The pin is drilled through the extension, into the threaded barrel shank, correct? Like a pinned muzzle device. Unless the pin shears, it will hold it in place as long as there’s a barrel nut keeping it from falling out.
I’m from Georgia, not Pa.
I’m not trashing your wares. Someone asked above what were the opinions on the barrels. I simply gave an answer and the conversation has evolved.
ETA: I’m open to being proven wrong.
In the M4 TDP, the index pin is just pressed into the extension. It isn’t supposed to breech the barrel tennon threads, but some manufacturers do that because they thought it was a reasonable idea to ensure the extension wouldn’t come loose. The real solution to extensions not coming loose is torque. The torque value on them is really high. Colt uses pneumatic ratchet guns when they install theirs, or at least used to.
With the AR-15, the barrel extension provides more meat to fight hoop stress, which you don’t see much of a problem with in 5.56 until you run the round count so high to slightly egg-out the chamber, at which point the barrel should have been replaced anyway. Look at the gap between the barrel tennon and the extension:
In the larger diameter cartridges, you now have a thinner chamber wall that needs all the support it can get from the extension.
We did a group buy several years ago with Faxon if I recall, and several of the chambers showed a perfect circular shadow at 12 o’clock that was allowing brass to expand into it, and was clearly visible on spent cases. It was right under the index pin location.
Faxon took care of it, ate any barrels that people were not happy with or had this problem, and I think went to the TDP approach afterwards.
I had this conversation with Bill A. many years ago when talking about how to set the working pressure limitations to the cartridge. Everyone was thinking that bolts are the driver in that equation, but the barrel extension and chamber wall thickness, as well as the extension teeth lengths also factor in heavily.
You really find out a lot about the AR-15 when you chamber it in something else. Things that nobody would have given second thought to pop up and let you know you’ve overlooked something.
I also talked with Frank from POF about this when he introduced the Revolution at SHOT (.308 in an AR-15 frame, same barrel extension OD). It was my first question about containing the hoop stress since the chamber is even hogged out more, with thin walls. He said the extension metallurgy and even the barrel nut (tightly fit), were how he addressed hoop stress in the Revolution.
It’s one of the things that made him afraid to even do the design back in the day, because it was in his mind years before he actually did it.
But yeah, if you drill into the threads when installing indexing pins, you’re not doing yourself any favors.
