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rsterne
For some time now I have been contemplating what my winter project should be, now that the Motel is slowing down, and the snow is fast approaching…. This is the time of year when I get to play in the shop, and bring to life things I have been thinking about all summer…. This year’s project is very ambitious, a scratch built rifle using only a few purchased parts…. the barrel, trigger group, scope, the fasteners, and a few small items like the gauge, picatinny rail and mounts, etc…. Here is a sketch of the general arrangement….
This gun is a monster…. It’s 48″ long, with a 33″ barrel, and will weigh an estimiated 12 lbs. plus scope…. It’s heart will be a 7″ twist, .257 cal TJ’s barrel from a new mandrel I am having made…. The fast twist is necessary to tame the new 113 gr. Bob’s Boattail bullet I designed, which is basically a scaled down version of the 200 gr., .308 cal “Whiteout” BBT that won the 200 yd. EBR in October…. This is its intended fodder…. Molds will hopefully soon be available from NOE….
In order to push that bullet into the mid 900s, I will be using 3800 psi, normally supplied by tethering to a regulator on a 4500 psi Great White tank…. However, the reservoir, which is made from 1.25″ OD x 0.095″ wall CrMoly, will be 400 cc, so I should be able to get a few shots off tether within a 1-2% ES…. The main tube runs the full length of the rifle, and houses the valve, hammer, hammer spring and preload adjustment…. and provides a mount for the PRod trigger group and a home-made adjustable stock, to be machined mostly from aluminum, and a short Picatinny rail up front to mount a BiPod…. The hammer has it’s own cocking knob, and it’s free-floating, ie when the gun is uncocked it rattles around in the ~0.1″ space between the valve stem and the captive spring guide…. The 1/4″ diameter steel spring guide slides through an adjusting screw mounted in a block bolted into the tube…. It carries the long hammer spring, preloaded to between 5-10 lbs. of force when uncocked, which maximizes the hammer velocity without requiring more than 20 lbs. of force when cocked…. I have used this arrangement before, and I really like it…. It makes for a lighter and more constant cocking effort, and not having the hammer spring in constant contact with the hammer, and yet lots of preload, reduces or eliminates hammer bounce…. The preload adjustment is made through the end of the main tube with a long socket wrench….
The exciting part of this design, to me, is what has been deemed the “Monotube” upper structure…. This is a 1.25″ OD x 0.065″ CrMoly tube that runs nearly full length…. The breech block, which will be machined from 2024-T3 round bar for a sliding fit inside the tube, extends from about 4″ ahead of the loading port to the back of the tube…. In that one piece, it mounts the barrel, serves as the chamber, transfer and loading port, and bolt carrier…. It will be solidly bolted inside the tube, and the barrel threaded into it securely…. On top of the tube will be a Picatinny rail for the scope, bolted to the tube and breech block, ahead of the loading port…. At the front of the tube is a short aluminum plug, which is a close fit on the barrel, also bolted into the tube, 1″ from the front…. The muzzle of the barrel (recessed just inside the tube) will be threaded 1/2″-20 NF, and a stack of Belleville disc springs will slide over it, and be tensioned by a simple 1/2″ nut, tightened by a socket wrench…. By using a stack of five, 0.073″ thick Bellevilles in a series arrangement, I can achieve an adjustable tension of up to 1300 lbs. on the barrel in two turns of the nut, with a corresponding amount of compression in the outer tube…. Once the tension is applied, the barrel, tube, breech block and scope mount will work as one solid piece…. The object is the maximum possible rigidity, approaching that of a solid 1.25″ diameter barrel with the breech machined into it, but at a fraction of the weight…. Not a lightweight, for sure, but light for how solid it is…. There is a simple, double-concave aluminum spacer at the back, between the two tubes, holding them 1/4″ apart (so the barrel tube is free floating), with through bolts securing the breech block in the upper tube to the valve and hammer spring carrier in the lower tube…. holding the two tubes parallel….
