. Much depends on the barrel length and velocity achieved. I've seen tracer keyhole out of a 1/9. I've also seen "1/9" barrels actually measure to closer to 1/10.5, 1/12 depending on the mfg. It's not easy to measure and plenty of smaller shops do it wrong - back to the "buy reputable" line. The problem for a beginner is how to determine who you can trust.
Sometimes 1/9 is better for an intended application, but 1/7 is less likely to ever have an issue and will just be easier for the beginner.
.