.300 Win Mag load development

[ThickPines]

I’m going to start load development on a .300 Win Mag hunting rifle soon. It’s likely going to be a slow process, but I wanted to document it here. Unfortunately, I don’t have a chronograph, so I will be basing this solely on the groups for now. I will begin at 100 yards, then eventually move further once I find a load that I’m satisfied with. Any input or advice is welcome.

The rifle is a Browning A-Bolt II that I recently bought used. I don’t have any idea how many rounds are through it, but it was filthy with dirt and grime when I got it. It looks well worn, like the previous owner got many good hunting seasons from it. I’m guessing (hoping) that it was hunted hard, but not shot a lot. It has a 26” stainless barrel and a Burris Veracity 3-15x50mm scope with Warne Maxima rings.

The components I’m beginning with are: Sig brass, 180gr Nosler Accubonds, H1000 powder, and CCI magnum primers. From reading load data and other users info, this seems to be a popular combination, so I’m hoping for good results.

I’ve been fiddling around with hand loading for a few years, but this is my first real dive into load development. My only concern at this point is the brass: since it has never been fired, I’m wondering how much my load development will change as the brass fire forms.

I’m planning to load a few this weekend and see how it goes. I’ll be loading in 0.5 gr increments, probably 3 round groups for now.

 

[Harvey_NW]

I’ve been fiddling around with hand loading for a few years, but this is my first real dive into load development. My only concern at this point is the brass: since it has never been fired, I’m wondering how much my load development will change as the brass fire forms.

2nd or 3rd firing will show pressure if you load anywhere close on new, so you may have to back the charge off a bit. Headspace off the shoulder, not the belt. Bump shoulders .002" once fully formed.

I’m planning to load a few this weekend and see how it goes. I’ll be loading in 0.5 gr increments, probably 3 round groups for now.

Don't do this, you'll just be playing in noise. "Load development" isn't really a thing as far as "tuning" a load. Don't waste the components unless you want to.

Any input or advice is welcome.

The quickest way to finding a good load is to do a quick pressure test working up from a mid range charge toward max in .5gr increments (1 shot each, you're only looking for pressure. If you get ejector marks or bolt lift, STOP.), then load some at a safe charge and shoot a group. 10 shots is usually enough to give you a good idea of how it will shoot. My goal is always 10 into 1" or less. If it doesn't do that, swap bullet or powder and repeat. That's it. No silly OCW, seating depth, ladder, meaningless testing, it either shoots or it doesn't. I personally prefer to stay a minimum of .040" off the lands. If the throat is super long I just seat the boat tail edge to the neck/shoulder junction and roll on. Hope that helps, good luck.

 

[ThickPines]

2nd or 3rd firing will show pressure if you load anywhere close on new, so you may have to back the charge off a bit. Headspace off the shoulder, not the belt. Bump shoulders .002" once fully formed.


Don't do this, you'll just be playing in noise. "Load development" isn't really a thing as far as "tuning" a load. Don't waste the components unless you want to.


The quickest way to finding a good load is to do a quick pressure test working up from a mid range charge toward max in .5gr increments (1 shot each, you're only looking for pressure. If you get ejector marks or bolt lift, STOP.), then load some at a safe charge and shoot a group. 10 shots is usually enough to give you a good idea of how it will shoot. My goal is always 10 into 1" or less. If it doesn't do that, swap bullet or powder and repeat. That's it. No silly OCW, seating depth, ladder, meaningless testing, it either shoots or it doesn't. I personally prefer to stay a minimum of .040" off the lands. If the throat is super long I just seat the boat tail edge to the neck/shoulder junction and roll on. Hope that helps, good luck.

Thanks for the tips. If it takes 2-3 firings to fully form the brass, will the first firing be more or less useless for development purposes?
My magazine limits my cartridge length, by how much, I don’t know yet. I know that I can modify the magazine, but I’d rather not deal with that if I can avoid it.
I know that I’ve seen a thread/article here about the load development process, but I can’t seem to find it at the moment.

 

[Harvey_NW]

Thanks for the tips. If it takes 2-3 firings to fully form the brass, will the first firing be more or less useless for development purposes?

Not at all, it will just show pressure quicker once fully formed so I recommend not to stand on it with new brass and then watch for it or make a small adjustment if needed later.

My magazine limits my cartridge length, by how much, I don’t know yet. I know that I can modify the magazine, but I’d rather not deal with that if I can avoid it.

Meant to add that, just load to mag length then and send it.

I know that I’ve seen a thread/article here about the load development process, but I can’t seem to find it at the moment.

Most likely this one, Painless Load Development. This is the method I use, except the author has tenfold more experience than me and goes right for a max charge, I still start a bit below max and pop a few shots off into the dirt pile even working up to make sure it's safe.

 

[SDHNTR]

Thanks for the tips. If it takes 2-3 firings to fully form the brass, will the first firing be more or less useless for development purposes?
My magazine limits my cartridge length, by how much, I don’t know yet. I know that I can modify the magazine, but I’d rather not deal with that if I can avoid it.
I know that I’ve seen a thread/article here about the load development process, but I can’t seem to find it at the moment.

Not useless as you’ll get a feel for what powder and bullet combo it likes or doesn’t like. Then you’ll just need to dial in the powder charge once fully formed.

 

[wind gypsy]

Juice aint worth the squeeze for everyone but I liked the idea of jamming the bullets on the first firing of a belted magnum to help get the shoulders blown out a little more and reduce case stretch on that first firing.

This is belted magnum specific since they typically have over 0.010" room to grow at the the shoulder datum when unfired and that isn't the norm for non-belted cases.

 

[pbroski]

Peterson is making a "long" version to minimize case stretch. So far they're available in .300WM and 7mm Rem Mag.

 

[ThickPines]

Thanks to all for the advice so far. Obviously, I still have much to learn, but that’s part of the fun, I guess. I have read some of the Painless Method posts and listened to the Hornady podcasts on load development and sample size. I haven’t had time to work on this much yet, but I’m hoping that this weekend will give me more time.