# