I made my first hammer today.... I wanted to make these builds with a separate cocking handle, but I knew the hammer had to be longer than normal and was concerned that they would be too heavy.... I decided that the only way to find out was to make one and weigh it....
I started out with a 5/8" diameter steel core 2" long that was drilled in 1-1/4" deep with a 7/16" drill.... It had to clear the stop rod and allow for 1" of travel, which I hope to achieve.... I knurled the outside and took a piece of 1-1/8" diameter MDS from Rocker1 (thanks David) and drilled it out 5/8" and pressed it on so that there was a socket for the flange of the spring guide to fit into (it is less than 5/8" OD).... I turned the back end to length, leaving just over 1/8" of the MDS protruding past the steel.... The Delrin spring guide will push directly on the steel core, while the MDS outer will prevent the sear from popping up behind the hammer.... This is the rear view of the hammer....

The MDS was turned to 1.2" long from the back to the lower corner where the sear engages.... If that proves to be too long to cock, then I will either move the sear location on the hammer back, or machine a bit off the front of the SSG mount, there is a bit available there in front of the screw locations.... The front of the hammer is just the steel core, and the overall length is 1.70", which is about 1/2" longer than an MRod hammer.... This leaves enough material between the front of the hole and the front of the hammer for a cocking handle to be installed.... The handle was made from a piece of 7/32" drill rod that is 1.7" long.... Both ends are turned down to 3/16" for 0.40" length, and threaded 10-32, leaving a shoulder 0.40" from each end.... The hammer is drilled 1.50" from the back, through and tapped to 10-32, and counterbored 7/32" to a depth of 1/8" so that the shouldered rod bottoms below flush.... The knob on the handle was made from a piece of 1/2" OD black Delrin rod 1" long.... It was drilled 5/32" to 0.8" deep and counterbored 7/32" for a depth of 0.30".... and then the lower portion tapped 10-32.... This allows it to thread into the Delrin handle, which gets its strength from the steel rod, but keeps the weigh down.... I usually use a piece of aluminum for the knob, and 1/4" steel for the shaft, but wanted a lighter approach.... Here is the front view of the hammer and cocking handle....

The last step was to mill two 3/8" long slots in the sides of the MDS to allow the hammer to vent during firing, to make sure there is no pressure buildup on the front, or vacuum on the back.... The total weight of the hammer and cocking handle is only 58.7 grams, only about 10% heavier than the Steel and MDS hammer I use successfully on my BRods.... I have two other hammers for the BRods, an all steel at 104 grams, and an MDS with aluminum core at only 27 grams.... so I can go lighter or heavier should the need arise....
This hammer is made for the 6mm bottle gun, I need to make the other one smaller in diameter to fit the thicker wall tube.... After making this, I have one concern, and that is the slots milled in the sides allowing the hammer to cock sideways in the tube more easily, since the cocking force is on the right side, and aligned with the slots.... When I make the second hammer for the .257, which will be running higher pressure, I won't be quite as concerned about weight, so I plan two changes.... The MDS will be full length, with a notch milled in the bottom for the sear.... and it will have a single vent slot in the top only.... This will leave both sides circular, in case there is any tendency to cock sideways and bind when cocking with this hammer.... That way I can determine if there is any difference, and which is better....
Bob