Thanks for the follow up report. If it was mine, I would be on the phone to Umarex asking for something in writing listing what was done to my firearm to insure the FA did not re-occur.
Mike, I'm confused. :o Please correct me if I am wrong, but I seem to remember an older post where you modified your firing pin, and/or spring, on this rifle and FTFs where a thing of the past. FYI, the reason for the spring on the firing pin is to inhibit forward movement of the firing pin when the bolt slams forward on a live round. The spring is utilized to overcome the forward inertia, thus preventing a "slam fire". My Colt M4 is all stock parts in the bolt and it has no problems with reliable ignition. Getting close to 8K rounds through it. :thumb:
Generally speaking, I have had the best overall experience with the Federal Bulk box. 36 grain, feeds through my Colt just fine, and about $18 - $19 at WalMart. It is plenty accurate out past 50 yards, minute of squirrel at least. But make sure to read the op manual of your firearm and use ammunition that fits the specifications listed there.