I'm totally out of small pistol primers. Can I use CCI 550 (magnum) primers in .38sp ?

Yes, just start low and work your way up.
https://www.google.com/search?...wAEB&sclient=gws-wiz

There are certain powders (HS-6 for one) where magnum primers are specified in a loading manual (Speer) for 38 Special +P loads. Some powders like HS-6 need a bit more spark to burn when it is very cold out.

If it were me I would use 158 grain bullets, HS-6 powder, and a magnum primer. If I didn't have HS-6 I would use a standard 38 Special load using a medium speed powder load specified (like Unique or Universal), a heavy bullet, and then check the velocity in a 357 Magnum revolver factoring in barrel length.

After I had a velocity that matched a standard 38 Special load I would use it in a late model 38 Special revolver that is rated for +P loads or better yet, a 357 Magnum. The difference in 357 Mag and 38 Spl +P max pressure is more than 20kpsi. Any issue would be 38 Special brass construction not the revolver itself.

Thanks guys. I tried a minimum load (125 fmj 4.8 of 231) and works fine.