With the WM using Federal brass I topped out around 3175-3200 with the 156 in a SAAMI chamber, if using ADG 7 mag brass I'm certain 3200-3250 would be doable for a max, not sure where it would end up for max though, especially if throated long. That is with a 26". Just for reference
If you're going to do bdl bottom metal on a long action, use a longer case. The max cases feed poorly in bdl format(my 7 max does anyway), mediocre at best from a tuned cfe box. My 6.5 max is built built on a 3.4" Tikka action, using 3.42" mdt detachable mags in an xlr element magnesium chassis. Hells Canyon should have them done in a few weeks.I assumed this would need to be magazine fed. Wouldn’t work through a BDL?
I have a lightweight 6.5 SAUM with Proof barrel, defiance deviant action. I took it to Alaska a couple of years ago for a sheep hunt. Never really fed that reliably. I’m considering doing a light week build with a titanium action, carbon fiber barrel, and was going to do a 6.5 PRC. I would like to shoot the 156s and I am considering a long action so that I’m not restricted with regards to OAL.