The different experiences with a straight hole are interesting. Bamma gets 100% reliability, Acer bails after two misfires, Dan gets variable results. No one has mentioned priming powder and its relationship to flashing, but I think it’s an important variable. So, what are you folks using to prime?
I have had very limited experience with straight holes. I have one rifle that I have shot many, many times with a Chambers liner. It is a .54 caliber, loaded with 2F and primed with 4F. Like Dan, my guns are hunting weapons, and I care only about the reliability of the first shot. Reliability with this gun has been 100%. The few times I primed with 2F, there were no ignition problems.
My last build was a 10 bore, with a straight 1/16” flash hole. I primed it four or five times with 1F, and it flashed every time. When primed with 4F, I had 100% good ignition for 20 rounds or so. To simulate actual hunting conditions, the bore was wiped between shots. Unfortunately, I had to send the gun off without conducting further experiments, but I think the flash hole would be better if it were drilled out to 0.070 or so.
Considering early muskets, if they were priming with 1F or similar coarse powder, it may well take an 1/8” hole to get reliable ignition. Does anybody have any experience with straight flash holes and priming with 1F? I appreciate that using a separate grade of powder for priming is probably a modern practice.