Chamber pressures in the Springfield .45-70 runs 24,000 to 28,000. Chamber pressure in the .45 Colt ran around 12,000 to 15,000.
70 grains under a golfball aint gonna get that wicked. I have a 1-1/2" cannon that I shoot 2 ounces in. It shoots a 2# ball. I suspect that it is getting some measureable pressure. It will poke a hole thru a car body at a quarter of a mile. Your golf ball will not create enough resistance to build that much pressure.
You would be wise to begin with some high pressure seamless tubing. It is very common in oil field and gas field operations. Then, build around it. The oil field tubing is pressure tested to 10,000 psi. It is often referred to as upset tubing. It also telescope fits as it has a 1/4" wall thickness. 1-1/2" slip fits in 2" fits in 2-1/2" which will press fit in 3". You build strength fast with it.
Golf balls are strictly regulated to be exactly 1.68" in diameter, and 1.62 ounces in weight. If you started with 1.5" upset tubing, it would be necessary to bore or hone it about ten thousandths to get the ball to go in. (0.010 of thickness off the inside all the way around creates 0.020 increase in diameter.)
Whittington Center Life