My first thought on reading this was ... Are there any 6.5 bullets that weigh under 100gn? Even 100gn is VERY light for the calibre. Most people here in the UK either use 6.5X47L as an out and out match cartridge or as a dual-purpose match and small deer species number. It's ideal for our small native roe deer and still man enough for some of the larger breeds. I'm sure these rifles are also used for 'foxing', our main varmint species, and I'd think people will mostly use bullets like the 120gn A-Max which has a reputation for being very frangible and fast expanding at decent terminal velocities. (I can email a guy I know who wins Factory Sporter class in benchrest matches and also uses the same Accuracy International rifle chambered for 6.5X47 Lapua for extensive deer and fox shooting, if you want to know. We don't have coyotes here, a large dog fox being our nearest equivalent, much larger and tougher badgers out eating other wildlife and doing damage everywhere in the British countryside, but they have 100% legal protection thanks to our breed of bunny-huggers!)
For competition use, the lightest bullet most people shoot here is Lapua's 108gn Scenar, a very long design for its weight - same length as the 123gn Scenar and new 136gn Scenar-L give or take a few thou'. The 108 is used in Tactical, McQueens (sniper / 5-second exposure match) and Civilian Service Rifle comps in shorter distance stages. It's a fantastic bullet for 300 yard stages - light recoil, very accurate, and bucks the wind better than almost any competitor. The cartridge feeds very well from box magazines when loaded with this bullet too for use in these rapid fire / snap-shooting events. A lot of these guys use 20-inch or shorter barrels with sound moderators ('moddy' use is the norm now in the UK for most deerstalking and fox shooting practice). The cartridge is very well suited to 18-20 inch barrels, so you're on the right track there. Once you go below 22 inches you start to lose rather a lot of velocity, maybe 30 fps or more per inch of barrel.
Anyway, moving on, given loads data for the slightly larger case and powder charge .260 Rem for bullets like the 100gn Nosler Ballistic Tip, there is absolutely no reason why the 6.5X47L won't perform just as well, better in fact, with this class of bullet.
There are two issues - the bullet holding together in flight when shot from a fast twist barrel if it's built as a light jacket varmint design, and loads. The second simply shouldn't be an issue. Looking at the 85gn Sierra on the company's website (a new bullet on me I must admit, but I wonder if any have made it across to us anyway?), it's described as ideal for medium velocity 6.5s such as 6.5X55mm, so Sierra obviously expect it to stand up to some pretty high rotational speeds as that cartridge uses 1-7.5" twist. (You might think why not use a slower than 1-8" twist anyway with 120gn as your top bullet weight? A lot of lighter 6.5mm bullets need 1-8" as they're very long for weight, and if you consider using Barnes for deer, you'll definitely need it.) I would likewise expect the 100gn Ballistic Tip to stand up OK to a really fast spin rate.
So with that out of the way, what about loads? Anything recommended for the 100gn Nosler BT should work well (with lower charges) in the smaller 6.5X47L. Nosler lists loads for a fair width of burning rates for its 100g BT from AA2460 and H. VarGet to numbers in the 4350 class with IMR-4064 listed as 'most accurate'. Berger lists a smaller number of powders all slower burning than I'd expect for its 100gn BT Match (now discontinued, I notice) - Norma 203B, IMR-4007SSC, Ramshot Big Game, Alliant Re17, AA-2700, Viht N550, H4350, and Win760 (same as H414). Rather a lot of barrel-burning double-base or high energy numbers there too, only 4007 and H4350 as standard single-base jobs. What these two manuals say is that the powders that people use in 120-130gn match loads apparently work well too with 100 grain jobs.
Moving on, let's see what QuickLOAD suggests. With the 85gn Sierra HP, COAL has to be short at 2.500" to have any decent amount of case-neck gripping the bullet. Depending on what chamber throat you go for, it'd likely have to make a fair old jump into the rifling which will lower pressures and velocities.
Most of the powders recommended for .260 Rem in the manuals are too slow burning for this set up and they'd likely be inefficient and unsatisfactory, even compressed loads reducing pressures and resulting in partial charge burns. 4350-class powders only see 85-90% of the charge consumed in a 20-inch barrel. Even the 4895s and VarGet don't get the 99-100% I'd prefer, running at 94-96% burns. (This is without a long bullet jump effect either that would make the problem worse.)
As expected, QuickLOAD suggests faster burners would be fine, including the following:
IMR-3031, AA-2495, H322, H335, Ramshot X-Terminator and TAC, Alliant Re10x, Viht N130, N133, N140/540
Not everything shown in QuickLOAD as suitable works as hoped, but H322, 3031 and the Viht powders look good, TAC and H335 if you prefer ball powders. fill-ratios range from low 90% to slightly compressed loads. MVs are calculated as high 3100s to mid 3200s from a 20-inch barrel, but that assumes the chamber is throated for the bullet and its short 2.500" COAL.
Looking at the 100gn Ballistic Tip, the list is as before, but some of the faster burners such as Re10X start to fall short on velocity, while slightly slower burners now become viable although they still don't achieve a full 99-100% charge burn - add in the 4895s, Re15, VarGet and IMR-4064. Predicted MVs run from just below 3,000 fps for N140 to 3,068 for IMR-3031. This assumes a slightly longer COAL than with the 85 Sierra HP, 2.600 inches providing a reasonable amount of bullet shank in the case-neck.
If any of these powders and combinations appeal, PM me and I'll tell you what sort of loads QuickLOAD suggests and the usual warnings that apply
