Author Topic: Any Interest in 6mm ? - Build Thread  (Read 2916 times)

rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #60 on: January 03, 2018, 08:41:14 AM »
I have been discussing it with Lloyd and we have been trying to analyze the mode of failure and the forces (did you get the photo I emailed you).... When we calculate the load at 4200 psi, and compare it to the shear strength of the Delrin at the bottom of the O-ring gland, we still have about a 2:1 safety margin to shear.... What Lloyd thinks is happening (and I agree) is that the load from the pressure on the O-ring is mostly near the outer edge of the gland.... Since the gland is only 0.060" from the edge of the poppet, that force is "levering" the gland, ie bending it at the base, and a tear starts at the bottom front of the gland, which then rips across the gland at that point.... The solution we came up with (I just need the time to make one) is to move the O-ring 1/16" further towards the throat (providing I have the room, which I think I do).... This will double the thickness of the side of the gland, greatly reducing its tendency to bend, and hopefully allowing it to live at over 4000 psi.... and increase the safety margin in shear to 4 or 5:1....

I looked at extending the brass front portion of the poppet aft and making the gland in that, and facing the back of it with Delrin as a seal.... The problem is that you have to have a 100% seal between the brass and Delrin, or the HPA would simply slide the Delrin back on the stem and it would leak.... I don't see how any method of "inserting" the Delrin would help.... The other solution is to make the entire poppet of steel (or brass on a steel stem) and then install an O-ringed Delrin seat in the valve.... something I would sooner avoid if possible....

My concern, of course, is that if this happened with a bullet loaded, the gun would fire it on its own.... It might not be lethal, because the air is venting through the tiny groove in the stem.... but I don't want to chance it.... I wish I had the ability to test the new poppet at about 8,000-10,000 psi, just to make sure it will stand that pressure without failure....

Bob
« Last Edit: January 03, 2018, 08:45:27 AM by rsterne »
  • Coalmont, BC

rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #61 on: February 07, 2018, 03:39:55 PM »
I have been cleaning up some leftover projects for the last month while I have been waiting for some 1.2 mm CS O-rings to arrive for the poppet in my version of the ART/SS valve.... They finally got here and I made a new poppet which worked out great.... The smaller O-rings allowed a shallower groove, which I moved further down the poppet towards the seat and now it seals great and seems to function fine, even at 4200 psi.... It is holding air pretty well, losing less than 100 psi per day, which is good enough for now, and often slow leaks like this improve with time, as they are often the poppet on the seat.... Anyways, assuming it is good enough for now, the next order of business is to make the uppers, and then ream the chambers and mount the barrels.... After that I can shoot these guns for the first time, as I cast some bullets last week as well....

I started on the receivers yesterday, cut them to length (actually 1" over) and machined the concave bottom to fit against the tube on both receivers, which will be interchangeable on the lowers.... Today I laid out and center-drilled both ends of the bore for the barrel and bolt, high enough above the top of the tube to allow for the 20 mm OD barrel sleeve, plus 0.20" to allow for the thickness of a piece of 1-1/4" ABS pipe which I will be using for the foregrip on the unregulated .257 cal.... That plastic pipe is 0.150" wall, so it will just nicely clear the Carbon Fibre barrel sleeve.... It will be captured between a recess on the bottom of the receiver and the tube, like I did with the foregrip on the Monocoque.... Anyways, the C/L of the bore is 0.60" above the top of the main tube.... Once the locations were center-drilled, I mounted the receiver between centers on my lathe, and then held the back in my 4-jaw chuck.... Having it sit on a 60 deg. center inside the spindle bore allows easy centering of the jaws, you just tighten them progressively on the four side of the receiver until it is secure.... Using a live center (ie one that spins on a bearing) in the tailstock, I machined a 1" long collar on the front of the receivers (which is why I left them 1" too long).... The collars will ride in my steady-rest to support the receivers on center while I drill them full length....

