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'''Bolt thrust''' or '''breech pressure''' is a term used in [[internal ballistics]] and firearms (whether small arms or artillery) that describes the amount of rearward force exerted by the [[propellant]] gases on the [[Bolt (firearm)|bolt]] or [[Breechblock|breech]] of a [[firearm action]] or [[Breech-loading weapon|breech]] when a projectile is fired. The applied [[force]] has both [[Euclidean vector#Length|magnitude]] and [[Direction (geometry, geography)|direction]], making it a [[Vector (geometric)|vector]] quantity.
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Bolt thrust is an important factor in weapons design. The greater the bolt thrust, the stronger the locking mechanism has to be to withstand it. Assuming equal engineering solutions and materiel adding strength to a locking mechanism causes an increase in weight and size of locking mechanism components.<br>
Bolt thrust is not a measure to determine the amount of [[recoil]]/[[free recoil]].
 
 
==Calculating bolt thrust==
With a basic calculation the bolt thrust produced by a particular firearms cartridge can be calculated fairly accurately.
 
===Formula===
 
:<math>\vec{F}_{bolt} = P_{max} \cdot A_{internal}. </math> <ref>[http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htm A Look at Bolt Lug Strength By Dan Lilja]</ref>
 
where:
*''F<sub>bolt</sub>'' = the amount of bolt thrust
*''P<sub>max</sub>'' = the maximum (peak) chamber [[pressure]] of the firearms cartridge
*''A<sub>internal</sub>'' = the inside area (of the cartridge case head) that the [[propellant]] [[deflagration]] gas pressure acts against
 
Cartridge case heads and chambers are generally [[Circle|circular]]. The [[Area of a disk|area enclosed by a circle]] is:
 
:<math>Area = \pi r^2 \approx 3{.}1416 \cdot r^2. </math>
 
where:
*''π'' ≈ 3.1416
*''r'' = the radius of the circle
 
Equivalently, denoting the diameter of the circle by ''d''.
 
:<math>Area = \frac{\pi d^2}{4} \approx 0{.}7854 \cdot d^2. </math>
 
[[File:Bolt thrust.JPG|thumb|100px|The green line denotes the internal case head diameter and the red line the external case head base diameter of a rifle cartridge case.]]
 
A practical problem regarding this method is that the internal case head diameter of a particular production lot of cartridge cases (different brands and lots normally differ dimensionally) can not be easily measured without damaging them.
 
===Friction effects===
A complicating matter regarding bolt thrust is that a cartridge case expands and deforms under high pressure and starts to "stick" to the chamber. This "friction-effect" can be accounted for with finite elements calculations on a computer, but it is a lot of specialized work and generally not worth the trouble.<ref>[http://www.varmintal.net/abolt.htm Stolle Panda Bolt Stress and Deflection Analysis]</ref>
 
By oiling proof rounds during [[NATO EPVAT testing]] procedures, NATO test centers intentionally lower case friction to promote high bolt thrust levels.
 
==Practical method to estimate bolt thrust==
Instead of using the internal case head diameter, the external case head base diameter can also be measured with a [[caliper]] or [[micrometer]] or taken from the appropriate [[C.I.P.]] or [[SAAMI]] cartridge or chamber data tables and used for bolt thrust estimation calculations.
 
The basic calculation method is almost the same, but now the larger outside area of the cartridge case head is used instead of the smaller inside area.
 
:<math>\vec{F_{bolt}} = P_{max} \cdot A_{external}. </math>
 
where:
*''F<sub>bolt</sub>'' = the amount of bolt thrust
*''P<sub>max</sub>'' = the maximum (peak) chamber pressure of the firearms cartridge
*''A<sub>external</sub>'' = the outside area of the cartridge case head
 
This method is fine for getting a good estimate regarding bolt thrust and assumes an overly large area that the gas pressure acts against yielding pessimistic estimations, generating a safety margin in the process for worse case scenarios which can result in increased maximum (peak) chamber pressure of the firearms cartridge, like a round that is chambered in an already very warm [[Chamber (firearms)|chamber]] that can result in [[cooking off]] (i.e. a thermally induced unintended firing).
 
