panabax

Here is a picture of a pair of completed pistons with the gas cuts made in the appropriate locations. Time to heat treat and then the fun part, testing. Panabax

Lest some of you think I am all talk and no game, attached below are pictures of the latest prototypes of a US manufactured M4 gas piston. This is a lot of 20 CNC turned pistons. I will need to make the radial cuts in the rings manually and do the heat traeting. Hopefully this will be done in the next two weeks so I can test them out, which I will do thoroughly (that's really the fun part, right?). I was very pleased with how these turned out. I will keep you posted. Panabax

I don't have any experience with the B&T, but I am confident that it is a very high quality piece. I suspect it probably has the edge on the KZ, but the price is a lot to swallow, especially since it does not help with the potential 922r issue. That said, the KZ is very well made and sturdy. I was pleasently surprised by its construction quality. However, all of these rail forearms suffer from excessive girth. Someone described the Surefire as similar to holding a pringles can. To be sure, the KZ was the same. Ultimately, I milled about 70% of the material off my KZ forearm to create a sleek aluminum forearm, US manufactured, with short trial rails. It is every bit as comfortable as the stock handguards. I would be concerned that, after spending the money on the B&T, you are not going to be satisfied with it, compared to the factory hand guards. My $0.02. Do with it as you like. Good luck. PanaBax

Yes, the S7 is very tough and very hard. Max hardness for 15-5 is attained at a 900 degree temp with a Rockwell C of 44 and a yield strength of 175,000 psi. At 1150 degrees is goes to Rockwell C 28 and a yield strength of 125,000 psi. I agree that a hardness test on the factory piston would provide very useful information. Unfortunately, with all my skill and good looks, I don't have the necessary equipment. Anyone out there have a hardness tester in their garage? Panabax

I am not planning on grinding the diameters on the rings. They will be polished. I agree that the wear part must be the piston not the cylinder. Panabax

I had not considered it. I understands that the original is stainless and I wanted to duplicate the original as closely as possible. If you go through the stainless alloys generally available, 15-5 and 17-4 look like very good candidates for the original material. S7 is tool steel. S7 is used in machin tool applications for its combination of toughness and hardness. However, I think it is would end up harder than I want it to be. In addition to potential stress and fracture issues, there are also wear issues. Toughness generally goes up as hardness and yield strength go down. Between the gas cylinders on the barrel, the bolt carrier face and the piston, I want the piston to be the softess of the three such that any wear occurs only on the piston. It should have only enough yield strength to handle max loads. Any unneccessary increase in yield strength and hardness will reduce toughness. I plan to heat treat to the low end of the hardness scale and then test for deformation and wear. The 15-5 also has the advantage of very good corrosian resistance in hot caustic environments which I think is a plus for the 15-5 over the S7 given the hot gas working environment. Panabax

Properties of 15-5 PH Stainless Steel 15Cr-5Ni is a martensitic precipitation hardening stainless steel offering high strength and hardness along with excellent corrosion resistance. Generally similar to Custom 630 (17Cr-4Ni) in composition and properties, 15Cr-5Ni is chemically balanced to eliminate all but trace amounts of delta ferrite, thus providing superior transverse toughness and ductility plus a higher degree of forgeability. 15Cr-5Ni has fabrication characteristics similar to those of other precipitation hardening stainlesses, and can be age-hardened by a single-step, low temperature treatment. It has been used for applications requiring high transverse strength and toughness, such as valve parts, fittings, and fasteners, shafts, gears, engine parts, chemical process equipment, paper mill equipment, aircraft components and nuclear reactor components.

No problem. I have a degree in Mechanical Engineering and a machine shop in my garage so it is more hobby than altruism. I am going to machine another prototype by hand and use the prototypes to test various hardness/toughness combinations. The material is 15-5 stainless which is a precipitation hardening material. That means you heat to a time and temp for a specific hardness. No quenching, no tempering. It is much easier to hit targets with than with more conventional alloys. The strength and toughness characteristics of this material are very good. It is used extensively in the aerospace industry for mission critical parts. I am sure it is very much up to the fairly simple task of transmitting energy from the barrel to the bolt face. I think I will apply different heat treatments to the two pistons, one softer one harder, and see if one bends or breaks. Really, you should see some of the materials I have seen used for semi-auto rifle pistons. I have an original Rhino M-16 piston conversion and the piston is quite simply 1/8" drill rod. It actually worked quite well, although it has long since been retired to the old gun parts pile. Clearly, the only reason for these pistons would be 922r compliance and, for that reason, it is a dumb project. Still, I try to do what I can to abide by all of the laws that govern our society, even the stupid ones. I will post pictures when I get the other made and then, to the range. Now that will be the best part, trying to break them. Panabax