The most expensive part of the whole thing is the Millet 6-25 x 56 SF Scope (obtained from Eric at http://www.scopesandammo.com) which my wife will be giving me for Christmas…. It is massive, 22″ long with sunshade, and weighs 2.2 lbs., built around a 35mm tube…. The MilDot-with-bar Reticle is calibrated for the maximum magnification (25X) which means the dots are far enough apart to allow sighting at 100 yards and have the top of the lower post be the point of aim at 200 yards…. The distance from the dot-to-bar becomes 1 Mil at 12.5X, and that point is marked on the zoom ring, so at that setting you have a MilDot reticle, but with 10 dots available in every direction…. The Elevation turret has 140 MOA of travel because of the huge 35mm tube, which should allow enough to reach out about 1/4 mile…. I will be machining a 20 MOA slope in the Picatinny rail so that the trajectory intersects the line of sight at 100 yards with the scope centered…. Hold under, using the MilDots, will be used at ranges under 100 yards…. Funny thing is that the scope doesn’t look at all out of place on such a long rifle….
I’ve ordered the barrel mandrel, and the CrMOly tubing and 2024-T3 bar stock…. and Mrs. Claus ordered the scope…. so this project is off and running…. and I can’t wait to start making chips….
Bob
rsterne
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This morning I finished up the Buttstock…. It turned out quite well, with lots of adjustment…. The LOP can be varied from 13-15″, and the drop is adjustable over a range of 1-1/4″ in 1/4″ increments…. The shaft in the butt pad is 1/8″ off center, so 1/2 turn changes the height of the butt by 1/4″ and then there are three holes on 1/2″ centers…. The threaded rod is glued and pinned into the butt pad, as well as threaded, and the wingnut locks the rotation….
This afternoon I did most of the machining on the Breechblock insert…. pretty much everything except the slot for the bolt handle…. I will have to duplicate everything you see here in the Monocoque tube, so accuracy and layout is the key here…. I’m quite pleased with the way it turned out….
The two holes with low profile 10-32 screws in them are to take the tension load of the barrel when the Belleville washer stack at the muzzle is compressed…. There are three screws that bolt the upper and lower tubes together, a 10-32 at the back and two 8-32s forward, either side of the transfer port…. The transfer port is milled at a 30* angle towards the muzzle to ease the flow transition into the barrel…. The Picatinny scope rail bolts to the Breechblock though holes in the Monocoque tube, with the forward screw tapped into the tube itself…. The opening for loading is milled flush with the centerline of the bore on the right side, and just past the vertical centerline, so just over 1/4 of the material has been removed…. This should make the assembly much more rigid vertically than milling out the entire top half of it…. There will be a steel thimble (actually in 2 pieces) that will have the barrel port, chamber, and loading tray machined into it…. The 8-32 screw under it will take the end force trying to push it back on firing…. It is set deeply so that the head is in shear for extra strength…. I will likely mill the slot for the bolt handle into both the Monocoque tube and the Breechblock in one operation after they have been assembled…. or possibly machine the tube first, then install the Breechblock into it and mill the slot through the slot in the tube…. I’m sure it will be the most difficult part of the whole procedure….
Bob
Today I machined the double convex Tube Spacer to match the breechblock, as the mounting screws and transfer port penetrate it to connect the two tubes…. The recess in the middle is a pocket to clear the barrel tensioning retainer screw….
On the bottom at the front I had to machine a recess that will hold the Forestock tube in place tight against the top of the Reservoir…. you can see it in the photo below…. I also got the Hammer made today, including the cocking handle, as shown….
The total weight of the hammer and handle is 182 grams, and it will have a total travel of 1.5″…. The captive spring arrangement and adjuster was shown previously, and the moving part weighs 53 g. for a total of 235 g…. That means that only 77% of the total energy ends up in the hammer, the rest is absorbed by the O-ring on the guide when it comes to a halt before the hammer coasts the last bit to open the valve…. That is the price you pay for easier cocking and less hammer bounce….
Bob
Today I attached the Front Cheekpiece Mount to the back of the Breechblock….
It is mounted below the centerline of the tube so that the inside of the Cheekpiece tube sits against the top of the upper Monocoque tube…. I also machined the front of the barrel to fit the 1/2″ ID Bellevilles and threaded it 1/2″-20 NF for the nut that will provide the barrel tension…. The aluminum sleeve that bolts into the front of the Monocoque tube is 9/16″ ID to fit on the larger part of the barrel, while the Bellevilles ride on the 1/2″ section….
