30-357/blackout - short chamber or shorten the die?

[cumminsnut76]

As many of you have probably read I have been working to put together a cartridge for my newest contender build. I loved the idea of the 30 badger but wanted to use 357 magnum brass to have a little more power. I ended up running the 357 magnum brass through the 300 blackout fl die. What i ended up with is a rimmed 300 blackout that is about .075 short in the neck. my question is do I have MGM just chamber the barrel for a standard length 300 blackout and have a fairly large jump with 125 grain bullets ( I suspect over .100 with a black out chamber) or do i take .075 off the fl sizing die and have the chamber cut short to match. This would give me a full length neck and eliminate the long throat other than the reamer has all ready. I know I can use 360 Dan Wesson or 357 max brass but i have some extra new 357 mag brass and it is a lot cheaper and easier to get ahold of than the other options.

[Jedman]

I built a break action rifle chambered in a shortened 300 Blk Out rimmed. I shortened the sizing die .060 and it sizes 357 mag brass perfectly in one pass. It doesn’t seem to leave the neck short but it does make the chamber from having a full pressure 300 Blk Out cartridge to fit and the gun close.
I did this as I built the rifle on a very lightweight folding 410 shotgun and didn’t want to use hi pressure rounds to be used. I handload the ammo to approximately 32 H&R mag velocity’s and it shoots 100 to 150 gr. bullets good from a 1-10 twist barrel.

If you do chamber a barrel for a shortened bottle necked cartridge I would form some dummy rounds to use as a headspace gauge. With the rimmed Blk Out I made some with the rim turned off so headspace could be set using the shoulder but also allow enough depth that the rim sets the headspace but the shoulder is within a few thousands also.
Jedman

[popper]

Is the neck short or shoulder short? My choice would be to use the std BO chamber so you can shoot heavier when/if desired. Fire formed brass. My BO necks are often short as I convert 223 cases. No accuracy problem.

[barrabruce]

You cut a short chamber with a std 300 blk reamer and shorten the fls die.
Cut the rim recess after using the rimless modified 357 mag case as a guage.



What Jedman was saying is the 60 thou is off the base of the case, so everything is the same above the now shortened and rimmed case chamber.

All the throat length ,angle;shoulder and dimensions are the same as a normal 300 blk down where the base of the 357 mag reaches.

It could hold 1 grain less powder but the case is made to handle 42k psi and not 55kpsi.
So more moderate loads are called for but higher than 38 special or 32-20 brass.

Hope that made you confused and muddied up the water some.

Jedman is the man.

[cumminsnut76]

Is the neck short or shoulder short? My choice would be to use the std BO chamber so you can shoot heavier when/if desired. Fire formed brass. My BO necks are often short as I convert 223 cases. No accuracy problem.

Popper, it is the neck that is currently where the shortness comes from. The shoulder is in the correct location. I’m just worried that shooting short 125 grain bullets out of a blackout chamber where the brass is that short will lead to some very long jumps.

[cumminsnut76]

You cut a short chamber with a std 300 blk reamer and shorten the fls die.
Cut the rim recess after using the rimless modified 357 mag case as a guage.



What Jedman was saying is the 60 thou is off the base of the case, so everything is the same above the now shortened and rimmed case chamber.

All the throat length ,angle;shoulder and dimensions are the same as a normal 300 blk down where the base of the 357 mag reaches.

It could hold 1 grain less powder but the case is made to handle 42k psi and not 55kpsi.
So more moderate loads are called for but higher than 38 special or 32-20 brass.

Hope that made you confused and muddied up the water some.

Jedman is the man.

No this makes perfect sense. In the back of my mind it made the most sense but I have not done anything like it before and just wanted to hear from more experienced guys on their opinion.

[dverna]

Trying to learn and understand so please do not take offense.

Why not just do the barrel in standard .300 BO? What is gained by using .357 brass and having an "odd ball"?

[Fitz]

Trying to learn and understand so please do not take offense.

Why not just do the barrel in standard .300 BO? What is gained by using .357 brass and having an "odd ball"?

I think that usually the break action of the Contender is considered to be more accurate with a rimmed case , but that being said I have several rimless cartridge barrels that are very accurate.

[cumminsnut76]

A little bit is for having a rim. Traditionally the rimmed cartridge is more desirable for the break action, easier to grip and remove, possibly a little more accurate but that is debatable. Realistically the biggest driving force is that I have having something everyone else has. 300 blackout is just bland. And wildcatting is fun

[Doughty]

I did mine, in a very customized Ruger #1, just as barrabruce describes. Using Accurate #7 and the Accurate molds copy of the 175 plain base. A fun and "accurate" cartridge.

https://accuratemolds.com/bullet_det...ullet=31-175BP

[popper]

I use 110, 125 and 135 in my BO, many are 'short necked' 223 cases, no problem at all. This is the silly little half jacket and you can see, really loaded long. Shot free standing from BO pistol, ~60 yds. Bigger holes are GKs and 9mm close.

[Rapier]

You may recall the Super Mag wild cat cartridge line, 33, 30, 7mm, etc. Once Elgin Gates got the 357 SM to work in a TC, with his silver soldered two piece brass, and then Remington started making the Max brass, he started working with necking the 357 SM case down. I owned a XL by RPM, in 7mm Super Mag. It was very accurate with the 145 RCBS using 4227.
The idea of cutting a 300 BO chamber short in an 8 or 10 twist .308 barrel, threaded for an action or stubbed into a single shot's barrel, with a 300 BO FL sizer cut off .060 is very functional. If the base cut is kept to 357 mag web diameteris you can just use a 38 Spl chamber reamer to cut the rim. This is a one off build, so brass trimmed, die formed and then fire formed is all you need. Reloading is by neck sizing with the FL sizing die.
The case and twist is perfectly capable of throwing 110s up to 230 grain cast .308 bullets.
Here is my 30x25-20 DW revolver made in a similar fashion as discussed. Basically the 25-20 FL dies were cut off at the top to the shoulder joint, created a rimmed and elongated straight walled 30 carbine when run with a .308 expander ball. The right load is a 200gr cast Saeco. The 200 cast was the most accurate of all the bullets tested and would do 1,500 fps.

[cumminsnut76]

I use 110, 125 and 135 in my BO, many are 'short necked' 223 cases, no problem at all. This is the silly little half jacket and you can see, really loaded long. Shot free standing from BO pistol, ~60 yds. Bigger holes are GKs and 9mm close.

Popper, these are what mine look like right now