I haven't heard about 169's
The 168 Sierra Matchking (SMK) was originally designed for 300 m competitions. It has a very steep boattail angle that causes dynamic instability past 700-800 yd. This can be overcome to some extent by pushing them hard and/or by using an excessively fast twist rate for its length (i.e. a 10-twist or faster). However, the simplest alternative for shooting to 1000 yd is to use a different bullet.
The 169 gr SMK is a recent addition to the Sierra line that has rectified the excessively steep boattail angle issue. By all accounts, it is an excellent design and should work for your intended purpose.
This bullet was designed with one thing in mind, and that was to shoot 1000 yards in a 308 Winchester. To accomplish that, we added length to the boattail and engineered a forgiving tangent ogive with a closed nose. Doing this gave it a .527 BC which keeps it super sonic past 1000 yards in a 308...
www.sierrabullets.com
Another good choice in the same weight range would be Berger's 168 Hybrid.
Berger Hybrid Target Bullets utilize their industry-leading hybrid ogive, which blends tangent and secant designs to optimize efficiency, reduce wind-drift, and minimize sensitivity to seating depth. The Hybrid Target Bullets are competition proven and a favorite amongst today’s top long range...
bergerbullets.com
Up front, I don't know what your background is, so don't take offense if this response seems overly simplistic for your experience level. Depending on how your RPR is throated, it is likely you could use a variety of bullets in the 155 to 175 gr weight range. For example #2156 155 gr SMKs, Berger 155 Hybrid, Berger 155.5 Fullbore, 175 SMK, 175 TMK, and the Berger 175 OTM would all likely work for your intended purpose...the list goes on and on. It's also possible you could go with 185s. However, I would buy any of those until/unless you know with certainty that the rifle is throated sufficiently long for bullets in the 185 gr weight range. Using bullets that long may also interfere with loading to mag length if you intend to feed these rounds from a magazine.
With a 20" barrel, you may encounter issues pushing heavier bullets fast enough to reliably stay supersonic at 1000 yd (i.e. >/= 185 gr). High pressure may become an issue even with bullets in the 168 gr weight range. Using the 169 SMK as an example, IF you can push them to around 2600 fps in your setup, it should be enough. Below is an output for the 169 SMK from JBM Ballistics (
https://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi). You can do a similar prediction with most any bullet you might want to try and determine approximately how fast the muzzle velocity would need to be for the bullet to remain supersonic past 1000 yd. At that point, it is simply a matter of making a reasonable estimate of whether the necessary muzzle velocity estimate with a given bullet is
realistic, and if so, think about which powder might get you there.
There are a number of powders in the area around the burn rate of Varget that would likely work just fine, especially with 168s or 155s. H4895, IMR 4895, Varget, N140, RL15, IMR4064, and a few others would all likely get you there. Availability can be problematic with any of those powders, so what's currently available on the shelf/website should be a consideration. Temperature stability would also potentially be an important consideration. I mention this because you didn't specify the brand of 4064 you wanted to try. In my hands, Accurate 4064 is quite temperature-sensitive and I would avoid using it unless you have no other choice. IMR 4064 should not suffer from temperature-sensitivity to nearly the same degree. Likewise, many of the older double-base powders can provide outstanding velocity, but that often comes at the cost of fairly high temperature-sensitivity. A few of the newer double-base powders seem remarkably insensitive to temperature, likely due to new and improved coatings. I would think it might be possible to push a 169 SMK or Berger 168 Hybrid loaded to mag length to 2600 fps from a 20" barrel, but you might need to run the pressure up a ways to accomplish it. So I will only say start low and work up carefully. If you find the pressure necessary to push 168s to one of the 155 gr bullet selections. The shorter length may allow for reaching the necessary velocity (2700-ish fps) to keep the 155s supersonic to 1000 yd at slightly lower pressure. The bottom line is that I think it's readily achievable, but I don;t like telling people they may need to run [relatively] high pressure loads on an internet shooting forum. So you will need to find out for yourself what you can realistically achieve with one of these bullets using IMR 4064 as a powder. Determining the minimum necessary velocity required for the bullet to remain supersonic out to 1000 yd is a place to start.
