Most .357 Magnum loads do not need mag primers, but some do. Your load book will give you an idea, although some just list mag primers with all loads. Speer # 13 & 14 only suggest using mag primers in .357 with HS-6, HS-7, H-110, W-296, & Lil' Gun. (hard to ignite and or very slow ball powders)
Your Remington 5 1/2 primers are mag primers, and will work just fine for your purposes.
Primer Identification by Manufacturer
CCI
• Small pistol = 500
• Small pistol magnum = 550
• Large pistol = 300
• Large pistol magnum = 350
• Small rifle = 400
• Small rifle magnum = 450
• Large rifle = 200
• Large rifle magnum = 250
Federal
• Small pistol = 100
• Small pistol magnum = 200 (My note: There are also some old Fed 200's out there that are small rifle)
• Large pistol = 150
• Large pistol magnum = 155
• Small rifle = 205
• Small rifle magnum = 250M
• Large rifle = 210
• Large rifle magnum = 215
Remington
• Small pistol = 1 1/2
• Small pistol magnum = 5 1/2
• Large pistol = 2 1/2
• Small rifle = 6 1/2
• Small rifle magnum = 7 1/2
• Large rifle = 9 1/2
• Large rifle magnum = 9 1/2M
Winchester
• Small pistol = WSP
• Small pistol magnum = WSPM
• Large pistol = WLP
• Large pistol magnum = WLP
• Small rifle = WSR
• Large rifle = WLR
• Large rifle magnum = WLRM
RWS
• Small pistol = 4031
• Small pistol magnum = 4047
• Large pistol = 5337
• Small rifle = 4033
• Large rifle = 5341
• Large rifle magnum = 5333
[BPosted by Bagheera on Ruger.net[/B]