I can't think of a situation within the normal scope of elk hunting I've seen where your 6.5 with the monometal bullet would be inferior to the 300 with a lighter-for-cal bullet.
I can. When the monometal fails to open up, and it happens, especially at slower velocities.
The lightest .308� LRX is 175g so �lighter-for-cal bullet� would mean a TTSX in the 130g to 168g range if keeping the bullet design as similar as possible. According to Barnes:
110g .308 TAC-TX = .166 SD / .289 BC
127g 6.5mm LRX = .257 SD / .468 BC
130g .308 TTSX = .196 SD / .350 BC
150g .308 TTSX = .226 SD / .420 BC
165g .308 TTSX = .248 SD / .442 BC
168g .308 TTSX = .253 SD / .470 BC
175g .308 LRX = .264 SD / .508 BC
180g .308 TTSX = .271 SD / .484 BC
I�m not sure what �slower velocities� means, but my hunting compadres and I have killed quite a few animals over the last few years with Barnes TTSX and MRX bullets weighing 100g to 180g and have yet to recover one. Most animals dropped straight down, others have taken a few steps at most. Ranges have varied from about 25 yards to hair under 400. During that time I�ve learned to trust the tipped Barnes bullets.
Any bullet can fail. Some will do so more often than others. I worry more about bullets coming apart at high velocity than failing to expand at low velocity.
Tipped Barnes bullets open at relatively low speeds. The 110g .308� TAC-TX are designed for the .300AAC Blackout which start out at around 2400fps. Videos show them expanding at 1900fps, not so much at 1300fps. Other photos on Barnes� web site show a 7mm 150g TTSX that expanded after impacting at a little over 1700fps. My own informal testing with water jugs leaves me with little concern about TTSX/MRX/TAC-TX expansion, but even so I prefer impact velocities above 2000fps as a rule of thumb for pointed bullets.
Using that admittedly arbitrary and conservative 2000fps limit, a 127g 6.5mm LRX , if launched at 2750fps would have an effective range of over 500 yards. If the velocity limit is lowered to 1800fps, the range increases to just under 700 yards. In 32 years of hunting Colorado big game, my three longest shots have been ~350 yards, ~400 yards and 487 yards. The vast majority of the rest have been under 300 yards. In short, I wouldn�t worry about a 127g 6.5 LRX expanding at the ranges at which I would use them.
So about the rare event when the bullet doesn�t expand at low velocity? In that event hydraulic displacement and temporary wound cavities will be minimal and the difference between the wound channels created by a .264� or .308� will be even more so. The 127g 6.5 has a higher SD than even a 168g .308 bullet. Whether that would translate to similar penetration or not only testing would tell. In any case, both would penetrate soft tissue pretty well and if heavy bone was encountered both would probably expand.
The 110g TAC-TX bullets from my .300AAC Blackout start out a bit under 2400fps. If launching a 127g 6.5 LRX at 2750fps, it will drop to 2400fps somewhere around 250 yards. Even hunting elk, deer and antelope in open sage, most of the shots my partners and I have taken have been under that range.
In short, the difference between a 6.5 127g LRX and a �lighter-for-cal bullet� in a .300 isn't something I would worry about
