I'm with you guys on the safety issues. I have only one gun tube that may be under your recommended wall-thickness specs. I blind-hole gun drilled a piece of 4140 chrome-moly 5in. OD. with a 2in. bore. Wall thickness is 1.5 with a 2in. thick breech. It's been fired approx. 400 times. Standard load is 1.975 patched lead ball with 1000grs. of 1F. No blank-loads have been fired. I proofed the barrel with 1lb. of goex. FF and two patched balls then took measurements and magnufluxed. We're going to have it phlorascoped (ex-rayed) next week to check for internal stress fractures, for my own piece of mind, after reading your forums. If a replacement tube is needed, I'll do a 6in.X34in.X2in. replacement. So far this gun is extremely accurate with no signs of fatigue, but SAFETY FIRST.
Well lets see. Safety first. There are some things you should consider to increase your safety threshold.
1.5 wall in 2 in bore. The safety guidelines we point to from AAA and N-SSA recommend 1 caliber. It helps that you used 4140 even though it doesn't meet the quidlines. But as you note thickness of the wall should be one caliber. I wouldn't change. There are other things you can do to reduce stress on the barrel.
Windage. The rule of thumb for windage is 1/40th of bore. 2 inch bore/40 = 0.05". 2- 0.05 = 1.95". The ball you use is 1.975". On top of that you use a patch of unstated thickness and a lead ball. Basically you have zero windage. You can decrease the ball size to 1.95 leave out the patch and use zinc balls. This will reduce pressure probably significantly. If needed you can add a sabot to stabilize the ball in the bore. Even though this ball shoots good, you have a barrel that is not built to full safet standards and this ball will stress the gun.
Load. Again guidelines suggest a maximum load of 2 ozs. per inch. 2 inch bore max load is 1750 grains. Your load is 1000 grains, so you are within parameters there. You are using Fg which has a higher burn rate and generates higher pressures than Cannon grade. Use cannon grade powder to reduce the pressure and reduce stress on the barrel.
If you have access to some 6" x 34" 4140, then that should be your next gun!