My references, Hornady and Lyman manuals plus an article in Rifle Magazine #133 all list the 348 model 71 with a 1 in 12 twist. Its not clear from my manuals what era their test arm came. Rifle Mag 133 ( Jan-Feb 1991) under the Spotting Scope column, Dave Scovill mentions using a Browning reproduction 71 that has a 1 in 12 pitch. He goes on to say that at first he was concerned whether that twist would stablize bullets of 165 and 180 grains - but succeeded in getting some loads that would shoot 2 inches at 100ys with several powders. Foactory loads ranged from 150 to 250 grains so the twist rate seems to work over a wide range of bullets.
Remember the 71 is no long range target gun and twist likely not as critical for "normal ranges" as say the 6mm and 7mm calibers that can need tight twists for large bullets.
I think your only worry on twist rate might come if you want to try 280 to 300+ grain bullets. Seems to me the problem in other bores come when heavy (long) bullets are used, they can't be driven as fast and then the common twist rate is too slow (long). For instance in .223/.224 cal, twists of 1/10 to 1/12 or maybe even 1/14 are common and work fine for bullets of 40 to 65 or 70 grains in typical cases. But going to 70-80+ gr bullets, especially extra long boat tail types run into stability problems unless twist rates of 1/7 to 1/9 are used, or unless an extra large case is used to get velocity cranked way up.
I've experimented with 180 to 250 gr 348 bullets - my major problems are: maintaining consistent sight picture with factory sights at over 50 yds, flinching, rough trigger, hold consistency and sensitivity of the 2 piece stock, barrel heating. Probably in that order. My accuracy ranges from 1.5 inches to 6 inches at 75 yds. Need to make a good square black patridge front sight for paper target work.
HTH
