Both are devices intended to ignite upon impact and to cause a primary charge to fire. A 209 primer is actually a primer, with an integrated anvil, contained within a cup. It is a complete device, which is intended to be held in place such that the primer surface is struck by a firing pin - this will cause the primer to ignite, and shoot hot flame and gas through the hole in the far side of the cup, setting off the main charge. A percussion cap is essentially a simple cup with an impact-sensitive charge held in place at the base. It does not have an integrated anvil, and is designed to be placed on a nipple in order to function. When the cap is placed on a nipple and struck by a hammer, the charge is trapped between the top of the nipple (which acts as an anvil) and the hammer, causing the charge to ignite, with the flame and gas going through the hole in the nipple, through to ignite the main charge. Typical percussion caps are not much more than 1/8" across (although there are larger 'musket' caps), and they are open on one end. A 209 primer is approximately 1/4" across, and enclosed, with a flange or rim on one end.
A 209 is a 209 regardless of what it is used for.
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Well......... As far as I know, a 209 primer isn't a paint primer at all. It is an ignition source for the powder in a bullet. There are a few different types, 209 being one of the most common. They're most commonly used in shotgun shells, and black powder rifles.
There are none produced that I know of, a 209 primer would be over kill in a revolver due to the small chamber and lower amounts of powder.
Among muzzle loaders, caplocks and flintlocks do not use 209 primers.
Federal 209A primers are hotter than Federal 209 primers and are hotter than Winchester or Remmington or CCI 209 primers. To duplicate your load when substituting Federal 209A primers in place of any other 209 primer, use 1 grain less powder for like performance.
Question can never be answered as currently written.
Depends on what caliber you are comparing it to.
Most areas will allow a loaded muzzle loader in a vehicle so long as it is unprimed, (no powder in the pan, percussion cap or 209 primer) though not a safe idea, and you should always check with the local jurisdiction to be sure.
Both would work, however a standard 209 should be plenty, and a magnum primer runs the risk of blow-back that could damage you and or the gun.
209. Double check with a gunsmith.
By 870 I assume you mean the Model 870 shotgun? If so, there would be no purpose for that type of conversion. Shotshell primers are of the 209 size. The 209 "conversion" is done with muzzle loaders. The idea is to replace the muzzle loader nipple (that holds the cap) with a conversion unit that uses the 209 shotshell primer to get more positive ignition of the black powder.