I first drilled a 1/4" pilot hole from front to back, using a foot long 1/4" drill bit, stopping just before I hit the 60 deg. center in the headstock.... I then used a long 1/2" drill to increase the diameter to the finished size, removed the receiver from the lathe (so the 60 deg. center wasn't in the way, and drilled through the last 1/2" at the back.... The hole was straight and true, and parallel to the top of the tube in both planes, which is what I wanted.... A piece of 1/2" CRS slides perfectly through the receiver, full length, so I know that the bolt and the barrel will be aligned and parallel.... I them mounted the receiver back in the lathe between centers, re-checked the steady-rest position, and then drilled the front of the receiver out to 25/32" to accept the 20 mm CF barrel sleeve.... The sleeve is about 0.004" larger than the drill size, but it just starts into the hole, so all it will need is a bit of sanding to be a perfect slide fit....  Here is what it looks like at the moment.... I only have one drilled at the present time, the other one will be done tomorrow, all going well....



The plan is to cut off the collar, it was only there to allow me to support the front of the receiver in my steady-rest.... I don't need it for support, as the barrel sleeve goes 2.5" into the receiver, which is over 3 diameters.... It will be located by two sets of 3 setscrews at 120 deg. so the assembly should be 100% rigid when complete....

Bob
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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #62 on: February 08, 2018, 04:22:31 PM »
I drilled the other receiver today, and then drilled and countersunk all the mounting holes and drilled through for the transfer port....



The small recess on the bottom just behind the transfer port is to clear the upper valve mounting screw, which is just proud of the tube.... The receivers, like the tubes, are drilled on the same pattern, so the uppers will be interchangeable on the lowers.... Although I intend to build the .257 on the long, unregulated tube with the thick wall (4200 psi MSWP) and the 6 mm on the regulated bottle version with the short tube (thinner wall with 3200 psi MSWP).... the interchangeability will allow me to test the other way around, and change my mind should I decide to....

Bob
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grumpy

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Re: Any Interest in 6mm ? - Build Thread
« Reply #63 on: February 09, 2018, 04:46:43 AM »
Definitely following this one. What molds do you plan on using for it Bob ?

Dave
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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #64 on: February 09, 2018, 05:02:59 PM »
I have three 6 mm moulds, Dave.... the 60 gr. 6mm Bowman from Arsenal (4 cavity with 2 HollowPointed by Erik).... the 74 gr. NOE.... and the shortened 64 gr. version of that.... both with Lyman HP pins in one cavity....

Today I milled out the loading port area in the two receivers.... Instead of milling it straight across, I left the left-hand side of the receiver intact from the boreline over, so that I don't lose so much rigidity.... These guns are single-shot, so no need to cut it away more than that.... I did this on my Monocoque and it works well, but of course you must load the bullet from the right side.... Drilling a 1/2" hole straight through the receiver to assure the alignment of the bolt and barrel means that when you machine the loading port you don't have a tray to guide the bullet into the bore.... so that is made from a separate piece of 1/2" diameter steel.... I drilled one for 6 mm and the other for .257 before milling them to shape.... You can see one of the inserts sitting on top of the receiver in the foreground.... along with a flat-head 4-40 screw which holds it in place.... The back receiver already has one installed....



With the loading tray in place, the right side is level with the centerline of the bore.... and the left side is vertical, tangent to the left side of the bore.... You don't need to look at what you are doing to load, just place the bullet on the right hand lip of the tray, and roll it to the left and close the bolt....

Bob
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Christopher

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Re: Any Interest in 6mm ? - Build Thread
« Reply #65 on: February 09, 2018, 05:27:51 PM »
Nice to see you back on the 6mm build since you got your metric orings in.  Everything is looking real nice.

Chris
  • dead end road KY

rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #66 on: February 11, 2018, 06:05:05 PM »
I got lots more done on the receivers today.... This reduced the weight of them quite a bit.... and vastly improves the appearance....



The top and upper edges are now milled down, leaving raised areas where the dovetails for the scope mounts will be.... I also bevelled the top corners as well.... When I machine the dovetails I will be angling them down 20-25 MOA at the front to put the barrel on POA at 100 yards with the scope still optically centered....