Fair enough on the eye-ball. If you get take your mics to it you will find other deviations that are harder to see. However, the diameters of the rings is right on and it fits perfectly. I will cut the radial grooves, heat treat it and see if I can break it. If the prototype works, I am already modeling a cad version for a small production run to get a more precise product. The 3d model is below. I will kepp you posted. Panabax

You're kidding, right? You should see me with my micrometers in hand trying to measure the distances on that damn thing and then trying to recreate it on my miniture lathe. The groove between the two rings at the end is only .055" There is not much room in there for a tool. In other words, any variation in the dimensions is purely non-intentional. However, I suspect the most critical dimensions will be diameter of gas rings and length of piston. Just a guess. I was just glad that it is clearly recognizable as an M4 piston. I will keep you posted as the project progresses. Panabax

Just thought you might want to see the gas piston coming into being. This picture is the work in progress prototype next to the factory original. I still need to make the radial cuts in the gas rings and then heat treat. Then, I need to make one more just like this one. The shame is that these would be super easy to turn on a CNC lathe and pretty cheap too, but the radial cuts add a lot of complexity to the CNC manufacturing. Perhaps if these work well, I will order a lot of pistons without the radial cuts (cheap to manufacture) and then do the radial cuts by hand. Might not be too bad. The lathe work on a manual lathe is a real time hog. I have many hours in this one piston. Panabax

Probably you are using 17-4 PH. The PH stand for precipitation hardening. It refers to the way the metal is heat treated. It is not heated, quenched and heated again to temper. It is simply heated to a particular target temperature and then allowed to air cool. The 17-4 is very similar to the 15-5 I decided to use. I will also do my best to break them. It should be fun. Panabax

I shot both target loads and 3" 00 buckshot without problems. If you look at the cuts in the gas rings you will see that the cuts are made with an ordinary end mill while the piston is turning. It is the rotation of the piston that creates the apparent complexity in the cuts. If you look down the angle of the cut you will notice that all the cuts are aligned on that angle. I don't believe the angle of the cuts is critical to function, while the offset from ring to ring may be. I will recreate the offset but probably not the angle of the cut. If I was programing it on CNC equipment, the angled cut would be the most efficient way to manufacture as you can make one cut on each ring with a single pass. Panabax

I will let you know how the project progresses. I have ordered a small amount of Stainless 15-5 to turn the pistons out of. I will need to figure out what hardness to shoot for. I went with the 15-5 because it has good tensile strength and toughness and is quite abit easier to heat treat that other steel allows. The PH stainless alloys only require heating to a particular temperature for a particular time and then are air cooled. There is no quench and no subsequent tempering involved. The material is used extensivley in areospace applications so I suspect it is up to the job of opening the M4 bolt. I am pretty sure I read that the factory pistons are stainless, but I have no idea how to figure out what alloy it is or what heat treatment was involved. Just curoius, from your name I assume you are in Texas, where? I live in Dallas. Panabax

I appreciate the confidence in my machining skills, but I am just a hobbyist with a mill and a lathe. Not looking for customers. The mod is very simple if you have access to a milling machine. I am happy to continue to offer my prototyping services. That said, I was considering having a batch of US made gas pistons made. They can be turned on a CNC lathe pretty cheaply, but in order to justify the cost, you would need to manufacture twenty or thirty sets to bring the cost per set down to a reasonable amount. I would consider pursuing this if there was enough interest. In the mean time, I am planning on turning a pair of gas pistons on my lathe and testing them in the next couple of weeks. It seems like the gas pistons are the next most obvious 922r part to manufacture domestically. If it were not for the three grooves cut into each piston ring, it would be a pure lathe job which would be very cost effective on CNC equipment. I wonder what would happen if the grooves were omitted. I am not sure what their intended function is. I assume to slow down the piston stroke. Panabax Panabax