Need some brain power...

I just built a .300BLK upper to hog hunt with. I've assembled some loads that I have no idea what the jump is. They cycle fine, but it bugs me not knowing exactly what's going on and as I experiment with different bullets, it's a PITA to find the proper length so they will function. So here's where I need some of the boards brain power:

There are absolutely no .300blk modified cases available...anywhere! I ordered an extra .223 case, figuring I'd just cut it down and form it to 300blk.
Now I realize that after forming the new case to 300blk, the neck will be too tight for a bullet to freely pass through it.
If I somehow stretch it out, it won't fit the chamber.
I do not have the capability to turn the neck, inside or out.

I know there's a way, but I can't think of it. Any ideas??

Use a cleaning rod. Insert in barrel with action closed until you you hit the bolt face,mark the rod at the muzzle. Then open the action and drop a bullet in the chamber and reinsert the rod to touch the bullet,mark the rod at muzzle again. The measurement between the 2 marks is your COAL. You might have to hold the bullet in there with something while getting this last measurement.

I cannot believe you cant find brass for a blackout when everyone and their brother is reforming and selling it. Do a google search for .300 blackout cartridge cases for sale.

A couple other options:

The shank of a 5/16" drill is .3125. You could use one to expand the case neck. You would probably need to do this with a pair of pliers and the case in a shell holder.

Use a Dremel tool to split the case neck so the bullet is held loosely.

Years ago, I made a 22/.243 Middlested modified case. I turned the neck down to weaken it then ran it over a K&M tapered expander. Took it out of the shell holder and worked it around the tapered expander manually until is would easily accept a bullet.

Quote:

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Originally Posted by jonbearman

I cannot believe you cant find brass for a blackout when everyone and their brother is reforming and selling it. Do a google search for .300 blackout cartridge cases for sale.

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Please read carefully:

"There are absolutely no .300blk modified cases available...anywhere"

Brass to load isn't an issue.

I may have figured it out. First I took my 308 modified case and measured the inside, .309" Next I took a resized/formed case and measured it, .306".
With nothing too lose but a case, I dremeled the inside case neck with a stone to about .308, then rolled up a piece of emery cloth and spun the case around it for a fine finish.
I ended up with .310", which a bullet slides nicely in. Now I just need to repeat the process on a modified case.

Thanks for the ideas guys.

WOW! That's a lotta work compared to just using the split case method and the Hornady bullet comparator. You did manage to prove there are many ways to skin a cat :)