Comments by "buddermonger2000" (@buddermonger2000) on "SIG M5 Spear Deep Dive: Is This a Good US Army Rifle?" video.
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@Jonathan Pfeffer I don't think it would've been. In real terms, the bi-metal case is just as new and untested. And frankly, while the XM5 is a mostly okay rifle, like I said, the requirements came at the cost of everything else.
Remember, this rifle is meant to be the replacement for the M4. So, how does it fare in this task?
First off: despite the fact that it is I believe 20" in total length, it only has 20 rounds of ammo per magazine and also is a heavier rifle with more recoil meaning that it is harder to maneuver with and has reduced situational awareness. This makes it very poor for the close range engagement area, and in fact, it's the reason why battle rifles were seen as poor for those ranges in the first place.
Secondly: while it does have a 3000fps velocity at the muzzle as well as a bullet with a superior ballistic coefficient, which makes it ideally quite good for longer engagement ranges, it also still has more recoil which means it is more difficult to get repeat shots on target and more difficult to be accurate. It would also greatly benefit from a longer barrel to get a complete burn, which it does not actually have.
For the purposes of replacing the M4, the goal was to gain capability while not losing capabilities or only losing them marginally. What has happened here is that the close range capability has been reduced to the point of almost being lost, and it is a sub-optimal setup for long range.
Now, to address the point of retraining 1.3 million troops: you don't have to. They firstly only are giving this to the close combat force initially. This means that they're only training 250 thousand troops with the other troops being trained presumably going to be recruited in the new cadre which comes up and not the current standing forces. While an expense, the manual of arms is actually the LEAST important part of training. The most difficult part in training is making sure that soldiers can learn to fire their weapons accurately, and bullpups don't significantly affect this. In fact, most people can adjust fairly quickly to the new manual of arms of a bullpup.
Finally, the polymer ammunition had far more economic benefits in that being a lighter case brings down logistics costs, which makes it a far better argument for long-term expense.
This is not to mention the battlefield utility of having lighter ammo you can keep more on your person and the utility in a case which keeps heat from both your fingers and your barrel which means that overheating becomes less of an issue from firing more.
Also important is that polymers tend to be easier to manufacture than metals with the case technology also better able to be retrofitted should the desire come once the factories are built.
Another thing to consider is future-proofing. A bi-metal cartridge is by no means future proofing as it is never tried because no-one is dumb enough to do so. No-one in their right mind designs a case with such insane pressures because they don't want to blow up their guns and waste money. A polymer cartridge that likely lends itself to production as well as shaving logistics costs and better exploring ammunition in that route as materials science improves IS future proof.
They went conservative, and in most cases that's a good thing. Here? It was a bit of a misstep.
There would've genuinely been a superior rifle if they were to try to take a hot loaded 6.8x51, put that into an AR-10 (with its buffer tube which helps absorb recoil), given it an 18" barrel (5" really isn't going to be a huge issue), and added some picatinny rails. That would've been a better replacement for the M4 in a way which lent itself to easy transition as you suggest the Sig bid was meant to be, than the actual XM5.
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@Jonathan Pfeffer Let me clarify some points:
XM5 with optic is a 14 lb rifle. That is heavy by virtually all standards for a rifle. An AR-10 from genuinely the initial production runs may start to hit the 10 lb mark and end up in the same weight class, but I do not think one with modern manufacturing would unless I simply forget the way the AR-10 is in which case I will accept the criticism.
The XM5 is based on the Sig MCX series which is a short stroke gas piston action. It is why the stock can actually fold over. The buffer tube would not let that happen, and in fact, I believe it is mentioned in this very video.
The polymer cased cartridges would not achieve the necessary velocities out of a carbine length barrel that is correct. However, the True Velocity offering was in a bullpup, giving it a full 20" barrel to work with to achieve complete powder burn and achieve the necessary velocities within the overall length requirements and thus not being an issue.
Polymer ammunition also provably absorbs more heat and helps reduce heating of the barrel. In fact if you watch the Task and Purpose video on the True Velocity bid, you see exactly this phenomenon taking place as the host Chris Cappy touches the barrel, the breech, and the case and noting that it is cool to the touch.
Finally, the point about situational awareness was in the section about close range combat for a very good reason. Full power cartridges out of rifles on full auto tend to lower situational awareness to a great degree due to excessive recoil. InRangeTV noted this with their video about G3 vs FAL full auto and is also noted by Ian in videos on the Soviet interwar full auto rifle (can't remember the name) as well as the video on the full auto M14. In fact, I believe the only one which didn't end up in that category was the full auto FG-42 due to the recoil being forced into the buttstock and having a buffer tube along with the compensator.
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@Jonathan Pfeffer On the point about full auto, yeah it's probably going to be less of an issue when outside but you're right full auto fire in those situations is that of the machine gun, except when they have to get close. In terms of the suppressors ability to reduce concussion, you're normally right but with 80k PSI I do wonder if you're running into a situation much like suppressing 50 BMG where yes it's lower than before but by no means low. Though that is 100% open speculation on my part so you are correct on that point.
To return once again to the question of polymer ammo, you're comparing the current XM5 to the True Velocity bid which are two totally different weapons. The True Velocity bid, having a long barrel and not necessitating the insane pressures to brute force the velocity requirement, did not have an especially heavy barrel capable of absorbing more heat. Not only that, but heat does not act like electricity. While electricity will take the path of least resistance with regards to heat, just because you heat an insulator doesn't mean the heat just doesn't go anywhere or doesn't transfer, it simply means that the insulator does not conduct the heat outside of it easily so it does not transfer it to yet another object with much ease. This means that you can heat the objects quite hot on average, but it won't transfer out and burn something else as easily.
To put it another way, because heat isn't like electricity, heat will simply go to the nearest object. And the factors which determine what makes a conductor vs an insulator for heat are not any innate ability to transfer heat, but how easily heat is retained and how difficult it is to raise the temperate per given thermal unit.
Metals, with their low specific heats, are simply worse at carrying out thermal energy as they cannot retain heat as easily nor can they absorb as much heat per unit. For a barrel, for the properties of cooling, you would want it to be metal since it would release heat to the environment instead of keeping it in the barrel. However, with regards to lowering temperature, which is transferred to the barrel via case ejection, polymer is superior.
In fact, much of these properties are used when using coolants. They're typically high specific heat, insulating liquids, so that the heat can be transferred to them and either provide a sink for the heat, or if need be, able to be quickly replaced with a new batch with which to continue cooling the system. It is of note the utility of liquid is the ability to take the shape of its container in this.
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