7" Twist .257 cal Barrel Mandrel on Order

[rsterne]

I have just placed an order with Mike Sayers at TJ's for him to make a custom mandrel to produce 7" twist barrels for my new 100 gr. & 113 gr. BBT's in .257 cal.... The 113 gr. is basically a scaled down version of the .308 "Whiteout" bullet that won the EBR.... It won't require Helium to achieve the power necessary to drive it into the mid 900s, however.... Something between 3600-4000 psi air with a 30-32" barrel should do the job.... I'll let you know when the mandrel is done and barrels can be ordered.... Below are what the bullets look like....

    

Bob

[jimreed1948]

I'm glad you posted this.  I was just getting ready to order a 10 twist but will wait to see your results.  What rifle will you put the barrel on?

[rsterne]

Nothing built for it yet.... and not much point if the 100 gr. and 113 gr. bullet molds never reach production.... but I didn't want guys to have the excuse of not ordering molds because there was no barrel available for them.... It's a "putting my money where my mouth is" kinda thing.... 

Bob

[jimreed1948]

I've been exchanging a few emails with Randy at Accurate Big Bore Ammo about making an AF .257 with a 10 twist.  Maybe I should get the 7 twist and see how it does first.

[rsterne]

It really depends on what bullet you are going to use.... If it's a heavy flat base bullet, the 10" will work just fine.... for a heavy boattail you will need the 7".... If you are using flat base bullets under about 80 gr. the 14" seems to be the standard....

Bob

[Job]

Bob, you should open a research institute of air weapons 

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[rsterne]

The Miller Twist rule does not take into account the reduction in stability from a boattail.... Here is the calculator I use....

http://www.geoffrey-kolbe.com/barrel_twist.htm

Bob

[Rizen 1]

I will be anxiously waiting for the barrel...

[rsterne]

Mike confirmed today that the barrels will be available in 7/16", 1/2", or 9/16" OD and are $4.75 per inch.... I'll let you know when the mandrel is done.... the specs are....

0.257" groove
0.250" land
6 grooves
7" twist

Bob

[Job]

Stability Factor would be excellent,but I think that the bore quickly dirty with lead.

[rsterne]

In order to stabilize such long bullets, you have no choice but a fast twist.... A slightly harder alloy might be necessary, and the velocity will be important, as will bore condition (smoothness) to avoid frequent cleaning....

Bob

[Gr8Gorilla]

I'm definitely going to be in on this!

[rsterne]

I received a PM yesterday from a shooter who was concerned about the OD of the barrels from Mike at TJ's, so he contacted 14 other barrel manufacturers to find out if they could or would produce what we need.... Only one, Pac Nor, was at all interested, and they quoted just shy of $500.... Here is the answer I sent to his PM....

Unfortunately, Mike cannot make a 5/8" OD in .257 cal, but it is a simple matter to sleeve the barrel, that is how they are intended to be used.... You could use either CrMoly tubing, or carbon fibre tubing.... The 1/2" could be increased to 5/8", and the 9/16" could be increased easily to 11/16", or larger on both sizes by using thicker tubing....

Two common sizes of CrMoly tubing come to mind, both are 3/4" OD.... The 0.095" wall is 0.560" ID and a 9/16" barrel could be turned/ground to fit.... The 0.120" wall is 0.510" ID, and the 1/2" barrel should slide right in (my choice).... I understand Mike can grind the OD of his barrels to any finished size you want.... Loctite 638 will do an excellent job of bonding them together....

Alternately, if you want to use carbon fibre tubing, there are lots of choices available.... High modulas tube by Rockwest Composites would my choice.... They have a 0.5" ID x 0.63" OD that is $117 for a 72" length (enough for two 1/2" barrels).... Alternately, you could double sleeve the 1/2" barrel with a unidirectional inner tube, with a 0.608" OD ($50 for 72") and then use a high modulas 5/8" ID x 3/4" OD outer layer ($125 for 72").... They also have some 0.553" ID uni tubes which would alloy you to start with a ground down 9/16" blank.... including one which is 0.793" OD for $116 for 50", which would give you that OD in one step....

