Any Interest in 6mm ? - Build Thread

[YEMX]

Excellent Bob!  I'm glad one of us is making progress!  I'm still waiting on funds to buy the Cobra block.  After that, I should be good to start modifying my breech, tube, then assembling it.  There's still a few things I need to decide though- what kind of stock do I want, and whether or not I want to add an LDC...  If I do add an LDC, how to go about it.  The little things!   

[rsterne]

Follow the air path from the port to the bullet.... that entire area is pressurized, along with the bolt face.... If there are any breaks in the surface, follow the air out to where it hits a seal such as an O-ring.... Best case is to calculate the end force based on the diameter of the O-ring.... However, if the O-ring fails (and the air has nowhere to go), you may need to use the OD of the barrel (spindle, etc.) to calculate the end force.... Let's say the end of a 16mm (~5/8") OD barrel might see pressure if an O-ring leaked.... At 3000 psi, that is 935 lb.f., so at a 3.5:1 safety margin you need to engineer the barrel retention for 3271 lb.f.... 

By comparison, if the only seal is barrel to bolt face, and it's a .257 cal, the forces on barrel and bolt at 3000 psi are only 156 lb.f.... and 3.5 times that is "only" 544 lb.f....

Bob

[Prouzy]

At 3000 psi, that is 935 lb.f., so at a 3.5:1 safety margin you need to engineer the barrel retention for 3271 lb.f.... 

Bob

Yowza!  I wonder what Marko's 20mm.............

[rsterne]

"Well engineered" is the answer....

Bob

[Prouzy]

"Well engineered" is the answer....

Bob

Tru dat! 

[rsterne]

I finished the machining to mount the stock and pistol grip today.... I decided to lengthen the locking pin by 3/16" and simply drill holes in the tube for the adjustable stock positions.... The locking pin is 5/16" diameter, so all I did to lengthen it was drill and tap it 10-32 and installed a short SHCS.... The head is 3/16" thick, exactly the length I needed.... I drilled seven 5/16" holes on 5/8" centres, so I have 3.75" of adjustment for the LoP.... In order to keep the stock from rotating I milled a short block of aluminum 1/2" wide x 3/8" tall, which I attached to the back of the tube with two low-profile 6-32 SHCSs.... The block prevents the stock from pulling right off the end while adjusting it, unless you lift up on the front of the adjusting lever, the same way the stock works on an AR buffer tube....



I laid out the location of the trigger group so that my LoP is adjustable from 12" (mostly for transporting it) to 15.75".... I drilled and tapped the mounting holes in the tube for the trigger group, and milled the slot for the sear.... The photo below shows what the stock and grip look like mounted to the main tube.... set for a 14.5" LoP, which is what I use most of the time....



I'm very happy with the appearance, simplicity, light weight and the ergonomics, and I will set up the 6 mm bottle gun with the same dimensions....

Bob

[YEMX]

That's gorgeous Bob!

[Prouzy]

Very nice!  I went back to review your post on Nove 30 where you mentioned simply turning down that portion, that will provide a nice rigid shooting platform!

[rsterne]

The bottle gun version uses a 0.095" wall tube, which would be too thin if I turned the outside down to fit inside the AR stock.... Therefore I have to use a buffer tube to mount it, and that requires an adapter to the MRod size tube.... Here is what it looks like....



It was made from a piece of bar stock, turned down to 1-3/8" OD for 1" of length, and that portion drilled out to 1-1/8" and then tapped 1-3/16"-16 TPI to fit the buffer tube threads.... I have a maximum of 5/8" thread engagement.... The adapter was drilled through to 19/32" and then tapped to 5/8"-28 TPI for my SSG gap adjuster.... The front half was turned down to 1.058" to fit inside the MRod diameter tube a total of 0.70", and mounting holes drilled and tapped for 8-32 screws top and bottom that will be 1/2" from the end of the tube.... This puts the rear trigger mounting hole 1.50" from the back of the adapter.... and that ends up in exactly the same place as the shoulder on the turned down version I made yesterday.... When assembled (without the tube) it looks like this....



With the stocks adjusted in the same holes, the LoP of both guns are the same, so they will share identical ergonomics.... I still have to make an insert for the straight through tube to mount the SSG and the rear of the trigger group.... and of course the SSG assemblies, but those will come much later when I have the valves and hammers made....

