Recently purchased a 7-300 Win Mag and will be staring load development in the next couple days. Haven't been able to find a ton of info on sizing it down. I will be using a Redding full length die. I saw one post on another forum where a guy used the bushings in order of .331, .324, .317, and ending with .309. Is it necessary to go in these stages,? Also will be trying N570 with the 180gr class bullets, anyone that has tried this combo, any info is greatly appreciated.
7-300 win sizing
- Thread starter DesertBoy
- Start date
Help Support Long Range Hunting Forum
Justice1327
Well-Known Member
Bushings will depend on your case neck wall thickness, but I only step down twice.
Using Peterson Long 300WINMAG brass, I step it down to .308, then down to the finished diameter for a .284, with 3thou neck interference. The final dimension is not sized all the way down the neck. I size only what is needed to give me a false shoulder. This, and leaving the bullets just touching the lands and removing my plunger, gives me great fireformed brass. I have great results with n570 and especially H1000 at 76.5gr for my 168 Bergers.
Mine is the 7mm Practical v2 on the Manson reamer.
Using Peterson Long 300WINMAG brass, I step it down to .308, then down to the finished diameter for a .284, with 3thou neck interference. The final dimension is not sized all the way down the neck. I size only what is needed to give me a false shoulder. This, and leaving the bullets just touching the lands and removing my plunger, gives me great fireformed brass. I have great results with n570 and especially H1000 at 76.5gr for my 168 Bergers.
Mine is the 7mm Practical v2 on the Manson reamer.
QuietTexan
Well-Known Member
I wouldn't move down more than 0.010" in bushing size at a time. If you try to do it all at once you end up with necks that are crooked and uneven.
I step down from .338 to .308 cal using three bushings, at about the same intervals as you listed for .308 to .284 cal. It takes a few extra steps but at the end of it you get better brass for the effort.
An upshot of this is that since you're necking down, you can intentionally leave the unsized portion of the neck slightly long and have a false shoulder to hold the case back against the head with instead of using the belt on the initial firing. That will give you better shoulder movement on the first shot, will add at least one firing to the life of the brass. All it takes is test fitting a case the the rifle to determine how much of the neck to size.
Pics in this post show about how much of the neck can be left unsized to get a good crush fit:
Ackley Improved Pickle Spot
I ran five boxes of Rem factory 250 Savage through my first AI and accuracy was so amaxing that I struggled trying to find as accurate a load with the fire formed brass. I used COW method in my other AIs as I’m just to impatient to get loading the fire formed brass.
QuietTexan
Well-Known Member
I tried to get pics of what I'm talking about. This is one case, on one side it looks all normal and good - the other is visibly a couple hundredths shorter and the shoulder/neck junction is bulged. This was 338 cal to 308 cal in one step, and is why I use multiple intervals now.
