This 300/221 Fireball Upper
is based on a Model 1 .750" 1:8 twist barrel with 1/2 x 28 threads.
Next I performed the following two steps to enhance performance:
I drilled a new gas port in
the pistol position and plugged the original port in the carbine position.
That gives you enough pressure to cycle heavy subsonic rounds without a
I used a Fulton Armory
adjustable gas tube to reduce the backpressure for lighter supersonic
loads; other ways they have way too much recoil with the gas port that far
In the photo above, you can
see the original front sight tower with no gas tube, the new gas block is
located under the SIR handguards and incorporates a Fulton Armory gas
adjustment as shown in the photo below.
This rifle functions great
with 220gr subsonic and 150gr supersonic loads, plus it has a 1/8 twist, so
it should stabilize up to 250gr bullets.
As far as reloading goes, I have the Redding deluxe die set (they call it
300-221 because the original parent case is .221 Fireball). This set
includes an expanding die that lets you form .300 whisper from .221
fireball. This is a time consuming process and is done in two steps. I made
a few hundred cases this way, but then I found someone who was making 300
whisper cases from .223 rem. He cut me a good deal on 1k cases, and I've
been using them ever since.