Okay, now we are getting some where.
Barrel diameter 2.75/3 = .917 equals the safe wall thickness from the guidelines.
Your barrel is 2.75 - 1.0625 = 1.6875 /2 = .84375. minimum.
This also turns out to be a good example to show why Barlow's Formula should not be used with muzzle loading cannon.
To keep it simple, here is a link to a Barlow's Formula's Calculator.
http://www.aerocomfittings.com/barlows.htmlYour barrel diameter is 2.75 inches.
We don't know what type of brass you gun is made from so I will just use the numbers for C360 brass. Which as a yield strength of 45,000 PSI.
I don't know what the actual pressure of blackpowder in you gun would be but a number often referenced for black powder in cannons is 25000 psi.
Barlow's Formula Calculator
Barlow's Formula - An equation which calculates the relationship of internal pressure to allowable stress, nominal thickness and diameter of pipe.
The formula is: P = (2*S*t) / D, where:
P = internal pressure, psig
S = unit stress, psi
t = nominal wall thickness, in.
D = outside diameter of pipe, in.
P (internal pressure): 25000 psig
S (unit stress): 45000 psi.
t (nominal wall thickness): 0.7639 in.
D (outside diameter of pipe): 2.75 in.
This gives a thinner wall thickness than safety guidelines.
Even though I missed the s factor in the Barlow formula, I knew it wasn't valid for this application, this proves it.