Buckfever
It is not that the 100NP will not work on Mule Deer, but you said large Mule Deer & there are better
choices.
You mentioned the fact that alot of people use 100 gr. NP in the 25-06, they do but this is apples
vs oranges, the Sectional Density is higher for a 100gr. 25 cal bullet than for a 100gr. 6.5mm. bullet.
BTW & a little off topic but the 25 does better with the heavier bullets on Big Mule Deer than the 100, the 100TSX was tried & it never failed but I liked the quick kills of the heavier bullets better at
longer ranges.
Back to the 6.5, the 100 part is a little short for caliber & the BC is not good, if you look at the
Nosler charts, the 125Gr. NP at 3,000 is FLATTER than the 100NP is at 3,100, in fact the 125NP will be flatter by a little at 2,950FPS(very doable) than the 100NP at 3,100, so there is no traj. adv. with the 100, so I know of no other advantage it could have except recoil & the 6.5X55 is mild with 140's so I don't think it would be noticed.
So for ANY size Mule Deer I would look at a 125Nos. Part. or a bullet like the 129Hor. or about any
good shaped 140 gr. This range, 125-140 is the range I would focus on.
The 125Nos. is a little flatter than the 140Nos Part.(about 5" at 400 give or take), but with a more
streamlined bullet like the 140SST or 129SST, then the traj. are getting VERY close.
So this part is not a big deal in this range.
In penetration the 125 will be a no brainer that it will be great, but a 129 gr Horn. Soft Point will penetrate just fine at 6.5 velocities, my Wife has used this bullet for years with great success.
Even a 140SST will be fine for penetration at 6.5X55 speed, I may use a little caution with the 129SST, just to be sure.
Another thing is some 6.5X55, even modern ones have a long throat to shoot the heavie's, keep this in mind & that may effect the 125.
For Elk, I would want to use a 140 Soft Point or the pre., which ever shoots best.
So, for me & for Deer at at 6.5 vel., the 125NP, 129 Horn Soft Point or any 140.