Bob
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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #67 on: February 13, 2018, 06:31:17 PM »
I worked on the receivers more today.... I drilled and tapped the six holes for the set-screws that will secure the barrel.... The original plan was for 10-32 screws at 120 deg. to each other.... but I was unable to get them of the proper length here in town, but I was able to get 5 mm x 0.8 mm Metric setscrews that are 10 mm long.... You can enlarge 10-32 threads to M5 by simply running the tap through them, as they are the same pitch (within less than 1%) and the Metric screws are 0.008" larger than the 10-32s.... I dislike mixing SAE and Metric fasteners on a project, but the alternative would have been to wait weeks to get the 10-32s.... I justified it by the fact that the barrels are 14 mm and the CF sleeve is 20 mm OD.... so at least everything to do with the barrels is Metric....  ::)



I had one other little glitch when setting up to drill the lower holes for the setscrews.... In order for the receiver to clear the carriage on my lathe (I was using my milling attachment to drill and tap the holes accurately) I could not drill them 30 deg. below the horizontal, I had to use 35 deg.... This is of no practical importance, but means that the angle from the top to each side screw is 125 deg. and the angle between the bottom screws is 110 deg.... instead of them all being 120 deg....

I also set up and milled the slots for the bolt handle today.... The front receiver is finished, but dinner hit the table before I got the vertical part of the rear slot done.... I'll do that tomorrow before I set up to fly-cut the recess in the front bottom of the receivers to locate the plastic pipe of the forestock....

Bob
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Alan

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Re: Any Interest in 6mm ? - Build Thread
« Reply #68 on: February 14, 2018, 05:27:05 AM »
I like the way you made the bolt cutout. The one on my Cobra allows the bolt to literally flop around, and seemingly always be in the wrong position for a quick shot. Maybe Jim Gaska will see this.
  • Roswell, New Mexico
Alan

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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #69 on: February 14, 2018, 08:04:08 AM »
Alan, the main reason for the "J-Slot" is to allow the bolt to be retracted completely out of the airflow to allow full bore-area porting throughout the airflow path.... There is an added benefit, which I have not 100% decided to take advantage of in these builds.... You can install a long pin with an adjusting screw at the back of the receiver, in line with the lower slot, to adjust how far the bolt face can be retracted.... This allows complete control over the velocity from maximum all the way down to about 200-300 fps (you can literally see the pellet in the air).... The further in you screw the adjuster, the smaller the barrel port....

Bob
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Alan

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Re: Any Interest in 6mm ? - Build Thread
« Reply #70 on: February 14, 2018, 11:13:26 AM »
I do understand the reason. The Cobra has a "T" slot. Once you seat the pellet, you pull back slightly, and then down. The way the detent works, is, well, backward. The bolt should have a grove along its whole path, with a bridge and a deeper "pit" for the detent device. All of this, as a way to keep the bolt in place. Since it doesn't have this, I constantly have to reseat the position of the bolt before shooting as the bolt is just plan loose! I thought about finding a cabinet door, plunger-type keeper, but haven't found one small enough.

Just this morning, I finished up my 27 tin through the .25 caliber Cobra. The edges of the vertical part of the slot are now rounded off, making things worse. It is the only thing I don't like about its design. Or most other bolt air guns for that matter.
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #71 on: February 14, 2018, 01:04:24 PM »
Yes, many airguns need a slight recess in the back bottom of the vertical bolt slot so that the bolt doesn't pop up.... The only disadvantage of the J-slot is if you switch back and forth from a gun that doesn't have one, you can forget to pull back to clear the bolt nose from the barrel port.... and the result is a very weak shot....

Here are the receivers with everything now done except to machine the 20-25 MOA slope in the scope mounts and cut the dovetails in them....



On the lower front corner you can see the semi-circular recess milled to act as a clamp for the plastic pipe forestock.... It pinches the pipe between the bottom of the receiver and the main air tube, mounting it solidly at the back in the lowest possible position so that it doesn't rattle around, and clears the barrel.... The velocity adjusting screws are installed in the back, in line with the lower bolt slot.... The 8-32 SHCS pushes on a 1.5" long steel pin that can prevent the bolt handle from retracting fully in the slot.... I have about 10 turns of adjustment, the back receiver has the adjuster fully retracted (maximum velocity) and the one in front has the adjuster set to minimum velocity.... I labelled the steel pin which is sticking out into the lower bolt slot, which holds the bolt forward so that the nose obstructs the barrel port, reducing airflow (to almost zero when fully closed as shown).... You can't see it in these photos, but there is a vertical hole coming up from the bottom, in line with the bottom of the adjusting screw.... It is tapped for a 6-32 setscrew which presses on a 1/4" long piece of thin Teflon rod which acts as a brake to prevent the velocity adjusting screw from moving by itself....