The front of the Monocoque tube is about 1/10″ ahead of the muzzle to protect the crown…. If I wish, I can use a Hatsan Air Stripper instead of the nut to tighten against the Bellevilles to provide the barrel tension and also strip the air away from the back of the bullet….
This completes all the parts that fit inside the Monocoque tube…. except to machine the bolt slot which I will do after machining the tube…. I am still undecided if that will be my next step, or if I will make the valve first…. and put off machining the two tubes until the end…. :
Bob
I couldn’t put it off any longer, so today I did most of the machining on the Monocoque Tube…. all the holes for the various bolts, plus the loading port and transfer port are done, plus the additional 10-32 tapped hole for the front of the Picatinny scope rail…. Here is the top view….
Everything fit really well, all the bolts fit into the holes, but a couple were tight enough to require a touch with a file or Dremel bit to make them easy to assemble…. Careful deburring was the order of the day to make sure that the Breechblock didn’t hang up when slid inside…. I was particularly proud of how well the loading port lined up…. Below is the bottom view, and you can see that the transfer port and mounting bolts also line up with the Spacer….
All in all, I am very pleased…. The key to the accurate machining was using the 5/16″ hole behind the loading port where the heads of the screws are that take the tension and compression loads of the barrel and tube…. Using a 5/16″ pin in the chuck of the lathe through that hole, both for setup (measuring everything from that point), and for indexing 180* for the holes on the bottom, did the trick….
Bob
Anyone interested in helping get the moulds for the 102 gr. and 113 gr. bullets underway that this gun is designed around…. please post in the following threads, or PM me…. I need to get the tooling underway so that the moulds can be produced ASAP….
http://noebulletmolds.com/smf/index.php/topic,1129.0.html
http://noebulletmolds.com/smf/index.php/topic,1113.0.html
I would like guys to help share the $200 tooling cost so that the moulds will become available…. Thanks….
Bob
Today I machined the top of the PRod trigger group to fit the 1-1/4″ OD x 0.095″ wall tube…. It normally fits a 7/8″ OD x 0.065″ wall tube, so one of the concerns is that the sear projects far enough into the thicker tube I am using to safely hold back the hammer…. I used the 1-1/4″ diameter router bit I have to cut the larger radius, and at the point it barely touched the center of the recess I stopped and checked the seat engagement…. It was about 1/32″, which although I feel was perfectly safe (after all, the hammer can’t raise up by more than the clearance in the tube), I decided I wanted a bit more…. I noticed that the boss in the casting that holds the sear pivot pin was about 0.010″ below where the router cut, so I mounted the trigger group back in the milling attachment and milled down until I just touched the edge of that…. The arrow in the photo below points to that shiny mark, right beside the sear…..
With the trigger group machined to that depth, there is over 1 mm of sear engagement (I’ll get an exact measurement after I cut the slot in the tube), and I’m happy with that…. One other thing that is important is that the slot in the tube to clear the sear must be slightly more than the full length of the sear…. The parts of the sear in front and behind the actual sear now project slightly above the bottom of the tube recess in the trigger group, so if the slot is too short, it would affect the engagement depth, and in fact could even prevent the sear from cocking properly…. All the other parts (the trigger and the intermediate link) are still well below the tube and do not interfere….
The PRod trigger is very strongly built, and I see no problem with it easily withstanding way more than the 20ish lb. maximum load I will be asking it to hold….
Bob
Today I made the front half of the Valve, and cut the O-ring groove in the Fill Fitting while I was set up for that….
You will notice that I am using a conical valve spring to eliminate any restriction on the inlet side of the valve poppet…. I hope to work on the back half of the Valve tomorrow….
Bob
I worked on the Valve today, and completed the machining on the body….
While the rear half was in my square Collet holder so that I could index it 90* to drill and tap the four 1/4″-28 mounting hole, I also drilled the port and the two 8-32 screw holes beside that which hold the two tubes together at the front of the Breechblock…. In addition, I screwed the valve front into place and drilled and tapped 1/8″-NPT for the gauge which mounts on the left side…. I was extremely pleased with how well everything lined up…. Here is a photo of the assembly (without the tubes)….
