I can now answer your question well, because i have purchased the same rifle, but mine has a nice wood stock.
First, have in mind this: there are two kinds of 6.5x55 rifles. Older military ones, where pressures must be kept low for safety, and newer stronger rifles, such us the Sako, Sauer, Winchester, Ruger, etc. Because of this reason, you can only buy American made ammo which is loaded midly, for older rifles with weaker actions. As far as published reloading tables, most were developed with 28" barrels, but with low pressures in mind, for safety when fired in older rifles. Handloading tables for older rifles are named "6.5x55 Swedish Mauser". Handloading tables for stronger modern rifles are usually named "6.5x55 SKAN or SE".
Your Sako has a shorter sporting barrel and a long freebore, to accomodate long heavy bullets. Combined with loads safe for old rifles, this will give you some slow and whimpy velocities in your rifle. For example, maximum loads at 2,800 fps advertised in several handloading books, but tested in 29" tubes and kept at low pressure for safety, only gave 2,400 fps in my Sako. With 129 grain bullets, Lapua brass, and IMR 4350, i went 4 or 5 grains over maximum published loads and i got 2,800 fps, safely. And accurately. My Sako shoots 5-shot groups of Speer 120 grain and Hornady 129 grain bullets into less than 0.5" ragged holes, on cold days. On warm days shots 4 and 5 wander a bit (0.5" off the main hole). Also, the best powders for 29" tubes are not the best in your shorter barrel, they are too slow burning and you will not achieve great velocity. If you handload, you should use slightly faster powders.
To simplify, if you want full power loads, buy ammo made in Europe by RWS, Lapua, Norma, Sellier and Bellot, maybe also Prvi Partizan. If you handload, use good quality brass (Lapua) and load data for modern rifles, from the Lapua web page or elsewhere.
As far as rifling twist in your Sako, it is 1:8", to stabilize 160 grain bullets. For this reason, the freebore or throat area of your rifle is also rather long, to allow chambering rounds loaded with those long bullets. If you handload, use a COL gauge and fit your bullets 0.020" off rifling and be ready to be amazed. My Sako shoots any bullet very accurately, from 100 grain to 160 grain. So will yours.
Here is a sweet shooting load: Lapua brass, CCI BR 2 primer, 34 grains of H4895, Speer 120 grain Hotcor, COL= 2.97", 2,450 fps, almost zero recoil, noise or blast, kills deer like lightning and it has a fairly flat trajectory (less than 3" up and down from muzle to 200 meters). Grouping was practically one hole, slightly oval, at 100 meters.
Also a word of caution: because of the long throat designed to accomodate long bullets, if you shoot a lot of high velocity high pressure loads with shorter, lighter bullets, on the long run (over 1,000 shots), you will experience faster than normal throat erosion/barrel wear. This is because a lot of gass will escape around the bullet before it has the time to seal the bore. To prevent this, allow the rifle to cool between shots, keep it cool. Also, use a COL gauge to seat short bullets closer to the rifling and fire them with Vihtavuori powders. Use hot high pressure loads only if needed, say, if you expect 300 yard shots. That will significantly extend barrel life.
