Should magnum primers be used with H380 powder? I'm sure I read that somewhere but my Sierra manual does not mention this.

No, not necessarily. Many fine groups have been shot with 22-250's using standard or bench rest primers with H380.

Thats good to know. When I did some 30-30 rounds a few years back the manual I used said to use magnum primers with H380 in 30-30. They shoot pretty well so I cant complain.

To get the best ignition for ball powders, some manuals recommend the use of the hotter magnum primers. What little amount of ball powder I've used have always had a standard primer and all fired to my satisfaction.

Been loading shooting and hunting with H-380 for about 15 years now. Often it's not the fastest powder but it sure throws great. It works well in all weather I used it in. It has always given me moa or better accuracy (includes firelapping) in 375 H&H, 30-06 and 7mm-08. All of this is with WW standard large rifle primers. Never even tried mag primers so I do not know what difference they might make.

Try it and see if it makes a difference.

I've found more consistent chrono results using magnum primers for any ball powder and anything equal to or slower than IMR4831 in my rifles.

As I've mentioned in other posts, flashhole deburring cut group size in my M1A by 40% when I was using 2520 ball powder. This was in combination with Federal bench rest primers. What matters most is that however you light it up, be consistent. Deburring helps that with ball powder (though it never made any difference with stick powders that I could see). Using a ball powder whose optimal charge lets you come close to filling the case helps that, too.

A couple of years ago, the benchrest crowd was raving about and scarfing up a particularly weak lot of RWS primers. This was in addition to the fact many of them design cartridges with large rifle caseheads made with small rifle primer pockets. This should tell you a couple of things: They want the powder they measure out to have as much influence over pressure as possible, and the primer composition to have as little contribution to the pressure curve as possible because that produces the best consistency of load performance. It also tells you than with carefully prepared brass and well-filled cases, the ball powders many of them use will work just fine off small ignition. It just has to be consistent, whatever it is?