Hi as most of you know who are in the gun community a bunch of AR 70/90 kits came into the country and theirs still no barrels or receivers in production. Since I cant find anything I decided that I'm just going to make my own barrel and I've found the measurements from a guy on reddit who lives in Italy.
As I was getting the measurements a curiosity ran through my head, how do you mathematically figure out the proper diameter of the gas port hole for the gas block in the barrel?
Much help will be appreciated!
So my mind wandered off the other day and I started thinking about bolt carriers, recoil springs and caliber conversions. I feel kind of ignorant for not getting this completely straight, but I'm wondering if I'm missing something.
Let's say you'd have an AR-style rifle chambered in .308, and you'd convert it to .223 with a swap of the bolt head and the upper receiver. Let's ignore the magazine issue for this discussion. I'd imagine that the optimal bolt carrier velocity is the same regardless of cartridge (within some reasonable limit). Thus it should be perfectly possible to compensate for the new cartridge only by changing the gas port location or size, and leaving the same bolt carrier mass, the same bolt head mass, the same recoil spring and buffer in there.
For some reason I've always had it in my head that a larger cartridge requires a heavier bolt carrier, but I just realized that that's not right. A larger cartridge requires more space on the bolt face, more space in the receiver, and a sturdier lockup. This tends to lead to a heavier bolt carrier group, but there is no need for a heavier bolt carrier per se. Is my understanding correct? Of course there is less volume to work with when running a gas system on a .223 versus having a larger cartridge, but it should be perfectly possible to fiddle with the gas port size and location to compensate. I could also imagine the larger surface area of the larger cartridges to increase friction during primary extraction, but the difference between different calibers should be negligible compared to the difference between dirty ammo and slightly oily ammo.
The Saiga rifles use the same bolt carrier and virtually the same bolt for all of the difference cartridges.
The Knights Armament SR-25 uses the same springs and buffer as the M16a2 (although they have a heavier carrier and had some issues)
The DPMS genII small frame .308 rifles use the same buffers and springs as the 5.56 rifles.
Bonus: Check out this thread from arfcom on bolt carrier velocity
Liquid fluorine has great potential as a rocket fuel; per the Encyclopedia Astronautica, LF2/LH2 has a specific impulse of 470 seconds. http://www.astronautix.com/l/lf2lh2.html Lithium/LF2/LH2 can get you over 500 seconds, but requires you to have molten lithium at over 450K stored near cryogenic liquid hydrogen.
Krypton difluoride (KrF2) is a compound with some interesting properties, aside from being a noble gas compound. Annoyingly, it breaks apart at temperatures above about 195K. More importantly for the purpose of using it as a rocket propellant, it is an incredibly strong oxidizing agent. In fact, it is a more powerful oxidizer than fluorine gas, a consequence of the extremely dissociation energy (delta Hf) of the krypton-fluorine bond (54 kJ/mol, vs 157 kJ/mol for F2, via https://labs.chem.ucsb.edu/zakarian/armen/11---bonddissociationenergy.pdf )
Hydrogen fluoride (HF), the product of burning hydrogen with fluoride compounds, has a bond dissociation energy of 568 kJ/mol. Modelling the combustion of KrF2 and H2, we get the following equation;
KrF2 + H2 = Kr + 2HF
-(2*55 + 436) = -2*569 + E
for a net energy production of 592 kJ/mol. Compared to 545 kJ/mol for hydrogen/fluorine combustion, this is slightly better. Also, it has the advantage that your fuel is at a higher temperature (195K for KrF2 vs. 85K for LF2), and KrF2 is more dense than liquid fluorine. Unfortunately, KrF2 is a solid with no known liquid phase, and as mentioned, is unstable above 195K.
A better option might be xenon fluorides. The xenon-fluorine bond has a bond energy of a mere 13 kJ/mol, and xenon compounds are much more understood than krypton compounds. Likely the best option is Xenon Hexafluoride (XeF6). Xenon hexafluoride melts at 322K, and boils at 348K. A fairly narrow temperature range, and unfortunately one that would require heating, but likely not insurmountable. (Sadly, I cannot find information on the liquid density of XeF6).
XeF6 would react with H2 according to the following formula.
XeF6 + 3H2 = Xe + 6HF
-(6*13+3*436) = -6*569 + E
for a net energy production of 2028 kJ/mol
Granted, this combination would almost certainly have lower specific impulse than LH2/LF2 due to the large size of the xenon atom (average mass of exhaust products is 35.8 vs. 19, so exhaust velocity would be roughly 37% less). Assume roughly 300 isp assuming flame temp is equal to LH2/LF2, possibly more if the flame temp for xenon hexafluoride is higher. However, xenon hexafluoride fuel would give a higher thrust, in addition to being more dense and making your first stage smaller. Although at that point, why not use kerolox and ditch liquid hydrogen altogether. Still, an interesting theoretical exercise, and I'd be keen to see data on xenon hexafluoride flame temps if anyone has it.