ORIGINAL: wabi
I could never understand why inlines need 209 primer ignition. Since the path for the flame is straight, it seems like a regular old percussion cap would be plenty. I've been thinking about switching to the brass 209 sized holders that use a small rifle/pistol primer.
But for an answer to the question, I've used regular 209 primes and the new milder (Winchester T-7 & Remington) primers & can't really see any difference in accuracy or cleaning.
Not too many years ago, there were several primer nipple gadgets on the market, (Fusil V, etc.) which took the place of a standard #11 size nipple on a sidelock. These let you use a SR primer instead of a cap. These had a screw-on top cap with a floating firing pin inside that you put on after insering the primer. I tried one out on a Hawken I was using at the time, shooting patched balls over a chronograph. The rifle was shooting erratically per the chronograph readings, so I put the #11 nipple back on and fired a ten-shot string with the same charge except using a #11 cap instead of the SR primer. The results were amazing. The #11's gave velocitiy variations of only 5-9 FPS instead of 75-100 FPS! Needless to say, I ash-canned the primer nipple!
More recently, I decided to try a thing called a Mag-Spark, which in many ways resembles the SR primer device but which lets you use a No. 209 primer instead. This device works much better, giving a pretty uniform MV and good accuracy. I think that the difference might be that the Mag-Spark has a tinyhole in the bottom for the flash togo thru to get to the powder charge, just like the regular #11 nipple has. The other thing was bored the same diameter straight through from the SR primerchamberdown, with no hole at all to focus the flash.