Heat is discussed fairly often on various internet forums, especially when two subjects come up: barrel profile and method of operation.
We often see comments about how light barrels heat up too quickly. This is partially true – a lighter barrel will generally heat up faster than a heavy barrel. The “too” part is where the problem lies. Too fast for a machine gun barrel? Most likely. When you’re putting out a sustained rate of fire that can reach several hundred rounds per minute, a light barrel is definitely unsatisfactory. However, if you have a rifle, and not a machine gun, a lighter profile barrel may not heat up “too quickly”.
Also, there are many comments about how cool piston/op-rod systems run. These comments seem to be applied liberally and generally; that is, you will often hear that all piston conversions “run way cooler” than standard DI weapons. However, it’s not as if the mere presence of the op-rod has a chilling effect on the barrel, which is a critical component of the rifle, to be sure.
So we have two schools of thought here: that a lightweight barrel profile is more appropriate for use on a carbine, and that the standard system of operation is not unnecessarily hot; and that a heavy (or fluted/heavy) barrel is more appropriate for use on a carbine (or maybe a carbine machine gun to be used for laying down suppressive fire), and that an op-rod allows the rifle to run cooler.
Recently, while doing some experiments with standard plastic handguards, I thought I’d also compare a civilian legal M4 clone, or close to it – a Spike’s Tactical M4 LE with a Knight’s Armament M4 RAS handguard – with two Patriot Ordnance Factory rifles. One is a P-415, which uses POF’s op-rod system, and the other is called the RDIK, and it uses a gas tube, just like a standard AR-15. However, it’s equipped with the same heavy fluted barrel, heat sink gas block, reinforced upper receiver, and single piece railed forend that the P-415 uses.
To complete this test, I fired 30 rounds of 5.56mm ammunition through each rifle, then measured the temperature of the handguards in four separate places, as well as the temperature of the gas block/barrel. I measured these temperatures immediately after firing, 1 minute after firing, 5 minutes after firing, and 10 minutes after firing, using an infrared thermometer. I also measured the temperature of the bolt face immediately after firing.
Here are the handguard temperatures:
Note that all three handguards got progressively hotter until sometime after the 5 minute mark. I found it interesting that the POF gas tube upper was within several degrees – up or down – of the P-415 op-rod upper during the whole exercise. I also found it interesting that every handguard had reached essentially the same temperature 10 minutes after firing.
Here are the gas block/barrel temperatures.
I was surprised to see how cool the POF RDIK gas block was after firing. There was a greater initial temperature difference between it and the P-415 gas block than there was between the P-415 and the Spike’s M4 LE. In addition, the M4 cooled faster than the P-415, as you can see, though each upper had roughly the same temperature loss profile after 1 minute.
This proves an often-overlooked point: while light barrels do heat up faster than heavy barrels, they also cool down faster than heavy barrels – apparently, faster than even a heavy fluted barrel. This also proves true a comment made to me by an industry professional while we were discussing this topic: that the gas block of a piston/op-rod rifle gets very, very hot.
As a side note, the temperature of the M4’s bolt immediately after firing was 94 degrees; the P-415’s bolt temperature was 88 degrees. The RDIK’s bolt was 89 degrees.
Although this was a very limited and rather unscientific test, it would seem that the vast majority of the POF rifles’ cooling ability comes from the heat sink barrel nut, handguard, fluting, etc, and not from the piston/op-rod system. I will do more extensive testing in the near future.