Bob

[rsterne]

Today I made the SSG adjusters.... In this style of SSG, the center stop rod is stationary (but adjustable) and a spring guide slides on it to compress the spring and then launch the hammer when you pull the trigger.... The adjusters were made from a 2" piece of 3/4" OD CRS, machined down to 5/8" for half the length and threaded 5/8"-28 TPI to match the inside of the SSG mounts.... The inside was drilled to 1/2" ID for 1.5" deep to provide a place for a longer than normal hammer spring.... The back part of the adjuster was drilled and tapped 1/4"-28 TPI for the adjustable stop rod.... With the adjuster screwed fully in, there is 2.5" available to the back of the hammer, and with it out all the way (about 3/4" of adjustment) I have enough room for a 3" hammer spring with no preload.... I have not yet made the spring guide which slides on the 1/4" stop bolt and propels the hammer.... Since both threads are the same pitch, you can hold the stop bolt with a screwdriver (you can see the slotted end in the photo) and adjust the outer sleeve to change the preload without changing the gap.... If you hold the outer adjuster, CW on the stop rod decreases the gap (and preload) and CCW increases the gap (and preload)....



You can see that I have milled flats on the back of the adjuster, and I will be making a custom socket wrench to fit that.... Three sides are milled flat, 3/16" from the center, and you can make the initial adjustments with a 3/8" wrench.... One side is left unmachined, but is drilled and tapped for an 8-32 setscrew which will have small Delrin plug under it, pushing on the side of the 1/4"-28 threads as a brake.... There is a similar brake in the outer plug (the straight one for the one-piece tube is shown) to act as a brake for the 5/8"-28 threads on the adjuster.... These allow you to adjust the SSG preload and gap, but insure that the adjustments don't move on their own....

The stop rod shown was made from a 3" long bolt, and is too short to use with a 3" spring.... I will be making longer ones when I make the sliding spring guides....

Bob

[rsterne]

Today I made the wrench for adjusting the SSG through the end of the main tube.... The wrench consists of three parts, an extension tube, made from a leftover front piece of a Disco reservoir, a front end machined to fit onto the SSG Adjuster only one way, and a back end which mounts the handle and has a guide for the screwdriver....



The handle is simply a 3" long 1/4"-28 bolt, which will be locked in place with a locknut.... It is threaded into the rear mount, which is drilled 1/4" ID and turned to fit into the Disco tube.... The front end (lower left) is turned from a piece of 1" aluminum, with a 15/16" OD to fit through the back of the buffer tube, which I drilled out for that purpose.... It is also drilled through 1/4" for the screwdriver, and to allow the threaded stop rod to enter it, because it sticks out past the end of the SSG adjuster (see bottom right).... I milled a slot in the end of the wrench portion that is 3/8" wide, right out though one side, but blind on the other, where I used a 3/16" mill to create clearance in the corners.... That left a bump between which helps to center the tool on the end of the SSG adjuster.... It will only fit on one way.... The back of the wrench end is threaded 13/16"-28 to thread into the front of the Disco tube....



I used some Loctite 638 (green) to permanently install the front (wrench) end in the threads of the Disco tube, and then drilled the hole for the handle in the same plane as the slot, so that the bolt which serves as a handle points to the open end of the slot.... This corresponds to straight up in either gun, when the SSG adjuster is tightened in all the way, making it simple to count turns out from full in to determine the position of the adjuster....



A long slotted screwdriver fits through the center of the wrench to engage the slot in the end of the stop rod.... This allow you to hold the rod and turn the adjuster (adjusting preload but not gap), to hold the adjuster and turn the stop rod (increasing or decreasing both), or without using the screwdriver simply turn the adjuster which moves the rod in as well (adjusting gap but not preload).... The bolt handle just clears the back of the tubes when the adjuster is all the way in, so all you have to do is slide off the stock to adjust the SSG....

Bob

[Nvreloader]

Great Bob

I was wondering about your method of madness, leaving the slot?
Now it becomes clear. LOL 

Outstanding work, I like it, waiting for the next chapter>>>>>>

Tia,
Don

[rsterne]

This afternoon I made the spring guides and longer stop rods.... The guides were turned from Delrin rod, and are 0.35" OD x 1.5" long where they fit inside the spring and 0.59" OD on the flange, which is 1/8" thick.... They were drilled out with an "F" drill (0.257") so that they slide easily on the 1/4" rod.... There is a # 010 O-ring between the flange and the stop on the rod to act as a buffer.... The stop rods were made from a 4" long piece of 1/4" drill rod, threaded 1/4"-28 TPI for 1.5" of length to provide lots of adjustment.... The other end was also threaded 1/4"-28, and a nut bottomed in the threads, tightened down with Loctite 638 (green) to glue them permanently in place.... The nut was then turned down to 0.40" OD and contoured to the same shape as the bottom of the 7/16" drilled hole which will be drilled in the hammer.... The flange on the spring guide is what allows the hammer to compress the spring, and transmits the spring force to the hammer on firing....



This style of SSG has a couple of advantages.... First, it can be adjusted from the back without disassembly to change the preload like some of my earlier ones.... Secondly, other than the hammer, the only weight which is being accelerated by the spring is the very light Delrin guide, so little energy is lost.... The top assembly in the photo above is adjusted so that there is no preload on the 3" spring, and the distance between the flange on the guide and the mounting block is 1", which will be the maximum cocking distance I will build into these guns.... As you increase the preload, the slotted end of the stop rod will protrude more from the back of the SSG adjuster.... I have enough thread to allow nearly 1" of preload, which is more than I will ever use.... 3/8" preload on the 14 lb/in spring shown works out to 5 lbs.... If I run out of preload it is a simple matter to add a shim to the end of the spring inside the adjuster.... I'm very happy with how the SSG part of this project has turned out, next step is to think about the design of the hammer and make those....

