Over the past couple of years, we have from time to time had someone ask why there were two different sized collars on the firing pins of ARs. I have usually chimed in with the answer I got from a parts dealer years ago, that the larger collar was to catch the notch on the hammer if the disconnector was removed to try for full auto fire in an AR.

In the wake of another such question a few days ago, and a couple of other posts from select fire owners that their pieces had come with the large collar, I finally did what I should have done long since. I checked it out for myself.

I took the one AR I own with a Colt cutaway carrier, puled out all of the firing pins I have (3 each of large and small collar), pulled the disconnector out, and tried them all.

THEY ALL HUNG THE BOLT/CARRIER ASSMBLY.

My apologies for having posted what was essentially misinformation about this. I should have known better than to simply take someone elses word for something that I could have checked out myself.

Thus, I'm in the same position as the other guys who have asked the question.  Given that either sized collar will hang the bolt/carrier assy, why are there two different sized collars?

1) I do not know the answer.

2) I am in the manufacturing business, and it seems to me they found a better way to make the firing pin, so they did. Early M16s came with the large FP, which to me, looks fragile. The smaller FP was probably a design change to make a better FP. They probably had thousands if not hundreds of thousands of the large FP, so they still show up on weapons from time to time, as it was not a recall item.

No different than other changes they made - reinforced mag well, reinforced rear receiver, FA, shell deflector, chrome bores, all designed to make the weapon function better.

Current M4s have even more design changes, a new selector, as well as the feed ramp, to name two. Does anyone make the argument that that M4 feed ramps are a full-auto design part?

If you let off the trigger quick enough the hammer rises higher during the return movement of the carrier than if the trigger is held and the disconnector holds the hammer. The higher hammer riding in the groove milled out on an sp1 style carrier will hang on the larger 16 firing pin collar. The collar was made smaller on the sp1 firing pin to stop it from hanging when you did not want it too. It was all done too make the sp1 parts work together correctly and produce the hang if the disconnector is removed. If you use the larger 16 pin with all the correct sp1 parts it will hang even when it should not.