. If I throated it out a little and seated out further then I could get a little more but if I pressure out that won’t be necessary.

87gr RL16 was 2730. Same pressure signs as 98gr N565

107gr H50bmg was 2560. No pressure. However, it was definitely compressed and no more room whatsoever. Also, was obviously slow.

N570 looks the most promising. Should break 2750 and maybe 2800. We’ll see tomorrow. I’d be really happy with either. Both RL16 and N565 aren’t bad either, especially if my barrel speeds up a little. 2700fps w/ a G1 BC of 1.01 does pretty well. Push comes to shove I might try the 350gr ICBM2. Would pick up a little velocity just not sure how much.
 

Chase723

Well-Known Member
Joined
Nov 22, 2009
Messages
620
Finished pressure testing. N570 is the ticket. Seems unbelievable but I was able to shoot up to 106gr today. Shiny ejector mark but easy bolt lift. 2833fps. I presume my load will end up somewhere between 2750-2800fps. As long as the accuracy is there, then I’d say that the 375 RUM will be a success and is a very “affordable” way to get into 2000+ yards on a standard magnum bolt face. Literally just a barrel swap and you’re in business. Here’s to hoping actual load development goes well!
 

Chase723

Well-Known Member
Joined
Nov 22, 2009
Messages
620
there are ejector marks on the rim......but the primers aren't flat so the pressure it's not so high
I agree…not that I can get anymore in to try though. However whenever I see a jump from 40fps/grain to 70gr/grain at the top end I know that I’m approaching the threshold. Usually it’s accompanied by a sticky bolt etc but not this time. I’m going to load a couple more at 106 and see if it feels the same and then I’ll start load development.
 
Last edited:

Outlaw6.0

Well-Known Member
Joined
Feb 18, 2010
Messages
3,100
Location
Gillette, WY
I'm really looking forward to seeing where accuracy/velocity settles in for you. Looks to be a stout combo & i'm sure it'll be a lot of fun.




t
 

Taylorbok

Well-Known Member
Joined
Apr 19, 2021
Messages
194
Location
Sask
That's really impressive for that RUM case. I'm assuming you have run the ballistics, what is your expected elevation correction for 2 miles?
 
Top