7mm Hayabusa Build

[rsterne]

Some of you will remember my ongoing project gun which I dubbed the "Hayabusa".... It has been through three versions, the Mark I was a regulated repeater shooting pellets in .22 and .25 cal, the Mark II was an unregulated version built for bullets in four calibers, .224, .257, .308, and .357, and the Mark III, completed a few months ago, was also unregulated, in .410 shotgun and .458 rifle versions.... I decided some time ago that the 7mm (.284) caliber had some advantages in long range ballistics over the .257 and .308 as a varmint rifle, and finally got a .284 liner with a 14" twist from TJ's and I have on order a 94 gr. Bob's Boattail bullet mold from Veral at LBT which will be sent to Erik at HollowPoint Service to have two of the four cavities converted to HP.... I have been collecting the parts for a while now, and today was the day to start making chips.... Here is the beginnings of the new breech, with the bottom rebate for the main tube machined....



The rifle will be built on the Mark II version, using the larger valve which was designed for the .308 and .357.... The valve has a 9/32" exhaust port and the throat is over 5/16", so the porting when used with the 7mm barrel will be full bore area to maximize the power possible.... I will even be using a probeless, retracting bolt to avoid any restriction to the airflow, the idea borrowed from my Disco Double which was the first gun I built to utilize that feature.... I originally planned to use the .308/.357 breech, but with the change to the retracting bolt, and some other changes I have planned for this build, a new breech became necessary....

Bob

[rsterne]

Spent another day on the breech.... got it drilled and the boss for the shroud turned, threaded the front for the ferrule which will retain the barrel (shown), machined out the loading port, the notch for the front mounting band, and drilled all the mounting holes and the transfer port hole....



Not a huge amount of major work left on the breech, just the dovetails on the top, the bolt slot, and the slot to clear the cocking handle on hammer.... Another good day should see the breech pretty much done....

Bob

[Spilled Milk]

Don't want to interupt this thread, but I just want to say, please keep the updates on this project coming. I am really appreciating and enjoying seeng it being put together. Awesome!

Hoping you can elaborate on a couple of things..

1 - The ammo mold.
2 - The retractable bolt probe.

Thanks.

[rsterne]

I finished the breech today.... Machined the "J" slot for the retracting bolt, the slot for the side cocking lever on the hammer, the dovetail for the scope mounts, and tapped the holes for the setscrews.... Oh, and prettied it up with a bevel on the top corners....



Now that this major chunk of work is done, I can get on with the small but interesting bits that I haven't done before, the thimble for the transfer port, machining the overlapping portion of the barrel, and install the retaining sleeve on the barrel....

Bob

[rsterne]

I made the thimble and machined the barrel to fit it today, and made the collar to hold the barrel into the breech.... Thanks to Voltar1 for the idea of threading the barrel and the collar instead of silver-soldering it.... it worked like a charm.... You can see how the tubular compression nut will push on the collar to hold the barrel in place in the breech.... Also shown is the new bolt, a retractable, probeless design.... It pushes the bullet past the barrel port and then pulls back clear of the port to maximize flow....



For those of you not familiar with a thimble, here it is slid off the end of the barrel.... Basically the port and bolt sealing O-ring are in the thimble, which is held into the breech with one setscrew.... The barrel is free to rotate inside the thimble, which will allow you to index it for best accuracy.... You want the barrel rotated so that it vibrates up and down instead of sideways.... Rotating the barrel will cause the POI to move in a circle on the target.... If it is locked in place at the bottom of the circle, the vibration has to overcome the weight of the barrel, so in theory the groups will be smaller.... The end of the barrel is just ahead of the barrel port, and the chamber and leade are machined in the barrel.... It seals to the thimble with a single O-ring.... In my case, the thimble seals to the valve with a tubular transfer port made of 3/8" Teflon rod that is drilled to bore diameter.... The barrel port is oblong to maintain the area without causing loading problems....



