Standard disclaimer: I’m not a scientist and this was not a scientific test. Any conjecture on my part is purely an uneducated guess.
As I’ve written before, POF-USA provided me with two of their upper receivers – one is of their standard P-415 design and the other is actually operated via a standard gas tube. It’s called the RDIK.
This gave me the opportunity to compare how each handled heat. That is, just how effective are all the design changes POF has made to the AR receiver, barrel nut, and handguard? Well, as I found out, they’re quite effective. However, that test was pretty limited – only 30 rounds per weapon – and I wanted to step it up a little.
Today I put 80 rounds through each of three ARs – the P-415, the POF RDIK, and an M4 type AR with double heat shield handguards – and will shoot more in the next few days with other weapons. I also took chamber and bolt face temperature readings, in addition to the handguard temp (average of 4 places on the handguards) and gas block/barrel temp.
The rounds were fired as quickly as possible, and the rifles were left with the bolt carrier group in the forward and locked position. Temperature readings were taken immediately after firing and at two minute intervals thereafter, out to 12 minutes post fire.
We’ll start with handguard temperature.
As you can see, the M4’s double heat shield handguards were much hotter than either POF offering. The POF RDIK, in fact, had a slightly cooler handguard than the POF P-415.
This was in part due to the very hot gas block of the P-415. Here are those temperatures:
It wasn’t quite as scorching as the M4’s 353 degrees immediately after shooting, but it was over 320. The POF RDIK was drastically cooler – it never exceeded 200 degrees.
Chamber temperatures were much closer for all weapons.
The P-415 did stay cooler than the RDIK, with a difference of roughly 10 degrees. The M4 was hotter than either of the POF weapons, due in no small part to the heat sink barrel nut used on the POF rifles.
The following graph shows bolt face temperature.
It would appear that a large portion of the heat reaching the standard AR-15’s bolt comes from the front – that is, the chamber. If we compare chamber and bolt temperatures, the RDIK and M4 hardly ever had more than a 2 degree difference between the chamber and the bolt (with the bolt normally being 1-2 degrees cooler than the chamber). The P-415 bolt, on the other hand, generally stayed about 10 degrees cooler than the chamber.
What does this all mean? Well, to me, it means that getting the heat out (circulating air) is more important than trying to keep the handguards cooler (insulating the barrel with double heat shields) – regardless of the operating system you choose. It would appear that the piston/op-rod P-415 does slightly reduce bolt face temperature – but the RDIK does a very fine job of keeping the chamber area cool in its own right, which in turn keeps the bolt cooler.
It seems that there is no free lunch, and the heat which is not present in the P-415 chamber and bolt is very present at the gas block. The heat sink features and wide open handguard with lots of cooling slots almost seem necessary to keep the barrel/gas block temperature relatively in line with that of the M4 type AR. I would really like to test an op-rod conversion that does not have the heat sink barrel nut, big handguard, etc.
I would assume, based on these graphs and the comparison of the three uppers, that the large majority of the temperature of an AR-15 bolt during sustained fire can be attributed to the “fire in the barrel”, and a minority comes from the gas which circulates through the action. In other words, with the piston/op-rod system, the chamber “heats” the bolt, whereas in the standard operating system, the bolt is heated not only by the chamber but in a small way by the gas coming through the gas key, which in turn causes the bolt to pass some heat back to the chamber. As a result, the temperature of the bolt and chamber on a standard AR are married to one another to a greater degree (ha, ha) than on the P-415.
Again, I’m not a scientist. If anyone has a better conclusion based on the above data, I’m all ears.