[RC one]

The 6.5 is nice because it does have a better ballistic coefficient than a .308 projectile of a comparable weight but the trend right now seems to be shorter barrels and I don't think you really enjoy the benefits of a better ballistic coefficient from a 16" barrel the way you would in a 20"-24" barrel.

That being the case, I'm presently working on a project that involves 130 grain-135 grain .308 projectiles from a 16" barrel in attempt to address the velocity deficit that result from the heavier 168-175 grain projectiles.

Ideally, I would like to see Hornady produce a 135 grain AMAX and/or BTHP for the .308 so you could enjoy increased velocity in a 16" barrel, better ballistic coefficients, and excellent terminal results.

[Hardastarboard]

I like loading 125 grain bullets intended for an AK into my .303 Brit brass. It gives you a much cheaper load, since the 125 grainers are highly available, and a flatter shooting / less recoil cartridge. Kind of like the .243 version of a .303. Of course, it’s definitely not a SHTF round, since the availability of .303 is almost like a European metric rifle round here.

[Jack Black]

That being the case, I'm presently working on a project that involves 130 grain-135 grain .308 projectiles from a 16" barrel in attempt to address the velocity deficit that result from the heavier 168-175 grain projectiles.

But the problem with that is that the 'velocity deficit' is a problem mostly at longer ranges, and light-for-caliber bullets like a 130 grain .308 will have relative poor ballistic co-efficient, so that by the time you get to further out it will be traveling slower (as a percent) than the 175 gr. would be, and will also (probably) have less energy than the heavier bullet does at that same distance.

In summary, the reason people like the 6.5mm bullet (in many different cartridges) is the high BC bullets, that are relatively heavy for caliber. The 140 gr. run about .61 BC. The 155 gr - 175gr.308's run about .51 - still pretty good, and the favorite of long range shooters. But the 135 gr drops all the way down to .390. Here is the Sierra information for their best 135 grain pill.

Good article on long range bullets in top calibers

Here's what a really great long range bullet looks like: the Sierra 197gr HPBT, a 7mm bullet:

BC of .780.

Here's what the 135 gr. .308 you are thinking of using looks like:
BC of .390

Notes:
pics only approximately to scale.
both bullets are Sierra Match King, their best LR pills.