I checked all of my B-17 manuals for reference to stops for the Waist Guns and there was no mention of stops in any of them. The Erection & Maintenance Manual gives instructions on setting the Fire Cut-Off cams for the Top and Ball turrets, but none for the Waist, Tail Guns or Radio Hatch Gun. There are considerable instructions on syncing the Chin Turret to the Sight assembly in the bombardier compartment and for aligning the sight in the Tail position, but nothing on firing cams for those positions.
I’ve read pretty much every book that I could find on the 8th Air Force in WWII and there are numerous references to waist gunners firing into their own horizontal stabilizers and wing tips not to mention into other B-17s in their formation.
This is from the B-17 Erection & Maintenance Manual on the Waist or Side Gun Positions
E56AEC01-CBCB-4350-8BB4-8619F49D4D68 by
tanker622001, on Flickr
7E57D547-466E-4305-B35F-40DF0141462D by
tanker622001, on Flickr
This is from the B-17E Flight Manual
5C3F8E92-5500-4B59-97FF-F02E9152387E by
tanker622001, on Flickr
1DB96F27-35AF-4FDD-919E-EAE52B48B7AA by
tanker622001, on Flickr
This is from the B-17F and G Flight Manual
CC50747D-DF76-4AE1-BAD4-B041BE966098 by
tanker622001, on Flickr
From the B-17 Pilot Training Manual
8E697188-EB32-4F5B-92C4-508B5759C15C by
tanker622001, on Flickr