BCG/Buffer/Spring system - Engineering perspective
I'm interested in doing a fairly serious study of the gas port-BCG/buffer-buffer spring system on M4 variants.
I am in the process of building a fixture system which clamps onto the end of the receiver extension (RE) and measures the maximum bolt retraction during an actual recoil cycle. This involves a rod which projects into the vent hole in the rear end of the RE. This rod is pushed backwards by the plastic end of the buffer as it approaches the rear wall of the RE. The rod has an adjustable drag which prevents it from continuing backwards as the buffer completes its cycle and moves forward.
I don't have pictures, but the fixture looks like a cylindrical cap on the end of the RE. It has a hole on the axis for the rod to slide in. At present I cannot use a buttstock while getting this data.
I have some qualitative preliminary results with this device, but at present I still need to modify a skeleton stock for use in these tests. The surprising thing (to me) so far is that the bolt retraction varies quite a bit from cycle to cycle with the same ammo. The extreme spread over a few shots is perhaps 0.120" Again, harder numbers will follow.
I suppose that this isn't a surpring result as a short-stroking carbine may sometimes lock back, sometimes not.
For now I'd like to present my physical thinking on the system for comment and criticism. Some high-school level physics will be necessary to understand this, and maybe a bit higher level if we get into the thermodynamics of the expanding gases in the BCG. If anyone differs with my measurements or my conclusions, I'd love to hear it. If anyone knows of quantitative studies that have been published, just provide a link and I'm on cloud 9.
First of all, some measurements. My Springco white spring has a relaxed length of 10.3" When installed it measures 6.375" This represents the state of the spring at a bolt retraction of zero.
The bolt must retract 3.05" to release the next round in the magazine. The bolt has a retraction of 3.55" when it releases the bolt catch, and it hits the RE at a retraction of 3.75"
This means that a BCG that retracts fully, until the buffer hits the RE, is moving .7" further than is necessary to release the next round. Of course it must move back a little farther than this to allow the mag spring to lift the next round for feeding. But the next round only pops up a tiny fraction when the bolt lugs release it, and the BCG is moving quite slowly at the point of maximum retraction, so I think this extra retraction will be quite small.
Anyway, this .7" extra retraction in a fully retracted bolt represents an energy reserve against increases in friction such as the presence of foreign debris, fouling, and hard extraction in general.
I estimate the Hooke's constant of my spring at 2 lbs/in. I estimate my BCG/buffer mass to be .027 slugs, and my initial buffer velocity at 210 in/sec. This gives an initial BCG/Buffer energy of 50 in.lbs.
Using the spring energy equation of E=.5*k*(delta x)^2, I get that retracting the BCG to the next round release point puts an extra 31 in.lbs into the spring (compared with the preload energy at zero retraction). Retraction to the bolt lock release point puts in 38 in.lbs, and the bolt must put 41 in.lbs into the spring for full retraction. Remember that delta x is the length change from the relaxed length.
This means that the energy reserve ( the .7" difference between round release and full retraction) is about 10 in.lbs out of an initial energy of 50 in.lbs, or 20%
This means that if the BCG/Buffer normally just kisses the back of the RE on a cycle, it can start out with almost 20% less energy than usual or lose almost 20% extra to increased friction or extraction work, and it will still feed the next round.
More on the implications of this energy budget in my next post. Again, I welcome criticism of my measurements or logic.