Bench-Talk

I am getting ready to build a HVY VFS rifle and the IBS rules say you can be 29' long with muzzle diameter of .900.  What do you all consider the best length with a 17/18 twist barrel? I plan on using the 6.5x47 brass expanded up.

Also, I have some BIB 127 grain 10 ogive bullets, will they perform in 17/18 tw?

Thanks,

Gary.

Gary Christian:

I am getting ready to build a HVY VFS rifle and the IBS rules say you can be 29' long with muzzle diameter of .900.  What do you all consider the best length with a 17/18 twist barrel? I plan on using the 6.5x47 brass expanded up.

Also, I have some BIB 127 grain 10 ogive bullets, will they perform in 17/18 tw?

Thanks,

Gary.

Gary, via 1:17" twist, the 127s should shoot very well - no so with an 18" twist. I would not used a barrel in excess of 24" long - I'd opt for rigidity over the very slight velocity gain/reduction - you can expect only about 6-8 FPS gain/loss per 1.0" of barrel. On a HV taper, each two inch reduction in length also DECREASES muzzle deflection by about 35%! Except where BALANCE is THE issue, I would no longer opt for a barrel longer than 22".  Reducing the amplitude makes for easier "vertical" [dispersion] control! Don't overlook H-322! ;) RG

OK Randy, thanks for the info.

If I were to go to the 18 tw. what weight bullet(s) would work best with that twist? 

Thanks,

Gary.

Gary Christian:

OK Randy, thanks for the info.

If I were to go to the 18 tw. what weight bullet(s) would work best with that twist? 

Thanks,

Gary.

For true 1:18" twist barrels, FB bullets should not exceed 1.050" long - bullets of this length, at sea-level, STd. conditions, Sg will be 1.4+ . Few, if any of the available 118 Gr. thirty caliber bullets will exceed this length - most will be shorter, which is a good thing. I also make 125 Gr. bullets, using the 1.00" long jacket,  which are excellent via 1:18" twist barrels - again, the length of the bullet is the critical attribute: bullet length should be <1.050". Note: the BIB 127 Gr. bullet (see thread above) is based upon the 1.080" long jacket (finished bulley is about 1.10" OAL), thus needs a 17" twist, which imparts the minimum desirable Sg - 1.4.

Some button rifled barrels which are sold as 1:18" [nominal] twist are actually somewhat faster or,slower - for these barrels, if you will be shooting in extreme cold, it is wise to either use bullets based upon the shortest jackets, or, go one twist rate faster. With cut barrels, you should expect what you order - that is, a barrel stamped 1:18" should be 1:18" - less to "worry" over. With button rifled barrels, it never hurts to opt for one twist faster than the rate that produces 1.4 Sg.

 For practical purposes, bullet weight is merely, "along for the ride" - compared to form/geometry, it (weight) has relatively little to do with the gyroscipic stability factor (Sg). It is always best to employ a twist/bullet length combination which results in [at least] 1.4 Sg. You can check out the various recommended (1.4+ Sg) twist rates for BIB bullets on the products page on the BIB web-site - www.bibullets.com

Good shootin'!  RG

Very interesting stuff RG., Thank you very much

Regards,

Gary.

Hey RG !  Forgive an old Geezer for asking another question.

Since the 127 gr./10 ogive (purchased from you) should shoot fine in a 17 twist, what about the 115-118s, will they also shoot well in a 17 twist? 

Thanks again,

Gary.

Gary Christian:

Hey RG !  Forgive an old Geezer for asking another question.

Since the 127 gr./10 ogive (purchased from you) should shoot fine in a 17 twist, what about the 115-118s, will they also shoot well in a 17 twist? 

Thanks again,

Gary.

Gary, yes - it should deliver top performance with all of the above. The 127 Gr. (about 1.10" long) is the LONGEST bullet suitable for the 17" twist rate. Assuming a usable freebore (lead) length, anything shorter should work GREAT. RG

Mr. Robinett, in your first reply to this string, you said "each two inch reduction in length also DECREASES muzzle deflection by about 35%!"  How did you arrive at that number?  Is there some kind of formula that you can figure it?  I am interested in applying it to longer straight barrels such as used in long range shooting.

Thanks,

Joe

Joe Duke:

Mr. Robinett, in your first reply to this string, you said "each two inch reduction in length also DECREASES muzzle deflection by about 35%!"  How did you arrive at that number?  Is there some kind of formula that you can figure it?  I am interested in applying it to longer straight barrels such as used in long range shooting.

 

Thanks,

 

Joe

Joe, this information used to be (and may still be) available on Dan Lilja's web-site - it was included in an atricle titled [something like] A look at the Rigidity of Benchrest Barrels  - I am not an engineeer and can only parrot the content. I do recall that the barrels considered in Dan's atricle featured the maximum "legal" 1.250"  shank (cylindrical section) diameter  and the various Benchrest tapers: HV; LV and Hunter: worthy of note - a Hunter taper, combined with a 1.250" diameter cylinder, in "stiffer" than the typical LV taper! Look for the aritcle on Dan's web page: www.riflebarrels.com .  Dan may have included the formulas. Good shootin'! RG

http://www.riflebarrels.com/articles/barrel_making/rigidity_benchrest_rifles.htm

Good luck and good shootin'! RG

Thanks for the link.  I appreciate it.

Joe