Jeriah wrote:The upper I am ordering has a mid-length gas system, and an auto BCG. Common Internet wisdom seems to be that a middy is fine with a carbine buffer, and it's carbines that need H buffers, BUT that the auto BCG changes things somehow. That's the part I don't understand.
The internet either doesn't understand things or what they understand is based off using cheap parts that aren't in spec. Either is very, very likely. A 14.5" barrel with carbine gas, properly executed, should cycle optimally with a CAR buffer. A 16" barrel with midlength gas, properly executed, should cycle optimally with a CAR buffer. In fact, I do believe a 20" barrel with rifle gas but a carbine buffer tube and spring should also cycle optimally on a CAR buffer. The reason for this is that the carbine, midlength, and rifle gas systems are all designed to dump the same amount of energy into the BCG when matched to the design barrel length. Note that the time domain of the energy dump is different, but the total energy transfer is supposed to be the same. The buffer in combination with the spring is designed to soak up a specific amount of energy over the length of the tube and then use that stored energy to return the gun to battery. The design basis has the spring bringing the buffer to 0 velocity just slightly past the end of the receiver tube. In other words, the system is designed to thump the buffer into the end of the tube a little bit to ensure that, on a light cartridge, it actually makes it all the way to the end of the tube.
This breaks down when you get the gas system out of spec. For instance, if you take a midlength gassed barrel and cut it down a bit, things get wacky (shorter barrel in front of gas port reduces energy transfer to BCG). If you put a suppressor on the gun, things get wacky (suppressor forces more gas back through the gas system, dumping extra energy into the BCG). If you make the gas port too large, things get wacky (large port=more gas=more energy). When things get wacky, we have to take remedial steps to make the gun run properly again. If there's too little energy going to the BCG, the spring+buffer will arrest the rearwards motion of the BCG before the BCG has traveled enough to extract the spent casing and allow the new cartridge to rise out of the mag, or it may have insufficient return energy to push the new cartridge fully into the chamber and lock into battery. The general advice in this case is "don't shoot shit ammo" but really, you should be trying to figure out why there's either not enough energy or too much friction. If the gun has the other problem, too much energy, the action can cycle so quickly a whole host of problems can go wrong. However, we have a set of "band aids" in the form of heavier weight buffers readily available. The heavier buffers require more energy to get them moving at a given speed. This slows down the action, giving more time for each step of the cycle to happen, making those steps more likely to complete successfully. There's some big potential reliability gains to be made from slowing the system down, especially in terms of extraction, since longer time before unlocking means more heat transfer out of the case.
So, you need a light enough buffer to cycle the action the full distance but a heavy enough buffer to allow time for things to happen.
OK, that's starting to make sense. I think I'm going to leave the H buffer off my order, and if the action seems to slam too hard (moving too fast) and/or I'm getting failures to feed or something, I will immediately order an H buffer. (I will bring my old upper to the range as a spare, of course, so I'm not out a day of shooting when I find this out.)
Since you already have a CAR, I'd buy the H. Your new rifle will probably
cycle just fine on an H so long as you don't use 40gr ammo loaded with powder made from the bones of the oppressed underclass. If, by chance, it doesn't, pop the CAR from your existing rifle into your new one, have fun with your range day, and then order a CAR when you get home.