My M4, after firing 10 rounds, is *way* dirtier than my Mosin after the same amount. All that gas that pushes the bolt back is filled with carbon and other filth. It makes for a weapon that *has* to be kept 110% cleaner than a old-school bolt action, or dare I say, even a Garand,
There is a significant operating difference between your good ole Mosin and a semi auto: cycle time. When you reach up and touch that bolt handle, the barrel and breech pressure are zero.
However in a gas operated semi auto, the weapon is designed to open up while there is "residual breech pressure". This pressure point is carefully timed and measured. The unlock is at a pressure that has to be less than the rupture strength of the cartridge, according to Col Chin's book, about 650 psia. According to AMCP 706-260, a 7.62 Nato bullet has left a 20" barrel at 1.5 milliseconds, and estimating from the data, unlock would be between 3.3 and 4.3 milliseconds. Here the pressure curve is between 1000 psia and zero. By the time the cartridge has extracted the pressure has dropped very low, but it is not zero.
This is done to increase the amount of time there is useable pressure to keep the action parts moving. Of course this residual breech pressure is hard on the brass, and it has the unfortunate effect of gas venting from the breech.
The M4 carbine, as mentioned, has a gas system that I am not in favor of, as it vents high pressure propellant gas directly into the breech mechanism. And then, as the carrier key disconnects from the gas port, well the chamber and the tube still have that residual pressure, and so even more powder residue is blown around.
Some of the powder residue will be blown away in an open top design, but with a closed top breech, most of the residue stays inside.
The roller bolt actions open up very early in the pressure curve, so much so that the chambers are fluted to keep the brass from sticking to the walls. If they did not do this, the breech friction would be such that cartridges would be torn in half and rims ripped off. Still, the G3 vents a lot of gas into the breech. I have found my PTR91, a copy of the G3/HK91, to be a filthy pig in the breech trunion. To prevent malfunctions, it is very important on this weapon platform to clean the residue out of the chamber.
With the gas port out on the barrel, the M1 and M14 are far cleaner than an AR. However, If you shoot enough rounds in the M14, M1 Garand, you will get plenty of residue to wipe out, but the nature of the direct gas impingement mechanism means that round for round, it is a dirtier mechanism.
And your good old Mosin, it is far cleaner than them all.
Does have the disadvantage of a shorter cycle time and all.
Borris: Thank you for your service to our Country. Take care of yourself and please come back with all your original equipment body parts.
