c-g-designs

THOUGHTS ON BARREL TWIST RATES  AND BARREL LENCHTS.

            Following a question raised on a french forum concerning  the eventual  use of a .308 barrel with a 8 inches twist intended for subsonic projectiles, I was led to ask myself some questions. They are only interrogations and I would like to have a discussion started on thoses subjcts.

---A) The twist rate (or pitch)  is a relative easy notion, it would seem to me more important to consider  the rifling angle  of this twist, as for an identical twist rate,  tha twist angle vary significantly  in relation with the grooves diameter and the calibre.

Twist lenght and twist angle relation :

        1°) :Calibre .243 (6,20mm),twist 1 in 7 1/2, twist angle = 6°.

        2°) Calibre.264 (6,70mm), twist 1 in 8, twist angle = 6°.

        3°) Calibre.284 (7,20mm), twist  1 in 9, twist angle = 5°40’. 

        4°) Calibre .308 (7,82mm), twist  1 in 10, twist angle = 6°.

        5°) Calibre .308 (7,82mm), twist  1 in 13 (Palma),twist angle = 4°30’.

        6°) Cal.308 (7,82mm), twist 1 in 8,  twist angle =  7°10’.

        It is to be noticed that, for the 4 calibres of reference , and for heavy bullets, the recommended twists have an angle close to 6°.      

        For the lighter bullets (Palma in .308) the twist angle is reduced to 4°30minutes.            

        B) Barrel lenghts :

        The barrel lenghts, as commonly expressed in overall lenght is  is also not too significant, and would prhaps be better, as in artillery, to be expresed in number of calibres of the useful bore lenght, from the leade entrance to the muzzle. This is what I would name the useful lenght of a barrel.

        For the examples , the leade position has been considered the same for the examples hereunder (55mm from the rear of barrel) for the 4 diameters, although they vary for each cartridge. Effectively, the ‘S’ dimension, which is the leade entrance to the cartridge base, is 37,42mm for the 6mmBR and 55,36 for the .308 Winchester.

        For a same OAL of 30 inches  (762mm),supposing a leade situated at 55mm, the useful lenght (707mm) expressed in number of calibres, would represent :

                1°)Calibre .243, bullet diameter  6,20mm :

                707 / 6,20 = 114 calibres.

        2°) Calibre .264, bullet diameter  6,70mm :

                707 / 6,70mm = 106 calibres.

        3°) Calibre .284, bullet diameter  7,20mm :

                707 / 7,2 =  98 calibres.

        4°) Calibre .308, bullet diameter  7,82mm :                        707 / 7,82 = 90 calibres.

Matters for thoughts :

        1°) While the caliber .308 in 90 calibres seem to receive general acceptance , the lenghts of 114 and 106 calibres for the smallers .243 and .264 are they not excessive, provoking friction, temperature and wear ?.

        Also, the said frictions and the progressive increase of the pressure chamber volume reducing the push of the gases against the smaller bullet section could well  be

resulting in the absence of velocity increase in the forward part of the barrel, or even having a negative effect on velocity ?.

        2°) Does the extra lenght  bring better performance, or the factors exposed 1°) above does not have a negative effect on general performances ?.

        3°) Would it not be reasonable to consider a upper limit on barrels lenghts, say 100 calibres as maximal lenght, to not overgo ?

        This still allowing :

         --A lenght of 32 inches for the .308 (96 calibres).

        -- The .284 just over the line at 31 inches (102 calibres) ?.

        --The .264 being 99 calibres for a lenght of 28 inches.

        4°) I do not think the shape and number of rifling grooves be of consideration here ?.

R.G.C