I have long been a fan of the 1886 and 71 Winchesters, acquiring my first 71 in 1966 and my first 1886 in 1967. My present 1886 is fitted with a model 71 breech bolt, hammer and coil mainspring, making it at the same time slightly stronger as well as slightly more reliable. The 1886 breechbolt has the ejector cut passing directly through the firing pin hole, reducing the primer's support, while the M71 ejector cut is placed away from the firing pin hole and thus provides better primer support & safety with high-pressure loads. My 1886 has a Shilen barrel in 45-70 and my handloads push a 535-gr Lyman 457125 to over 1700 fps from a 22" barrel, hot loads to say the least but the old rifle is still tight.
IMO your 1886 will be OK for the 348 factory loads IF the breechbolt, ejector and firing pin are in good shape with no gaps. Please check your headspace, battering can occur on the locking lugs if they're a little soft and the headspace is a little loose. The M71 locking lugs were redesigned with a taper in order to reduce this battering tendency but the original 1886 ones should be OK if their case-hardening is still hard enough to resist the battering. IMO this is a semi-marginal chambering for the 1886 and so you should make sure that your ammo is dry & free from grease or oil, and you should NOT hotrod any handloads.
Regards, Joe
