.257 cal Monocoque PCP

[Buldawg76]

Bob
Yea the lathe is very appealing and is calling my name as my friend used it to do a lot of gun barrel work years ago but has since gotten up in the years and is not capable of using as much anymore. It is quite a bit more than we need in the modding of our air guns but then for the price with all his years of tooling bought to go with it makes it very tempting.

Just got to clean out the garage of clutter to make room for it and will look into the suggestion of Ebay for a milling attachment as its got to be more stable than my bench top drill press is and certainlu much safer to boot since a drill press is not really designed for milling.

Keep up the good work and eagerly waiting for the first shot to be heard around the world.

Mike

[rsterne]

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....



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....



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

[Matt15]

Glad the hole turned out the way you wanted. Like they say "Measure twice, cut once."

[rsterne]

Only TWICE ?!?.... That must be what I'm doing wrong.... *LOL*....

Bob

[Matt15]

Only TWICE ?!?.... That must be what I'm doing wrong.... *LOL*....

Bob

LOL!!

[Rizen 1]

Very nice Bob!!!  I am blown away at what you have done with such limited equipment!

[rsterne]

Gee, guess what!?!.... I found a beautiful Millet 6-25 X 56 SF scope under the tree this morning!.... Things were quiet around here this afternoon, so I decided to cut the tubes to length and assemble the Monocoque PCP for the first time.... Here is what it looks like....



I still have to add the bolt, fill fittings and barrel stretcher, and it already weighs just over 13 lbs.... without the scope!.... It is 49.25" long, although it looks longer.... It does balance nicely, in the back half of the forestock....  Then I removed the buttplate, and installed the BiPod (actually a TriPod, I guess) and the scope.... I think it looks pretty awesome....



This is pretty much a bench gun, although it could certainly be used prone for varminting, using the BiPod and with or without the buttstock.... I really like the easy adjustability of the BiPod for height and tilt, using the handwheels.... It was a pretty cool Christmas present to put it together and look at it for the first time....

Bob

[YEMX]

Ah!  Now I see what you mean by "monocoque"...  I had an idea like this a while back- of course, I'm still lacking the skills to bring something like this to life!  Well done Bob!!

[rsterne]

Yes, the upper tube is the main structural member (in compression), holding the barrel in line via tension, and containing the breech, and the scope mounted on top.... As you can imagine, a 1-1/4" OD CrMoly tube is quite stiff.... The reservoir doesn't touch the Monocoque tube forward of the breech and scope, in fact it clears it by about 1/4".... The hope is that the barrel will act like a free-floating barrel that is 1-1/4" in diameter.... but without the weight associated with that.... The Belleville washers at the muzzle, when compressed, will put up to 1300 lbs. of tension on the barrel.... and if that isn't enough, I can double them up and double the force to over a ton.... Did you know that a properly torqued 1/2"-20 thread (45 ft.lb.) develops a axial load of 5400 lbs. (more if lubricated)?.... Threads are amazing !!!

Bob

[Job]

Looks great  ,when you're done, please make a video to see how it works.
How much would the total cost to make one of these for sale?
Merry Christmas everyone

[Rizen 1]

That is AWESOME!!!!  I love it Bob!   

[Matt15]

Looks awesome!!!

[rsterne]

Total cost for a duplicate?.... see my comments at the top of page 6.... not going to happen....

I think this is probably the first of my builds I can truly call a "Custom" gun.... The only major parts, other than the scope and mounts, that I purchased are the barrel and trigger group.... Everything else (except for things like fasteners, etc.) was machined from steel or aluminum.... Oh, except for the level and pulleys for handwheels on the BiPod.... It's been a long time getting to this level, and I have to admit, it feels like a right of passage....

Bob

[scottyhazzard]

Perhaps, Bob, not so much a right of passage but an exercise demonstrating a mastery of every component of an extreme air rifle. The aeronautical components of the round it will fire. The deep understanding and mastery not just of the materials and engineering but also of the tools used to render metal into a distinct form with a focused purpose. It is really amazing. I can do none of these things but I understand some of what goes into each of these things. Each of these things on their own when mastered are impressive, all together is entirely something else. I appreciate humility but in this case I really think you might be shorting yourself or under appreciative of all the different fields of science required to accomplish the design just for someone else to build. Yet you built it yourself! This is closer to some sort of Zen. Just an amazing thing to follow along.

[Motorhead]

Well well ... see now this mass of parts & pieces has morphed into an actual Air Weapon ...


Going to be of interest "IF" the P-rod spec trigger group is upto task of holding the heavy hammer loads required to make this rifle fire at it's potential ?

Wild looking rig you have here Bob !!
Looking good 

[rsterne]

Scott, the PRod trigger doesn't concern me in the least.... I already put the hammer against it and pushed MUCH harder than the 20 lbs. maximum force when cocked.... The sear engagement, which is between 0.030-0.040" with the hammer up solidly against the top inside of the tube, likewise is not an issue.... I am, however, using a square front on the hammer, with maybe a 0.005" radius on the edge, to reduce the vertical load on the sear when cocked.... The back of the sear has plenty of angle on it to kick it down out of the way when it fires.... Interestly, this gun is much easier to cock than some of my modded Discos, or my Hayabusas, because of the preloaded spring arrangement and long hammer travel of 1.5".... but I expect little or no hammer bounce because of the gap between the uncocked hammer and end of the guide, which takes about 7 lbs. to make it move at all because of the preload.... This gun is the smoothest, easiest cocking big bore I have ever had....
 
Today I drilled the holes in the front of the tubes for the retaining screws for the Fill Fitting and Barrel Tensioner....



Here is a photo of the muzzle, showing the recess for the Belleville washers and nut that will tension the barrel....



That completes the machining on the Reservoir tube.... All that remains on the Monocoque tube is the slot for the bolt handle....

Bob

[Matt15]

Will you be making your own bullets?



Never mind!! I see you will be using the Bob's Boattail bullet mold.

[rsterne]

Yes, the tooling for the moulds is now being made at NOE, and I plan to get a mould for both the 113 gr. and 102 gr BBTs I designed for them.... I already cast my own bullets, and I have a .257 sizing die, so I should be ready once the moulds are done.... Here is what they will look like....

 

I will need the actual bullets in hand before I can cut the chamber in the barrel to the proper length.... I have a chamber reamer that I made previously that is the correct size for this barrel, fortunately....

Bob

[Bill G]

Holy cow!  That 113gr javelin, is it about 1.05" long?

[Bryan Heimann]

Looks like a sweet rifle, perfect for long range varmint hunting.  Very exciting

Edit- i wouldn't be concerned with the weight.  A sniper rifle weighs around 16 pounds these days.  As long as you can keep it around 20 pounds or less, it will be fine to sling.  The advantage of not requiring a tether is worth it.  Keep a buddy bottle in a backpack to top up every few shots.  You'd be a fool to take those long shots on critters off-hand anyway.  I bet this gun will be rock solid and steady on a bipod.  Maybe put a smaller bipod with elevation screw in the rear, for that benched effect in the field.  Obviously not a woods gun, but on a big farm it would be so sweet...