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:

1. 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 suppressor.

2. 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 back.

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.