===Bolt thrust estimations for various pistol/revolver cartridges===
{| class="wikitable"
|-
! | '''Chambering''' || '''P1 diameter''' (mm) || '''A<sub>external</sub>''' (cm<sup>2</sup>) || '''P<sub>max</sub>''' ([[Bar (unit)|bar]]) || '''F<sub>bolt</sub>''' ([[Kilogram-force|kgf]]) || '''F<sub>bolt</sub>'''''
|-s
| [[.22 Long Rifle]] || 5.74 || 0.2587 || 1,650 || 435 || {{convert|4268|N|lb-f|abbr=on}}''
|-
| [[9x19 mm Parabellum]] || 9.93 || 0.7744 || 2,350 || 1,820 || {{convert|17847|N|lb-f|lk=on|abbr=on}}''
|-
| [[.357 SIG]] || 10.77 || 0.9110 || 3,050 || 2,779 || {{convert|27248|N|lb-f|abbr=on}}''
|-
| [[.380 ACP]] || 9.70 || 0.7390 || 1,500 || 1,130 || {{convert|11085|N|lb-f|abbr=on}}''
|-
| [[.40 S&W]] || 10.77 || 0.9110 || 2,250 || 2,050 || {{convert|20101|N|lb-f|abbr=on}}''
|-
| [[10 mm Auto]] || 10.81 || 0.9178 || 2,300 || 2,111 || {{convert|20701|N|lb-f|abbr=on}}''
|-
| [[.45 ACP]] || 12.09 || 1.1671 || 1,300 || 1,517 || {{convert|14879|N|lb-f|abbr=on}}''
|-
| [[.454 Casull]] || 12.13 || 1.1556 || 3,900 || 4,507 || {{convert|44197|N|lb-f|abbr=on}}''
|-
| [[.500 S&W Magnum]] || 13.46 || 1.4229 || 4,270 || 6,076 || {{convert|59584|N|lb-f|abbr=on}}''
|}
 
The P1 (cartridge case base) diameters and P<sub>max</sub> used in the calculations were taken from the appropriate [[C.I.P.]] data sheets.
 
===Bolt thrust estimations for various rifle cartridges===
{| class="wikitable"
|-
! | '''Chambering''' || '''P1 diameter''' (mm) || '''A<sub>external</sub>''' (cm<sup>2</sup>) || '''P<sub>max</sub>''' ([[Bar (unit)|bar]]) || '''F<sub>bolt</sub>''' ([[Kilogram-force|kgf]]) || '''F<sub>bolt</sub>'''''
|-s
| [[5.45x39mm]] || 10.00 || 0.7854 || 3,800 || 2,985 || {{convert|29268|N|lb-f|lk=on|abbr=on}}''
|-
| [[.223 Remington]] || 9.58 || 0.7208 || 4,300 || 3,099 || {{convert|30396|N|lb-f|abbr=on}}''
|-
| [[7.62x39mm]] || 11.35 || 1.0118 || 3,550 || 3,592 || {{convert|35223|N|lb-f|abbr=on}}''
|-
| [[.308 Winchester]] || 11.96 || 1.1234 || 4,150 || 4,662 || {{convert|45722|N|lb-f|abbr=on}}''
|-
| [[.300 Winchester Magnum]] || 13.03 || 1.3335 || 4,300 || 5,734 || {{convert|56230|N|lb-f|abbr=on}}''
|-
| [[.300 WSM]] || 14.12 || 1.5659 || 4,450 || 6,968 || {{convert|68334|N|lb-f|abbr=on}}''
|-
| [[.300 Remington Ultra Magnum]] || 13.97 || 1.5328 || 4,480 || 6,876 || {{convert|67341|N|lb-f|abbr=on}}''
|-
| [[.338 Lapua Magnum]] || 14.91 || 1.7460 || 4,200 || 7,333 || {{convert|71914|N|lb-f|abbr=on}}''
|-
| [[.300 Lapua Magnum]] || 14.91 || 1.7460 || 4,700 || 8,339 || {{convert|81776|N|lb-f|abbr=on}}''
|-
| [[.50 BMG]] || 20.42 || 3.2749 || 3,700 || 12,117 || {{convert|118829|N|lb-f|abbr=on}}''
|-
| [[14.5x114mm]] || 26.95 || 5.7044 || 3,600 || 20,536 || {{convert|201387|N|lb-f|abbr=on}}''
|}
 
The P1 (cartridge case base) diameters and P<sub>max</sub> used in the calculations were taken from the appropriate [[C.I.P.]] data sheets.
 
==References==
{{Reflist}}
 
==External links==
*[http://lutz-moeller-jagd.de/Waffen/Blaser/R93/Blaser-R93.html Blaser R93 Unfälle - zur  Verschlußbelastung {{de icon}}]
*[http://lutz-moeller-jagd.de/Waffen/Technik/Zylinderverschluss/Zylinderverschluesse.htm Zylinderverschluß - Verschlußkräfte {{de icon}}]
 
[[Category:Firearms]]
[[Category:Firearm actions]]
[[Category:Firearm terminology]]
[[Category:Ammunition]]

Latest revision as of 21:13, 1 April 2014



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