Multi-caliber & different barrel lenghts

[WhisperFan]

I want to ask the forum a couple questions ......

I have a registered SBR. It started out it's life as a Rock River Arms "Pistol Only" lower with a 7.5" .223 barreled upper on it. It cycles just fine and everything was cool.

I bought a Model1 300 Fireball barreled A2 style upper with a 16" length and a 1-in-8 twist. I never fired it as a 16" barrel and cut it down to 10.5" and threaded the last .500" at 1/2x28 the day I got it. That puts the shoulder of the barrel just in front of the bayo-lug.

The problem now is that the 220 grain SMK is out of the end of the barrel before the gas has a chance to make the action cycle.

I am using a standard CAR length collapsable butt-stock with a std spring and I don't want to make changes to the lower - because it will eventually house the .223, the Whisper and someday a 9mm as well.

My plan is this -
1. Disassemble the upper.
2.Turn down the barrel's OD to accept a low profile gas block. Leave the shouder for the block at a pistol length postion.
3. Plug the old gas port and drill a pistol length gas port.
4. Put on a low profile gas block with a carbine length free-float tube
5. Re-install the A2 sight tower.

My questions are -
1. What is the proper postion for a gas port on a 300/221 10.5" barrel (pistol length)
2. What diameter should I start with? .080" ???
3. I plan on using my suppressor on it (pretty much all the time) Will this change amy of my measurements?
4. Is there anything I haven't thought of that I should be?

I don't dislike the idea of an adjustable gas tube, or and adjustable block - but if I can dial it in w/o them - that would be ideal.

This upper will be a dedicated subsonic upper used primarily with 220 gr SMK's pushed by AA#9 (I'm still working on my load - I heard 9.9 grains, but I break the sound barrier at that loading - the AA sheet I saw says 8.2 grains, so I will be slowly loading them down.

Thanks for any help