GUNSMITHING: A ri­fle buyer’s guide to twist rates

Jonny con­tin­ues his ri­fle se­ries with this in-depth ar­ti­cle look­ing at twist rates and im­prov­ing ac­cu­racy. Be warned, there is some high-level maths thrown in, you know, just for fun!

Sporting Shooter - - Contents -

Twist Rates

To max­imise the ef­fects of ri­fling, the spi­rals must be tai­lored to the spe­cific cal­i­bre, type of shoot­ing and type of bul­let. This tai­lor­ing can be done us­ing twist rate, which refers to the tight­ness of ri­fling in the bar­rel.

The num­bers in­volved are de­scrib­ing one rev­o­lu­tion per num­ber of inches. For ex­am­ple: ‘1 in 8’ refers to 1 com­plete rev­o­lu­tion in 8" of bar­rel. This can be checked eas­ily on any ri­fle by push­ing a clean­ing rod down the bar­rel, mark­ing the rod, and al­low­ing the ri­fling to nat­u­rally twist it.

Once the rod has turned 360°, the length of rod can be marked again, and the dis­tance can be mea­sured. Some guns will have it printed on the side and most man­u­fac­tur­ers will have gun specs on their web­site.

The gen­eral rule of how high the twist rate should be is de­pen­dent upon the size and weight of the bul­let. The smaller/lighter the bul­let, the more quickly it needs to twist, while big­ger/ heav­ier bul­lets need less twist or they will over-sta­bilise and start to tum­ble. See page 50 for the av­er­age twist rates on some pop­u­lar cal­i­bres.

Some ri­fles come with a stan­dard twist rate based on this av­er­age, oth­ers give op­tions of a few, and it is pos­si­ble to have ri­fle bar­rels cus­tom-made to en­sure the per­fect twist rate for bul­let sta­bil­i­sa­tion. For most peo­ple, the twist rate a fac­tory ri­fle is sup­plied in will be more than ad­e­quate, given that the ef­fects of a non-per­fect twist rate to bul­let match can rarely be seen in dis­tances of up to 100m.

So, how is this all cal­cu­lated? Well, a Bri­tish math­e­ma­ti­cian named Al­fred Ge­orge Green­hill (1847-1927) de­vel­oped a for­mula to work this out us­ing a mix­ture of known fac­tors. Here is his equa­tion, plus an ex­am­ple us­ing Hor­nady .243 Win 58gr V-Max.

AL­FRED GE­ORGE GREEN­HILL

EQUA­TION

Twist Rate = CD² — SG L x — 10.9 Where: C = 150 (180 when muz­zle ve­loc­ity > 2,800 f/s) D = Bul­let di­am­e­ter in inches L = Bul­let length in inches SG = Spe­cific Grav­ity = 10.9 for lead-core bul­lets

EX­AM­PLE Hor­nady .243 Win, 58gr V-MAX = 11.17

150x0.243² — 10.9 0.793* x — 10.9 = 11.17

This was later dis­puted by an­other chap named Don Miller. Miller be­lieved that the for­mula should be more pre­cise, and de­signed a fur­ther for­mula with the as­sump­tion that the ve­loc­ity is 2,800fps, stan­dard tem­per­a­ture is 59°F, and pres­sure is 750mm Hg with 78% hu­mid­ity.

These as­sump­tions tech­ni­cally should change based on ex­act mea­sure­ments, but the dif­fer­ence is so lit­tle that it can be counted as in­signif­i­cant and there­fore ig­nored. His equa­tion and an ex­am­ple us­ing the same am­mu­ni­tion as be­fore can be seen on the next page.

As­sum­ing that twist rates will al­ways use whole num­bers, Hor­nady 243 58gr V-max would work best through a 1 in 10 or 1 in 11 twist ri­fle. There­fore, a shooter who knows that this would be the best bul­let for their ac­tiv­ity would be best to own a ri­fle with this twist rate.

This can also help to ex­plain why ri­fles have bul­let pref­er­ences, and can be re­verseengi­neered to find an ad­e­quate bul­let for a spe­cific gun. See, it was worth read­ing this far through all that maths!

