Problem solved.

I tried the "oversized" pellets I have in the smaller barrel - no problem they slide in firmly but easily.

I removed the rifle barrel - pellets would not fit by hand pressure.

Solution one HSS drill marginally bigger than .22 - (7/32") - a vice and a very slow speed twist just at the entrance and a couple of mm deep a quick clean up of swarf (dust really) and - Bingo - the pellets now press in firmly just like the smaller barrel.