Base to datum with a .010" slop. Thanks
Headspace for 300 ultra with 35deg?
- Thread starter Mram10us
- Start date
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First, I really appreciate the help. The number you came up with is less than normal headspace, so it isn't right. Again, thank you.
I see the error. So for ackley or any Improved, you have to go backward from the neck/shoulder junction toward the shoulder since it is getting blown out. Your math was perfect if we wanted to find the headspace for a cartridge with a 35deg shoulder starting at the original shoulder measurement. Again, thanks for taking the time.
birdiemc
Well-Known Member
Lol wrong planet...so how in the world do you how much you're moving the shoulder?
You have to work the math backward from the neck/shoulder junction or.... just shoot the stuff and let it form trusting your headspace measurement
Are you trying to chamber a barrel? I am not a gunsmith but it is my belief that Ackley always set a slight crush on the parent cartridge. The only Ackley I have been around was set up this way. I know I am not helping, just curious.
I already did it last night 
Got it finished and math is good now after triple checking. It is a 300wsm/300rum hybrid I've been thinking about. I'll post results in the next week.
I asked because I like to make my own headspace gauges for hybrids. I like to have my numbers checked by others to see what mistakes I've made.
Got it finished and math is good now after triple checking. It is a 300wsm/300rum hybrid I've been thinking about. I'll post results in the next week.I asked because I like to make my own headspace gauges for hybrids. I like to have my numbers checked by others to see what mistakes I've made.
Correct, There is slight crush at the neck/shoulder junction on ackleys.
birdiemc
Well-Known Member
I've been thinking about this quite a bit, and I just spent the past half hour sipping whiskey, eating cheetos and staring at saami drawings of 280rem and 280ai trying to find anything that would provide a basis for doing math, and I dont get it.
I do understand now why the headspace lengthens instead of shortens like my math showed, cuz you're not compressing the brass into the new shape, you're blowing it out to the new shape. But I cant figure out the math that got you to the numbers you put up so please educate me sir.
That is a great example. It gets you thinking since everyone told me that "you shorten the headspace on ackleys". I scratched my head on my first one wondering why even bother till I noticed they were saying that without referencing the one saami example we have, the 280ai. I became familiar real quick with a set datum point rather than the neck/shoulder junction.
Regarding the math, when I get time I'll write it out and take a pic.
I copied the neck shoulder junction and headspace measurements from the 300rum, here
Then compared the base to shoulder length of the 300wsm, here
2.5383-1.7967=.7416
Add .7416 to the headspace for the 300 wsm and you get 2.4676-2.4776 for a .4450" datum which is different than the .420" datum of the rum.
To avoid doing the math, I let saami do the work by using the 30nosler print since it uses a 35deg shoulder and has a .420" datum.
Take the neck/shoulder junction measurement minus the headspace and get the difference:
2.2405-2.188=.0525
Then minus that number from the neck/shoulder junction in the rum:
2.5383-.0525=2.4858 for the min headspace.
The easiest way would have been to just use the 30nosler and the 300rum from the beginning, ignoring the 300wsm since it had a different datum measurement.
Another easy way would have been to just use a fl sized case, verified by a Comparator, fill it with epoxy to avoid crush then use it as a headspace gauge, avoiding math all together.
Sorry for my longest post to date, but I wanted to show where I could have made it easier on myself than I originally did. Hopefully it saves someone a bunch of time trying to switch datum points and simply use what saami already has in front of you.
Then compared the base to shoulder length of the 300wsm, here
2.5383-1.7967=.7416
Add .7416 to the headspace for the 300 wsm and you get 2.4676-2.4776 for a .4450" datum which is different than the .420" datum of the rum.
To avoid doing the math, I let saami do the work by using the 30nosler print since it uses a 35deg shoulder and has a .420" datum.
Take the neck/shoulder junction measurement minus the headspace and get the difference:
2.2405-2.188=.0525
Then minus that number from the neck/shoulder junction in the rum:
2.5383-.0525=2.4858 for the min headspace.
The easiest way would have been to just use the 30nosler and the 300rum from the beginning, ignoring the 300wsm since it had a different datum measurement.
Another easy way would have been to just use a fl sized case, verified by a Comparator, fill it with epoxy to avoid crush then use it as a headspace gauge, avoiding math all together.
Sorry for my longest post to date, but I wanted to show where I could have made it easier on myself than I originally did. Hopefully it saves someone a bunch of time trying to switch datum points and simply use what saami already has in front of you.
Sorry my last post is such a mess. I'm trying to hurry to go shoot and didn't edit it. Hope it makes sense
Leaving neck/shoulder as is.