For those of us who are maths shy, there are many on­line cal­cu­la­tors ready to work out the per­fect twist rate without show­ing the for­mu­las used. Al­ter­na­tively, like me and many oth­ers, buy an av­er­age twist rate (and there­fore more ver­sa­tile) ri­fle, buy some av­er­age weight for cal­i­bre am­mu­ni­tion, and go and use it!

Bar­rel length

The min­i­mum le­gal bar­rel length for a Sec­tion 1 firearm in the UK is 30cm (11.811"), so a shorter bar­rel than this is not only im­prac­ti­cal, as you shall see, but il­le­gal!

There is a long-stand­ing de­bate about the per­fect bar­rel length among ri­fle shoot­ers. Many fac­tors, in­clud­ing cal­i­bre and in­tended use, are of­ten de­cid­ing fac­tors in the ex­act length suit­able for in­di­vid­ual shoot­ers.

Ma­noeu­vra­bil­ity and han­dling

When hunt­ing, ma­noeu­vra­bil­ity is vi­tal; avoid­ing be­ing no­ticed by prey is dif­fi­cult enough without try­ing not to hit trees and bushes with a cum­ber­some 28" ri­fle. Shorter bar­rels are both eas­ier to con­trol when walk­ing or crawl­ing, and more light­weight so you don’t wear out your shoul­der; they are also eas­ier to free hand or take ‘mildly sup­ported’ shots.

The bar­rel is tra­di­tion­ally the heav­i­est part of a ri­fle so, as length in­creases, the cen­tre of mass will move for­ward dra­mat­i­cally. For high seat, prone or bench shoot­ing, the longer bar­rel and as­so­ci­ated weight can be an ad­van­tage, mak­ing the ri­fle less prone to out­side move­ment.

Burn­ing pow­der

Some pow­ders burn faster than oth­ers. Slower burn­ing pow­ders tend to pro­duce more ve­loc­ity at a more pre­dictable rate, while faster burn­ing pow­ders reach a high ve­loc­ity quickly be­fore rapidly de­creas­ing.

Pre­dictabil­ity of the bul­let’s ve­loc­ity, course and tra­jec­tory is es­sen­tial for ac­cu­rately aim­ing over long dis­tances. A longer bar­rel has more time and dis­tance for a pow­der to burn en­tirely, mak­ing a slower pow­der the prefer­able choice.

How­ever, a faster pow­der will burn more ef­fi­ciently in a shorter bar­rel, which has less dis­tance for the pow­der to fully burn. Fac­tory bul­lets do not cater for lengths of bar­rels or burn rates; this is im­por­tant, as when un­burnt pow­der is left in a bar­rel re­peat­edly, the bar­rel’s life­time will de­crease. It is pos­si­ble that a shorter bar­relled ri­fle will burn out faster than a longer bar­rel. This mat­ters lit­tle if you don’t home­load, though!

The length of the ri­fle bar­rel has a pos­i­tive cor­re­la­tion with ve­loc­ity ob­tained from the bul­let used. Pres­surised gas in the bar­rel forces the bul­let for­wards, caus­ing ac­cel­er­a­tion for the en­tire time that it is in­side the bar­rel. Upon ex­it­ing the bar­rel, the ex­pan­sion of the gas is no longer un­der pres­sure, and the ve­loc­ity of the bul­let sta­bilises. The longer the bar­rel is, the more time the bul­let has to ac­cel­er­ate in. There­fore, as shown in the graph, longer bar­rels pro­duce higher ve­loc­i­ties.

While there is no known point in which bar­rel length no longer af­fects ve­loc­ity, it al­ways be­gins to even out, mak­ing a less sig­nif­i­cant dif­fer­ence to ve­loc­ity, and il­lus­trat­ing a sweet point for com­pro­mis­ing. In the graph, the as­cent can be seen flat­ten­ing at 24.75".

