JS,
An SR22P should chamber a round from the magazine using the slide release.
You are using the worst ammo on the planet, ThunderBolts, which often coat the inside of a barrel with lead. The other Remington is an unknown.
By 350 rounds of ThunderBolt, you gun is rather dirty and crudded up. I would suggest this:
1) Clean the entire firearm including disassembling the magazines.
2) Lube the pistol and leave the inside of the magazine tubes dry.
3) Find some other ammo, CCI MiniMags, CCI Blazer, WW36HP, WW40RN, NOT any Federal as it is usually too weak and not any modern Remington.
4) If your gun is not leaded up, set the ThunderBolt aside and don't use it. Likewise if it is leaded up. Clean out the lead and don't use the ThunderBolt.
5) Clean and lubed, try it again with different ammo.
You might see the references to ammo used in: http://www.ar15.com/forums/t_5_4/148288_Beginner_Level_Set_Up_Of_A_New_Pistol.html
The other real question is: Does it chamber a round if you retract the slide and just let it fly from fully rear? If it does that, it should do it from slide lock. If it won't chamber a round from fully rear, clean and lubed, it time to ask Ruger CS.
The subject of the other post was a SR22P. It was only loaded from slide lock and worked every time with the ammo noted in the post. A couple thousand rounds by now.
LSA