Leave the small magnum primers alone. .223 Rem uses normal SR primers. I think you'd be inviting pressure problems (accompanied by safety problems).
I shoot a couple different bullet weights from .223 with Varget powder. I use Federal GMM primers, and CCI primers. Just "Small rifle" primers. Nothing magnum. Very similar weight charges to you. I know that Varget is very "temperature tolerant" and this is important to me, since I tend to test in the cold, and compete in the hot. (Canada!) Varget has only ever shown minimal variations in pressure and velocity, such that I have never adjusted a load for climate extremes. Even the most compressed load of Varget I can cram into a 223 casing never goes overpressure in 30 plus degrees competing in the Southeast US or Ottawa in the summer, and I've never had problems in minus 10 and minus 20 winter testing and training with the same load back in Ontario...
My barrel is probably shorter than yours because I'm loading 223 for National Match AR15 (20 inches) so that (might) be a factor - but I doubt it. The gas system of the ARs function flawlessly in all temperatures with this same load. If Varget was burning any different in the cold, the gas gun would be the first to frig up. I also load a much bigger varget charge in my wife's 22-250. With her rifle, even in the cold, we've never seen unburned powder like you describe. And that 22-250 has almost 32 grains of varget burning to drive a bullet down 26 inches of barrel.
I'm thinking your problem might be with a bad lot of of primers, or something else in the load, rather than with the type of primer...
Avoid small magnum primers with 223. Just try different brand. I know that CCI shoots different that Federal with the same load. I can't say which is "hotter" - just that they make different velocities in the same load if all else is same-same.
Don't want to insult you here, and you probably know this - but trying to cover everything -- if you are messing with the primers and handling them too much, the priming compound might be degraded from oils on your skin. (That argument could go on for days) That could neuralize or diminish the flash temperature if you want to beleive the guys that say so... Maybe some crap in the casings left over from cleaning. (Or wet... horror!)
I've loaded several thousand pistol loads with Winchester primers and have had THREE misfires in the past year with punched primers, but unburned powder inside after pulling cartridges apart to see... I have loaded and fired so many rifle cartridges I can't even begin to count, but easily in the hundreds of thousands. Nothing but Federal Gold Medal Primers and perhaps 2000 CCI I tried out last year. I have NEVER loaded a cartridge that didn't fire in any of my rifles.
I think I'm suggesting that not all primers are equal in terms of quality control - and THAT is what you should be looking at.