Most ve­loc­i­ties out of a 16" bar­rel are still very ca­pa­ble – but make sure you put the ri­fle over the chrono­graph as with cer­tain cal­i­bres/loads you can fall be­low 1,700ft/lb muz­zle en­ergy, mak­ing it not le­gal for deer in Eng­land.

Ac­cu­racy

It is im­por­tant to note that you should be able to out­shoot your ri­fle be­fore wor­ry­ing about mak­ing it more ac­cu­rate. That said, there are three main ways in which longer bar­rels are known to im­prove ac­cu­racy.

The first is less rel­e­vant to­day, but through­out his­tory, iron sights were the pri­mary method of aim­ing a weapon. Longer bar­relled mus­kets would there­fore be more ac­cu­rate as the dis­tance be­tween the fore sight and the rear sight were marginally longer, and there­fore more pre­cise. This is still a con­sid­er­a­tion for some ri­fles, although the vast ma­jor­ity of ri­fles are now able to hold tele­scopic sights that are far su­pe­rior in ac­cu­racy than iron sights!

The se­cond fac­tor is heat trans­fer. As a bar­rel be­comes too hot from suc­ces­sive shoot­ing, it can cause warpage in the bar­rel. Even slight warpage through­out a shoot­ing ses­sion can be detri­men­tal to con­sis­tency (and there­fore ac­cu­racy). A longer bar­rel has more mass avail­able to trans­fer heat in be­tween shots, thereby keep­ing the bar­rels cooler and less likely to warp. Gun­smiths have been try­ing to find other so­lu­tions for heat trans­fer. Varmint bar­rels are thick bar­rels de­signed to stop warpage and don’t have to be par­tic­u­larly long. Be care­ful with flut­ing, though, as some flut­ing can ac­tu­ally ac­cel­er­ate warp­ing.

Lastly, a longer bar­rel pro­vides a longer path for the bul­let to sta­bi­lize prior to ex­it­ing the bar­rel.

How­ever, with the above the­o­ries in mind, a large pro­por­tion of bar­rel ac­cu­racy can be put down to the stiff­ness or rigid­ity of the bar­rel. When

a shot is fired, the bar­rel vi­brates, start­ing at the ac­tion and in­creas­ing to­wards the muz­zle. The vi­bra­tions, though small, make a dif­fer­ence at the muz­zle to the di­rec­tion in which the bul­let will con­tinue its path. A more rigid bar­rel will vi­brate less and will be more ac­cu­rate.

While all these the­o­ries on ac­cu­racy are con­cep­tu­ally sound, lit­tle ev­i­dence has been found to show any dif­fer­ence in ac­cu­racy with re­gards to bar­rel length – de­ter­min­ing fac­tors of ac­cu­racy in ri­fle shoot­ing are more likely to be bar­rel qual­ity, trig­ger weight, bul­let qual­ity and wind strength.

Ef­fec­tively, the ar­gu­ment comes down to ve­loc­ity vs ma­noeu­vra­bil­ity. Ve­loc­ity nec­es­sary for tar­get shoot­ing at ex­treme dis­tances; ma­noeu­vra­bil­ity nec­es­sary to move around the wilder­ness when stalk­ing.

The bot­tom line in my mind is sim­ple – shoot­ers that mostly go stalk­ing and lamp­ing shoot stan­dard ri­fles with stan­dard ammo, be­cause the av­er­age set-up will eas­ily shoot 1 to 1.5 MOA, which is enough for shoot­ing deer and fox at 100-200m.

For long-range tar­get shoot­ers, twist rate and bar­rel length is vastly more im­por­tant, but these guys are shoot­ing at tar­gets I can’t even see. And be­fore I ac­ci­den­tally in­sult any­one else, I shall stop. Thanks for read­ing!

Choos­ing a long or short bar­rel is not as sim­ple as you might think

The weight, length and con­struc­tion of the bul­let you use will af­fect which twist rate is best

The length of the ri­fle bar­rel has a pos­i­tive cor­re­la­tion with the ve­loc­ity, which you can test us­ing a chrono­graph

Fluted bar­rels can dis­si­pate heat quicker

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