If you have a look at my earlier posts there are pictures of the magazines I got with the rifle they are original M55 magazines. As I said before I can use the magazines but that will restrict me to a cartridge over all length of 2.940. If I load each round individually I will be able to seat the bullet out to a longer over all length. But we will see. Once I have a range day I will give it a whirl but that could be a couple of weeks away. But I live in hope.A 308 / 7.62 x 51 magazine is about 71mm internally , a 6.5 x 55 round ( cartridge case with bullet loaded ) 139gr bullet is around 71mm .A 6.5 x 55 round won't fit a 308 mag. . A M55 Tikka is a short action typically for .243 / 308 and their derivative rounds . I am struggling to understand how a different mag would fit unless new bottom metal was fitted perhaps modified from an M65 , all very odd .I'd like to see some photos of the mag. and the bottom of the action . All the early Tikka M55 and M65s are superb rifles , my M65 super sporter is a long action but with a short mag in 308 , you appear to have the opposite , who knows ?
Enjoy your excellent rifle .
(By the way mm means nothing to me I still work in pounds shilling and pence.) I am to old to learn all this foreign stuff.