I'm with Petander, there is little or no danger of primer detonation by impact with the balls in the tube magazine.
After reading posts on chainfires in tube magazine rifles where round nose or pointed bullets were used I was curious because I used to have a Marlin 1895 and shot thousands of Lyman 457124 385 gr. round nose bullets from it with loads from BP to top end Marlin loads and no problem.
I tried setting primed cartridges off in a vice using round nose bullets cast from ACWW and a hammer. I could not set off a primer.
Not saying it hasn't happened or that it can't happen but using a standard round nose bullet cast from a moderately hard alloy I couldn't make it happen. Possibly a pointier bullet cast from hard alloy then heat treated might set off a primer but I wouldn't worry about a big round ball.
Longbow
BP | Bronze Point | IMR | Improved Military Rifle | PTD | Pointed |
BR | Bench Rest | M | Magnum | RN | Round Nose |
BT | Boat Tail | PL | Power-Lokt | SP | Soft Point |
C | Compressed Charge | PR | Primer | SPCL | Soft Point "Core-Lokt" |
HP | Hollow Point | PSPCL | Pointed Soft Point "Core Lokt" | C.O.L. | Cartridge Overall Length |
PSP | Pointed Soft Point | Spz | Spitzer Point | SBT | Spitzer Boat Tail |
LRN | Lead Round Nose | LWC | Lead Wad Cutter | LSWC | Lead Semi Wad Cutter |
GC | Gas Check |