My advice would be to pick a different rifle and cartridge for subsonic use. Any load you come up with for a 6.5x55 with a 22.5" barrel using jacketed bullets WILL have large velocity dispersion and poor accuracy. If you must use that rifle then you would do best with heavy cast lead gas check bullets,. TrailBoss is a bulky low energy powder which allows high case fills. It's fairly fast buring and is a good choice for low velocity loads, but it's still not ideal in a case as large as the 6.5x55. which has 57 grains H20 case capacity. Generally you want very small case capacity for subsonics. The ideal barrel length for shooting jacketed bullets subsoinic is typically 8 to 12 inches, though 16 is practical with large bores. Lubricating jacketed or solid bullets helps.
The problem is to get enough pressure to engrave the bullet into the rifling reliably, but then to get rid of the pressure before the bullet accelerates to above subsonic velocity. Its a fine line between sticking the bullet in the barrel and launching it in the 950 to 1050 fps range. Higher bore friction makes that more difficult, Larger case capacity makes it more difficult, a smaller bore makes it more difficult, trying to keep the pressure low makes it more difficult. The advantage of cast lead is that it has much lower engraving forces and much less bore friction, particularly if it's well lubricated.
Rifles which can shoot heavy cast lead subsonic fairly well include most pistol calibers like 45 ACP, 40 S&W, 44 Mag and 50 AE, though they don't handle low drag bullets. If you really want an accurate subsonic rifle with decent range get a bolt action made for the purpose in 300 Whiper, 300 BLK, etc or for more downrange energy in 458 SOCOM. Just about any rifle can be used with light carges of fast pistol powder and lightweight cast bullets to over 100 yards.
