Re: AR-10 (Style Rifle) Slam Fire Lesson learned..
I know what you meant...I was just ribbing you a little for the description.
Didn't mean any ill-will toward you/your post and I am glad that neither you (most importantly) or the rifle were injured.
With that said, the Armalite pattern rifles are not immune from slam fires even with the firing pin spring although it does in fact reduce the likelihood. The Armalite design actually still allows the pin to travel forward and contact the primer, but it minimizes the amount of travel, thus reducing or minimizing the impact on the primer. If you get a soft primer, or if you chamber a round that has been chambered (but not fired) and ejected several times, the likelihood of a slam fire, even on an Armalite, increases. In any event, while the firing pin spring has its disadvantages, it is IMHO a nice "safety" upgrade versus the more common "free-floating" pins found in many AR-platform rifles/carbines.
The other part of the lesson on slamfires with ARs in addition to your info above is: 1) maintain a clean, well-lubed weapon, including insuring there is no crud or debris causing the pin to protrude any which ups the chances of a slamfire; and 2) be wary of the primer being used with the rounds you are using, especially if reloading (i.e. - federal are softer than say CCI, etc., etc.).
ETA: A member here PM'd me re: my above comment on Fed primers being softer than CCI primers. I should have probably qualified that statement a bit better than I did instead of making the blanket assertion above. In any event, that is a premise that I have operated under for some time. The following is a link to a compilation of a couple different sources about primers that you may find useful:
http://www.sksboards.com/smf/index.php?topic=56422.0 There are numerous resources concerning primers being "soft" or otherwise having "thinner" cups than others. Just be advised, especially if you are reloading, that not ALL primers were created equally.