The two long 8-32 screws which hold everything together dropped right through the holes and screwed in with my fingers…. I still have to make the poppet, and install an O-ring seal for the stem in the back of the valve, but this is the last major internal part…. It won’t be long I will be able to machine the lower tube to hold the valve, hammer, and fill fitting…. Unfortunately, I am still waiting for the 1/4″ long 1/4″-28 SHCSs which hold the valve and fill fitting in place against the air pressure, so I won’t be able to pressure test it until they arrive….
Bob
I made the rest of the valve parts today…. Here is the finished poppet and stem seal….
The poppet is made from 3/8″ diameter PEEK, and I threaded the end of the 1/8″ drill rod stem to 5-40 thread, drilled the PEEK with a #38 drill (tap size for 5-40) and counterbored it 0.10″ to 1/8″ and then just wound the stem into the PEEK, letting it cut its own threads until it stopped…. It was so solid I couldn’t back it out…. I then cut the head to length and turned it for the spring seat right on the stem…. It sealed first time…. The stem seal is a #006 70D O-ring sitting in a 1/4″ diameter recess in the valve body, held in place with a short piece of 7/16″ bolt faced off flat on both ends and a screwdriver slot ground in the end for installation…. It has only enough drag to barely hold the weight of the valve body, so considering the forces on this valve shouldn’t even be noticed…. The inside of this valve is huge….
The throat (visible in the bottom of the rear section on the left) is 5/16″ ID, so the seat margin is only 0.030″…. The valve is drilled 11/16″ ID, leaving a 5/32″ gap all the way around the poppet head (4 times the throat area)…. The front of the valve is drilled 13/32″, the same as the front ID of the conical valve spring, and so has twice the area of the throat, allowing the air to enter the valve from the reservoir much easier than it can get out…. I don’t think there should be any problem with this valve flowing air….
Bob
Today I worked on the Thimble which contains the Barrel Port and Loading Tray…. The 3/8″ diameter stub on the barrel fits into the front of it to insure alignment with the bore, and the joint is sealed with an O-ring, which also seals it into the Breechblock in front of the Transfer Port…. I haven’t yet machined the O-ring groove behind the transfer port in it…. The barrel port in this part was machined with a 3/16″ end mill starting at 30* to match the transfer port at the back of the port, and then angled to 45* to make the front of the port longer to match the bore area…. The port ends up just over 3/16″ wide x 5/16″ long at the boreline, but blends into the 17/64″ round transfer port…. The finishing touches were done with a Dremel….
The hole at the back fits over the head of an 8-32 screw in the bottom of the Breechblock, that takes the end load from the barrel screwing against it, and the air pressure inside it…. You can see that screw in the bottom of the loading tray in the photo below….
Once again, I am extremely pleased with the way everything fits and lines up…. This relative small part took about 6 hours to make…. Next up is the Bolt, which will carry the O-ring that seals the breech inside this Thimble…. It is the retractable style I developed a while back, with no probe to restrict the airflow…. Then comes the complicated job of machining the “J” shaped bolt slot in the Breechblock and Monocoque tube…. I’m not looking forward to that….
Bob
Today I made the Bolt, except to fit the handle, and cut the O-ring groove in the Loading Tray / Thimble behind the transfer port…. The Bolt still has to be cut off about where the line is drawn, which will be flush with the back of the Breechblock when fully open….
The Bolt has a very long travel, when it is fully forward the shoulder on the Bolt is up against the Loading Tray…. This is necessary because of the long bullet, necessitating a long loading port, plus the nose of the bolt has to go past the transfer port, which had to be far enough from the front of the loading port to allow for the O-ring to seal when retracted so that the nose of the bolt clears the back of the transfer port, without the O-ring dropping into the port when fully forward…. It will retract about 3/8″ from the full forward, loading position to the firing position….
It’s time to do some more thinking to make sure I don’t forget anything before I continue machining the tubes…. The Monocoque tube and Breechblock both need the slot machined for the bolt handle…. and the Reservoir tube has yet to have any machining done…. although all the internal parts are now complete….