I found the correct location for the collar, marked it, and installed it with Loctite 638 (green) retaining compound, so it now permanent.... I will dress the OD true to the barrel tomorrow and then I can fit the barrel to the breech for the first time.... I still have to chamber it and cut the leade, plus do the machining at the muzzle to hold the Bellevilles and Air Stripper....

Bob

[rsterne]

Today I pretty much finished up the barrel.... I fitted the sleeve to the breech and machined a groove in the middle of the threads for the setscrew to limit barrel rotation once I find the right spot.... Then I turned down the muzzle for the Bellevilles and threaded it for the Hatsan Air Stripper, and made the collar to put the shroud in compression.... Here is the assembly....



Here is the assembled breech....



Here is the muzzle assembly....



and here are the parts used at the muzzle....



The shoulder that the shroud will sit on is 1" long at both ends for stability.... The tension on the Belleville washers is provided by tightening the Air Stripper.... Those are the stiffest Bellevilles I could get, capable of 1335 lbs. force when flat.... although I don't expect to have to use that much to find the "sweet spot".... I have another set of Bellevilles that are 600 lbs. flat load.... The only jobs I couldn't finish were machining the O-ring groove in the thimble (the 7.5mm x 1.5mm O-rings aren't here yet) and chambering the barrel (no bullets yet).... I'm also waiting for the 25mm OD x 22mm ID Carbon Fibre shroud tubing.... So, now we wait....

Bob

[rsterne]

The Carbon Fibre tubing for the shroud came in, and I cut it to length and installed it.... I think it looks quite nice....



In the photo below, the Air Stripper is tightened exactly one turn from when it first contacts the Bellevilles.... That applies 750 lbs. of tension in the barrel (and compression in the shroud).... I plan to leave it under that load for a week and see if the gap in the Bellevilles increases, which would indicate that the CF tubing is compressing over time....



I don't think that the Bellevilles are unattractive at all, in fact in combination with the Stripper it gives quite a unique and purposeful appearance to the muzzle, IMO....

Bob

[rsterne]

The mold arrived today for my 94 gr. Bob's Boattail bullets, with two cavities converted to HollowPoints....



The Mold is available from Veral at LBT under # 284-94-BST.... and the HP conversion was done by Erik at HollowPoint Mold Services....

Bob

[rsterne]

Cedric got one of the first three molds from Veral at LBT when we had him make the pattern for the 7mm BBT.... Here is his first shooting session at 75 and 205 yards....

https://www.youtube.com/watch?v=iMXr...nfjfprvoHuqE6w

Now more than ever I am looking forward to getting this build completed....

Bob

[rsterne]

I finally, after a month, got the Metric O-ring I needed to seal the bolt into the thimble.... Last week I had the opportunity to cast some of my new 7mm Bob's Boattails, and that gave me the samples I needed to use the chambering reamer I got from Sean Pero to chamber the barrel.... Here are the results....



In the photo above, you can see the system that retains the barrel into the breech, a threaded sleeve that presses against a collar threaded onto the barrel, the O-ring that seals the barrel to the thimble, the thimble itself, and the chambering reamer.... I ran the reamer in until when the bullet was properly seated the base was flush with the back of the barrel, which sits against a shoulder in the thimble just ahead of the transfer port.... The bolt pushes it home to that location.... Below is a closeup of the thimble, showing the internal 7.5mm x 1.5mm x 10.5 mm O-ring that seals on the OD of the bolt, and a bullet which has been chambered....



You will notice that the rifling marks on the front driving band are approaching full depth, but the middle band is just entering the tapered leade.... There are no rifling marks on the rear band, it sits in the parallel section of the chamber, which is about 0.001" over the groove diameter of the barrel.... You can feel resistance for about the last 1/8" of bolt travel, but it is not hard to chamber, you are just aware of the bullet engaging the rifling, which I like....

My replacement ShoeBox Compressor (a Freedom 8 ) has been shipped, and with this work completed, I should be ready to test the 7mm version of the Hayabusa once I get it and top up my tanks....

Bob