With 296 a magnum primer is needed and a good crimp with this double base powder. The deterrent coatings make ball powders harder to ignite and with the wrong primer and a light crimp you can end up with a squib load and a bullet stuck in the barrel.

Once I some how did not give one of my .357 magnum cases a adequate crimp and I ended up with a bullet stuck just past the forcing cone. Bottom line a magnum primer and a good crimp makes sure you have a high start pressure and good ignition. And if you do not do this not all the powder will be ignited and you end up with a squib load.

Below is from the Winchester reloading data booklet.

*Note: 296 powder is considered to be one of the best powders for use in magnum handgun cartridges. Recommended for these loads are the use of a Winchester or Winchester magnum primer and a very heavy crimp (high bullet pull).

Failure to follow this procedure could result in poor ignition and/or squib loads under extreme circumstances, particularly in loads
where less than 90% of the available powder space is being used (low loading density).

Do not reduce powder charges with 296 powder. Any further reduction in powder charge or change in components can cause dangerous pressures.