300 AAC BLACKOUT 12.5 inch = 300 Fireball 16 inch?
 

It is looking like a 12.5 inch 300 AAC BLACKOUT matches the velocity of a 16 inch 300 Fireball. This is probably due to the extra freebore (like 5.56mm compared to 223 or 6.8 SPC-II compared to 6.8 SPC SAAMI).

I was noticing that I could extra grains of powder before signs of pressure in a load that others were using with 18 grains. Previous loads will need to be re-worked up if you have a real 300 AAC BLACKOUT chamber. And remember - don't shoot 300 AAC BLACKOUT ammo in a previous chamber as it will have additional pressure.

Having a little difficulty following what your trying to communicate to me(us)...

Comparing velocity of two completely different cartridges, with different barrel lengths means pretty much, oh well.....

Barrel #1: 14" 300Whisper
Barrel #2: 24" 300Whisper
I can manipulate the powder charge to make the 14"(#1) out perform the 24"(#2) all day. Even if the powder charge is the same, the projectiles can be manipulated to make #1 look superior....even the seating depth, crimp/no-crimp, number of lands and grooves, twist rate, etc, etc.....

Even two completely indentical cartridges just being chambered in two different barrels(with the same twist rate, lands-n-grooves, length of freebore, length of barrel) can give some very large differences.


Or are you saying that the 300AAC is a better cartridge then the 300Fireball?

Scott

Well put SDB777. I think barrels are like snowflakes no two are the same. I have seen two barrels by the same manufacturer made at the same time using the same ammo that produce different velocities.

I have two of my 338 THUMPERS one 12" one 16". I have loads that are faster in the 12" then in the 16". They were made from the same blank with the same reamer one right after the other.

rsilvers are you talking about subsonic or supersonic loads? What bullets and what powder are you using? What are the twist rates of the barrels? We need more info.

But aren't your recommended seating depths extremely short to function correctly in the AR15 mag? Seated to your recommended OAL, seems like there is a lot of powder space wasted.
-Dan

I am saying that if you fix the OAL of two cartridges and load the same bullet - one to be 55,000 PSI in a 300 AAC BLACKOUT chamber and load the other to be 55,000 PSI in a 300 Fireball or Whisper(R) chamber and use the same barrel blank for each, the 300 AAC BLACKOUT will be higher velocity.

So in effect, the 300 AAC BLACKOUT can get 16 inch 300 Fireball performance from a 12.5 inch barrel.

The difference being like comparing 6.8 SPC-II to 6.8 SPC SAMMI.

True with longer bullets. But that is optional and applies to 300 Fireball and 300 Whisper also for best feeding. You can load to full SAMMI length of 2.26 if you want to give up some feed reliability in existing mags. We are going to make some custom mags for people who want to shoot long ammo.

So does the extra freebore that positively affects velocity also negatively affect subsonic accuracy as the bullet needs to jump slightly further to the lands?

It may negatively effect accuracy that you cannot set up a load to contact the lands, but we did not want the lands to be jammed if someone loaded a 220 or 240 Sierra MatchKing to full magazine length and got stuck bullets. So the lands ended up where they are based on the fact that the SAAMI max OAL is 2.26 inches, and people would be loading Sierra bullets. Military levels of reliability was the most important criteria (I personally got a stuck bullet in a 300 Fireball barrel), and the boost in supersonic velocity was a nice plus.

It would be interesting to see a subsonic accuracy comparison between two barrels of the same length but one reamed with the 300blk and the other with a 300/221 reamer.

I have always been told that you get better subsonic accuracy with the ability to get close to the lands (more even pressure and velocity) but would be curious to see the results to prove or dissprove that theory.

Of course I am assuming you would need to seat your 220/240 300blk loads longer than your documented COALs for an apples to apples comparison so that might not be what you want.

You can seat longer if you want but 300 Whisper(R) / Fireball has a poor reputation for reliability and that is due to the bullets contacting the USGI rib. Remington ammo is loaded shorter and so is very reliable. A custom magazine will allow both a long OAL and feed reliability if people don't mind buying them.