Bob
Only got a few hours in the shop today…. but I got the holes in the top and bottom of the lower tube machined…. That allowed me to bolt together some of the sub-assemblies for the first time…. It actually is beginning to look like something other than a pile of parts….
The barrel and Monocoque tube are not in place in the photo, because I wanted you to be able to see the general arrangement of how the parts fit together…. It was great to actually be able to shoulder it and see how the ergonomics work…. I’m quite pleased, although the gun is definitely long…. The LOP as shown in the photo is 14″…. I still have to machine the side holes for the valve screws, the hole on the left for the gauge, and the slot on the right for the cocking handle for the hammer…. Then it remains to machine the slot for the bolt handle in the Monocoque tube and the Breechblock…. The next time I put it together it should look close to the finished product…. Long, thin, and heavy….
Bob
Today I had to figure out how the drill the side valve holes at exactly 90* and aligned with the top and bottom valve holes…. Normally I use a 5C collet in a 4-sided holder, but the maximum diameter collet is 1″, and the tubes are 1.25″…. In addition, they are 4 feet long, and difficult to handle when clamping in my short, 2.5″ wide milling attachment…. The trick is to make sure that the hole drilled through top to bottom is exactly vertical, ie square with the C/L of the chuck…. I got the idea to make a fixture that I can clamp in the chuck of the lathe with two 3/8″ holes at exactly 90* to the chuck, that will straddle the diameter of the tube and allow me to put a 3/8″ bar through the existing 3/8″ holes in the tube…. I didn’t have any metal tubing large enough, so I used a piece of 1-1/4″ ID plastic pipe, mounted it in the milling attachment, and drilled a 3/8″ hole straight through at 90* to the wall of the pipe, and exactly on the centerline…. It’s crude, but it should work….
The next step is to make sure that when the tube is clamped in the milling attachment that the centerline of the tube lines up with the vertical centerline of the chuck…. I put the tube in the milling vise temporarily, and using an edge finder found the top of the tube…. It is then a simple matter to raise the vise up the radius of the edge finder (0.100″) plus the radius of the tube (0.625″) and the tube will then be vertically centered relative to the chuck….
Next I put the alignment tool in the chuck, lined the hole up roughly vertical, and put the tube loosely in the vice and lined the hole in the tube up with the holes in the tool….
To be continued….
Then I carefully slid a 3/8″ diameter bar through the vertical hole, while rotating and sliding the tube around until the bar passed through all four holes…. and then I tightened up the vice, checking as I did so to make sure the rod would still slide easily….
At that point, I removed the tool from the lathe chuck, and checked the bar to make sure it was vertical relative to the ways of the lathe…. As you can see, it was….
The last set was to use the edge finder again to locate the center of the existing holes in the tube, so that when I drilled the side holes, they would line up perfectly….
To be continued….
This gives you an idea of what can be involved just in setting up to drill a hole accurately in a piece of tubing…. I’m pleased to say that it paid off, and the side holes ended up nearly perfectly aligned with the ones on the top and bottom of the tube…. One hole need a touch with a file on one side, and the other needed to be moved over about 0.005″ with a Dremel on one side so that all four valve screws could be inserted and will take equal loads when the tube is pressurized…. When I can use the 5C collet in the square holder, I usually don’t have to correct the holes at all, but this worked well, considering the multi-step setup required…. Once I had the cross hole drilled, I then used that, with a 3/8″ pin in the chuck, to lay out the hole for the gauge in the left side, and the slot for the cocking handle in the right side…. Here is a photo of the top and left side….
and here is a photo of the bottom and right side of the reservoir tube…. You can see the hammer through the cocking slot and sear slot….
This was a time-consuming process to go through just to drill three holes and cut one slot…. but that’s the way it goes when you have equipment with severe limitations like a milling attachment on a lathe…. You can usually do it, but figuring out HOW and getting set up is the hard part…. The back end of the Reservoir Tube is now complete, the only machining left to do on it is to cut it to length and drill the four holes in the front to bolt in the fill fitting….
Bob
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