You have to remember, that what Mike produces are barrel LINERS, they are intended to be installed inside a thicker tube to add rigidity.... Another method that can be employed is to install them inside a much larger tube and tension the barrel by compressing the outer tube.... That is the method I will likely use in my own rifle....

Bob

[jimreed1948]

0.257" groove
0.250" land
6 grooves
7" twist

Tell me the difference in having a barrel of 6 grooves vs one with 3 grooves.

[jimreed1948]

You have to remember, that what Mike produces are barrel LINERS, they are intended to be installed inside a thicker tube to add rigidity.... Another method that can be employed is to install them inside a much larger tube and tension the barrel by compressing the outer tube.... That is the method I will likely use in my own rifle....

Bob
[/quote]

This is very interesting.  I found a good explanation of the compressing here if anyone else is interested:  http://www.google.com/patents/US4211146

How do you plan on doing the compression?

[rsterne]

Here is the latest version I used on my 7mm Hayabusa.....



The Hatsan Air Stripper compresses the Belleville washer stack, which in turn pushes on the collar on the end of the carbon fibre shroud while the stripper pulls on the muzzle.... The breech end of the shroud is a slide fit on a collar that is part of the receiver.... 



The barrel is held in place by a threaded ferrule....



Here are photos of the assembly with and without the shroud (compression member) in place....





While I haven't had the opportunity to fully test this system, QVTom has used a tensioned barrel and finds that once enough tension is applied to the barrel that group sizes shrink.... I believe he uses this system on the Slayer, but without the Belleviles, simply torqueing the stripper to apply the necessary force....

You asked about the number of grooves / lands.... I chose 6 lands to allow enough grip on the bullet to sustain the approx. 8 lbs. of peak rotating force required to accelerate the bullet circumferentially in the 7" twist.... In addition, I think that 6 lands centers the bullet better when it enters the rifling....

Bob

[jimreed1948]

I just modified this post after rereading what you listed.  So the shroud slip fits over a collar attached to the breach and when tightening the air stripper it then puts pressure on the barrel to help it stop flexing.  Is this correct?  So while testing, you either tighten or loosen the air stripper to adjust the barrel tension? 

[rsterne]

Screwing in the stripper pushes on the Bellevilles, which in turn push against the shroud, putting it in compression.... At the same time, the stripper pulls on the threaded barrel, putting it in tension.... One turn on the stripper, which is 1/2"-20 threads, is 0.050", and there are 5 Belleviles, so each one is compressed 0.010", which takes about 800 lbs. of force.... The maximum load if compressed flat is 1335 lbs., that takes 0.018" per Belleville, so a total of 0.090", a bit less than 2 turns of the stripper.... Bellevilles collapsed completely flat tend not to spring back to their original thickness, however, so in practical terms, about 1.5 turns on the stripper is all I would use, which is about 1200 lbs. of tension.... If you need more than that, the Bellevilles can be doubled up.... For instance six could be used, arranged )) (( )) and that would allow about 2400 lbs. of tension to be applied to the barrel with just under 1 turn of the stripper....

The idea is that as you increase the tension in the barrel, you increase the resonant frequency while decreasing the amplitude.... This is exactly what happens if you make the barrel larger in OD.... In both cases, the barrel is stiffer.... and stiff barrels tend to be more accurate, or at least easier to tune to be accurate.... QVTom estimated that anything over about 800 lbs. of tension didn't seem to decrease the group size further on the gun he was experimenting on.... If other barrels act in a similar manner then running the Bellevilles about 20% tighter than where the group size reaches a minimum should keep the group small over a wide range of temperatures....

Bob 

[rsterne]

The mandrel is finished.... Barrels can now be ordered from TJ's at his regular price of $4.75 / inch.... They are available in 7/16", 1/2", or 9/12" OD.... Mike @ (859) 635-5560....

Bob