Bob

[Matt15]

Wow. I some how missed this. Looks good so far!

[rsterne]

I worked on the PRod trigger groups today.... Lloyd Sikes of AirGunLab makes taller sears so that the PRod pistol grip style trigger can be used with the thicker tube of an MRod.... The sear is about 0.030" taller, to make up for the 0.095" wall compared to the 0.0.65" wall of the PRod.... Lloyd's taller sear is on the left....



It is made from hardened steel, and extremely well made.... On the 6mm bottle gun, which uses a 0.095" wall tube, all I needed to do was install it in place of the stock one.... The straight tube I will be using on the .257 is 0.120" thick wall, however, so even the tall sear would be too short.... Fortunately, I had a solution.... The first gun I made with a PRod trigger group on a 0.095" wall tube (my Monocoque .257) I didn't have a tall sear, so I machined the top of the trigger group instead.... For the 0.120" wall, I simply did both....



The radius on the top of the PRod trigger group is intended for a 7/8" tube OD.... When used on a 1.25" OD tube, the center of the arc doesn't touch, only the sides of the groove bear against the tube, and there is about a 1/32" gap in the middle because of the mismatch of the radii.... Using a 1-1/4" router bit (shown) I milled the groove out to a larger radius, removing metal from both sides of the top of the trigger group until the bit JUST touched the center of the hollow.... This eliminated the gap in the center of the groove when bolted to a 1.25" OD tube, and moves the trigger group up 1/32".... By using the tall sear, and machining the trigger, it works perfectly on a 0.120" (even a 0.130" wall tube)....

I'm getting closer to working on the hammers....

Bob

[rsterne]

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

[rsterne]

I made the second hammer today, for the .257, which has a slightly smaller tube ID.... The lengths are all the same as the hammer I made yesterday, and I have both in the photos for comparision.... Here is the top view....



Note that I moved only used one vent groove, on the top instead of the sides, so the hammer has more bearing area to resist any binding in the tube due to the side cocking handle.... In addition, I made the MDS full length instead of stopping it behind the cocking handle.... Here is the bottom view....



Instead of turning a 20 deg. bevel where the sear catches, I milled a flat to clear the sear and then milled the catch point on a 20 deg. angle to match the sear.... The new hammer is less than 2 grams heavier, and I like the appearance better, it looks like it should resist binding in the tube better than the first version with the shorter bearing length.... It's not worth changing the first one unless there is a problem, and having the two different versions will tell me if there is an issue with binding when cocking....

Bob

[rsterne]

Well today it finally starts to look a little bit like a gun.... and I got to find out that the hammer works as designed.... I measured two (dozen) times and then laid out and milled the cocking slot in the right side of the tube.... Here is what the long tube (intended for the tethered .257 cal) looks like now....



The hammer cocks and still has a bit of leeway for rearward movement before it hits the SSG mounting block that mounts the rear of the trigger.... When adjusted for 1.0" of hammer stroke, the front of the cocking handle rod is nearly 1/4" from the front of the slot when uncocked (as in the photo), so if I have the valve stem protruding 0.20" from the back of the valve (limiting the maximum lift to that, which is plenty).... and the stem touching the hammer when set for that 1" of stroke.... the hammer will hit the back of the valve body before the cocking handle hits the front of the slot.... which is what I wanted.... The hammer cocks smoothly and with relatively low effort with the 14 lb/in spring installed and about 5 lbs. of preload....

Now to lay out and machine the shorter tube for the regulated 6mm version to the same stage of completion....

Bob

[rsterne]

I got the trigger mounted on the short (regulated) tube, and milled the slots for the sear and cocking handle this afternoon.... This hammer works well also and I milled both the cocking slots 0.050" longer at the front.... With the handle at the front of the slot, you can see the back of the sear flush with the back of the hammer, so that is absolutely as far forward (maximum stoke) as I can go.... It is critical that the back of the hammer keeps the PRod sear pressed down, if it could pop up behind the hammer, the hammer would jam the valve open and dump the reservoir, and likely damage the hammer (or trigger) as well.... The length of the top face of the sear (about 1/8") is my safety margin....



This completes the stock, SSG and hammer assemblies on both guns.... Once I make a decision on the valves I will know where to mount them, and only after that can I proceed with the receivers.... Until I know where the valve exhaust port will be I can't make those.... The next steps will be making the front plug with fill fitting for the unregulated tube and the reversed tank block for the regulated one....

Bob

[YEMX]

Beautiful work so far Bob, as always!!