Well, the optimal barrel length for a 6.5 Swedish Mauser may well be 29". Typically, these issues were all carefully considered by ordinance boards and commissions before barrel lengths were decided upon. I no longer own a 6.5 SM but as I recall they came in several barrel lengths - maybe 29 and 23/24, something like that. Where and what you shoot probably has an much to do with your choice of barrel length as the ballistics of the cartridge. Generally speaking for typical hunting situations anything from 20-26" will do. If you intend hunting in scrub or broken forest cover a 20-22" barrel would be fine. If mountain hunting 22-24" may extract a little more velocity from the round (make it shoot slightly flatter) and make for a more steady aim (due to the increased weight up front). Of course, the rifle will now be heavier to carry for extended periods and tend to hang up on vegetation a little more. The whole thing is a compromise. Only you can really know what works best for you. Most go for something in the middle - 22/23/24" A 24" barrel on a deer rifle would be considered quite long by today's standards. Generally speaking, the deer won't know the difference.