Have a 26" barrel Rem 700 to work up a long range load for, but having difficulty finding supersonic load data.
My Sierra manual only go to 200gr for 308 Win.

Only found one reference indicating Sierra provided them 35.0 to 37.0 grains of IMR 4064 for the 220gr SMK. Would like to confirm this and get other references and options as well.

Sounds about right. My node was 40.6gr IMR4064 under 190gr BTSP, max 42.5gr. I figure the difference is about 2.0gr powder per 10gr bullet, about in line with the case capacity rule of thumb. Expect about 2400fps.

Popnfresh has used 41.5gr W748 and 47.7gr W760 under 230gr Bergers. From a 20" barrel, 2300fps and 2400fps respectively.

Work up to it.

Those are at 2.840" oal for the W748 and 3.245" for the W760.

You'll find there is no data for heavier than usual loads. I haven't used published load data for years, I just gave up and rely on Quickload and previous experience with my powder in other loads.

One thing when loading heavy bullets mag length, you cannot just take a light bullet charge weight and use it with the heavy bullet thinking it will be low pressure. That heavy bullet takes up so much case volume that the pressure  could be much higher.
Case volume is a very valuable feature of Quickload that I rely on all the time for starting charges.

According to Quickload:
At 2.800" oal with a 1.515" long 220smk and 56gn case capacity.....33.0gr of IMR 4064 puts you at 87% load density (plenty to start IMO) and 39k psi.

Me, being unfamiliar with the powder, and having no info would probably start at 33-34 grains for one or two rounds just to get a feel for it......

38.0gr shows to be 100% density at 57k psi.
I generally don't trust QL pressures until I adjust the burnrates to match my measured velocities. Doesn't look like you will get much speed with that powder.

Call Sierra themselves.  Their tech people are great!

Thanks for the input and direction on working up a load for this. Figured this might be outside the box.

@popnfresh
Much appreciate you running this through Quickload. I definitely want to tread softly working this up.

I'll have to measure the maximum COL that will fit in the internal mag well.

I have some W748 as well, but it sure seems high on the burn rate chart compared to IMR 4064. I was thinking it would require going to a slower burn rate to keep chamber pressures down and rely on the longer barrel to pick up a little speed.

That's a good idea calling Sierra. I've got 500 of their 220gr SMK to work with so they should be willing to help me out on this. I'll find out what powders they recommend.

.308 is a relatively small case and magazine length eats up a bunch of it, so powders with a lower density and typically faster burn tend to be better.

Example, .308 with 190gr BTSP and H4831. Even at minimum depth, you can't squeeze enough powder in to get to ideal velocity, let alone blow it up. Shoots pretty good at short range though.

Called Sierra - Nice guy with not much data to support the combination of 220gr and 308 Win.

He gave me this load:

A2520  gave 34.2g - 38.0g
22" barrel had 2154 fps

Also mentioned IMR 4064 and use 36.5g for the starting load and work up from there.

He had some Varget data, but said IMR 4064 showed a faster speed.

He didn't have any data for W748.

https://imrpowder.com/wp-content/uploads/2017/02/burnratecolor.pdf
http://www.accuratepowder.com/wp-content/uploads/2010/12/burn_rates.pdf
http://www.lasc.us/BurnRatePrint.htm

Maybe IMR 4320? German Salazar used it in 155gr Palma loads, but it has a burn rate similar to 4064, Varget, and W748. He said it has small granules and meters well.

http://bulletin.accurateshooter.com/2012/12/imr-4320-yields-great-results/

The problem is you have a smaller amount of fuel to burn at a lower pressure over the length if the barrel. You won't get the speed.

I'll just put this idea on the shelf for the time being and get a 168gr SMK load worked up for this rifle first.