I thought that type of Diana fired slugs or pellets. BBs were an American thing and they are 4.4mm not 4.5 hence them rolling out. Try some round-nose pellets in it.
Oh, I just read your post properly. I have no idea what it can fire if it cannot be loaded from the back. Garvin will be along to tell you how it works soon enough...
Looks like my first air rifle, breaks like a shot gun to pull and compress spring in barrel, then front part of barrel unscrews as you have shown. Mine accepted lead Diablo pellets or darts, which would just about cross a small room! Perhaps the rear has been blocked to prevent use? Try pushing a cleaning rod through the removed tube? HTH David
Thanks for input folks, just measured the breach, it must have been burred down to 4.04 mm. Will clean it up and try again.
Baz
BE AN INDEPENDENT THINKER, DON'T FOLLOW THE CROWD
Darts or pellets should be ok, have seen round nose ones pushed in by thumb pressure & then to seat them fully pushed down on a wooden surface, fence post etc. Is there a possibility the 'breach has been squashed, dropped to go out of round? Or damaged by pushing it on a firm/ rough surface to seat a pellet?
A mate had one of those many, many years ago, he used it as a spud gun by sticking the narrow end into a potato, no idea if that was the original purpose.