Bob
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Alan

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Re: Any Interest in 6mm ? - Build Thread
« Reply #72 on: February 14, 2018, 02:22:49 PM »
The screw is a good idea for more reasons than one. I often shoot inside the hay barn. As it is, I use my 397, and just 2 or three pumps to keep from shooting holes in the roof. Even then, a miss results in a dent. This "feature" would allow for that without retuning the pressure and/or hammer on my PCP.

I see things like this (mostly from you!), and I say to myself... "Why didn't someone else think of that?"
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Alan

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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #73 on: February 16, 2018, 04:12:24 PM »
I got to work on the barrels today.... The first order of business was to machine the breech end to 1/2" OD for 1" of length so that it will fit into the receiver.... In addition, I had to machine down the OD of the carbon fibre tube slightly to fit inside the 25/32" forward section which is drilled to a depth of 2.5".... I did that with the end of the tube running on a live center in the tailstock of the lathe to insure concentricity between the ID and OD, and it worked out great.... Both the CF sleeve and the barrel slide perfectly into the receiver and can be rotated individually, showing they are concentric with each other and the holes in the receiver are as well.... This was a bit of a nervous time, as any inaccuracy there would have been hard to correct.... At the muzzle end I turned the barrel down to 1/2" as well and threaded it 1/2"-20 NF so that it can accept either a Hatsan Air Stripper (as a backup) or my intended Harmonic Tuner, which I have yet to make.... Here is a photo of the barrel ends, breech on top, muzzle below....



and here is a photo of them with the CF sleeve in place (but not bonded yet)....



You can see where I had to skin down the outside of the CF tube to fit the 25/32" hole in the receiver, just a few thou was all that was required.... The next job will be to cut the chambers in the barrels before bonding on the CF sleeves.... I think the reamer I made for my .257 Monocoque will work, but I will have to make a new chamber reamer for the 6 mm.... I received some samples from Nick at Nielsen Specialty Ammo in the mail today (perfect timing, thanks Nick), as I would like to make sure the chamber in the .257 will accept as many different bullets as possible.... Here is a photo of the selection of .257 bullets I now have to try....



Left to Right they are.... My 98 gr. BBT HP, the 91 gr. Noble from Nick and his 85 gr. rebated boattail HP, an 88 gr. RCBS 82301, an 82 gr. NOE 260-80 FN, and a 73 gr. Lyman 257420.... I can also cast HP versions of the 82301 and 257420, so I will have lots of bullets to choose from.... I don't expect the 98 gr. BBT (or the FN version of it) to work in the 10" twist of the TJs barrel I have.... so if the chamber has to be too short to fit all the other bullets for it to chamber, so be it.... but just looking at the position of the ogives in the above photo, I think I can cut a chamber that will work with all of them.... If anything, the bullet with the Ogive starting the furthest forward is Nick's swaged HPBT.... Here are the 6mm Bullets.... the TJs barrel I have is also 10" twist....



Left to Right they are.... 58 gr. Bowman HP from Arsenal, 59 gr. NOE 245-64 HP and 69 gr. NOE 245-74 HP.... I can also cast FN versions of all three bullets.... The weights of the FN versions are 61 gr., 63 gr. and 73 gr. respectively.... The 63 gr. NOE is a shortened, FB version of the 73 gr. beside it, with the GC shank removed.... I also have a mould for a 65 gr. shortened version of the NOE 260-80, done the same way, but that will be too light for the power of this .257 cal (I hope)....

Bob
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grumpy

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Re: Any Interest in 6mm ? - Build Thread
« Reply #74 on: February 16, 2018, 05:18:36 PM »
Never know about that 98gr in the 10" barrel. Lloyd  had pretty good accuracy with the 93gr in my .257 with a 10". Ironically, he tried the 100gr, and it was only so so. I want to try some a little heavier for giggles.

Even tho I hardly understand much of what you say, I follow along closely ! lol

Dave
  • Michigan