New 257 assembly

[Hobbyman2007]

Here’s my list of parts . 29” TJ’s barrel liner in 1:14” with 15mm OD carbon fiber tubing, 30” Cothran stainless disco tube , Cothran stainless breech , marauder trigger group,  Cothran Valve .25 outlet, a couple AR2078 barrel bands ( fit real snug ) and a Boyd’s AT-one thumbhole stock in pepper laminate and a few custom parts like a rear cocking mechanism . Some of these parts were repurposed from my AR79 conversion that I just didn’t shoot mostly due to some of my own previous hack jobs .this one should come out a lot cleaner . I’ll post pics as the project comes together.

[PeterL]

Sounds interesting.

Peter

[mackeral5]

I look forward to this one. 

[Hobbyman2007]

I’m presently waiting for a few of the parts to come in ,I do have enough parts to assemble a test rig and see what I can wring out of the valve and this barrel length. I’m really hoping to hit 160 fpe , I know that’s a bit of a stretch but one can hope .

[rsterne]

You should be able to hit 160, I did with similar specs.... However, the gun is detuned to 130-135 FPE to get decent efficiency at 3000 psi....

Bob

[KnifeMaker]

Where's the popcorn? 


Looking forward to your .257 build. 


I have an unmolested .257 here from OneShot. (Where in heck has he been lately)? I have a "Plan" for it.  Gotta get Travis involved. 


Mike

[Hobbyman2007]

I’m not worried about efficiency as this will be used on the bench mostly . I bought a new set of carbide cutters for my lathe and the quality of my parts has gone up three fold using them . I pretty excited to see the stock for it from Boyd’s .

[mackeral5]

It keeps getting better!

[KnifeMaker]

Man, that stock is out of this world!


Sure wish you were a little further south. I would twist your arm from time to time on a few projects. LOL!


Mike

[Hobbyman2007]

Got the test rig up and running . I removed the ballance spool in the valve for faster closing and hopes of getting a bell curve .i did run into a snag on the first assembly , using a crosman .22 breech I had to ream out the opening into the barrel as it was about .01” too small . With no real base line it’s hard to say if the valve is closing faster , definitely no bell curve . Here are preliminary results shooting some 257420.
2000 psi , 873, 840,821
2500 psi , 973, 938, 900
3000 psi , 1018,920, 845
 So I did hit the intended target of 160 fpe on my first try . Another minor issue was the clearance between the barrel and air cylinder . I made a .1” spacer to get the spacing needed for clearance and this also allowed the QB78D  barrel bands to fit . Made another longer transfer port from 1/4” plastic plumbing tubing , which I’ve used in the past with great results . Now to wait for parts and decide what glass to install . I made my own probe using a grade 5 bolt , drilled and taped to 10-24 for a bolt handle . This I repurposed a 1-2” burr bit , which I always have lots of , this one had the edges dulled out from all the use. I’m not entirely pleased with my rear cocking knob , I find it looks like a drawer knob . I’ll be making a new one . So far I’m pretty pleased . I’ll be removing the barrel and hand lapp it to a mirror finish . I’d also like to work on the hammer spring set up , maybe get it to have some free flight .

[Hobbyman2007]

Mike I’m not sure what kind of project you have in mind , I wouldn’t mind helping with making some small parts if need be .

[darkcharisma]

I see a breech riser. What did you use? It looks like something from a hardware store?

[Hobbyman2007]

The breech riser was made from a sheet of plastic , actually a lid from a storage container . Had the lid but lost the container if that makes any sense.

[rkr]

The breech riser was made from a sheet of plastic , actually a lid from a storage container . Had the lid but lost the container if that makes any sense.

Plexiglass is useful for making breech risers. Heat it up to soften and then press inbetween the breech and tube and let it cool down.

[Hobbyman2007]

The breech riser was made from a sheet of plastic , actually a lid from a storage container . Had the lid but lost the container if that makes any sense.

Plexiglass is useful for making breech risers. Heat it up to soften and then press inbetween the breech and tube and let it cool down.

There’s a novel idea . I’ll give it a try. I think I have some 1/8” plexi somewhere in my mess.

[mackeral5]

Got the test rig up and running . I removed the ballance spool in the valve for faster closing and hopes of getting a bell curve .i did run into a snag on the first assembly , using a crosman .22 breech I had to ream out the opening into the barrel as it was about .01” too small . With no real base line it’s hard to say if the valve is closing faster , definitely no bell curve . Here are preliminary results shooting some 257420.
2000 psi , 873, 840,821
2500 psi , 973, 938, 900
3000 psi , 1018,920, 845
 So I did hit the intended target of 160 fpe on my first try . Another minor issue was the clearance between the barrel and air cylinder . I made a .1” spacer to get the spacing needed for clearance and this also allowed the QB78D  barrel bands to fit . Made another longer transfer port from 1/4” plastic plumbing tubing , which I’ve used in the past with great results . Now to wait for parts and decide what glass to install . I made my own probe using a grade 5 bolt , drilled and taped to 10-24 for a bolt handle . This I repurposed a 1-2” burr bit , which I always have lots of , this one had the edges dulled out from all the use. I’m not entirely pleased with my rear cocking knob , I find it looks like a drawer knob . I’ll be making a new one . So far I’m pretty pleased . I’ll be removing the barrel and hand lapp it to a mirror finish . I’d also like to work on the hammer spring set up , maybe get it to have some free flight .

Excellent results thus far.

As a comparison---my 3/4 Cothran with a 18.5" barrel  pushes Eunjin 43.2 gr 975fps at 3200psi.  So 90fpe out of 18.5" of barrel. with 29" of barrel the sky's the limit.

When I run the rod in the stem it gets one shot at 975fps then 1 in the 700's.  Without the rod I can get somewhat of a curve, but it is unpredictable----the first shot after sitting a while is subject to be slow. 

Carry on, I look forward to seeing where your journey takes us.

[Hobbyman2007]

Just finished sizing some bullets for this rifle . The barrel was fitted to a QB based gun previously so I know it was accurate . I’ve got 260-63- FN , 260-63-WFN  from NOE and 257420 ,257388 form Arsenal sized to .2578” . I don’t have a stock to put this test rig in but I might just clamp it down and shoot it anyhow . This is the most power I’ve got from an air rifle yet . Without knowing the actual specs on the Cothran tube that I hope is coming soon , I’d like to try a higher fill but not worth the risk for a few FPE.

[darkcharisma]

What do you guys think about the Walmart cheap plastic cutting boards? It is a bit thicker and cheaper...I need 1/2 riser.

[Hobbyman2007]

I think any plastic will do as long as will take the shape of both the breech and tube . I think I saw breech risers online just can’t remember where.

[KnifeMaker]

I'm thinking that I saw them at Magnum Air power or Hill, just can't remember which.   


I do remember that BNM had them at one time, however, far too unreliable to trust on getting what you order nowadays.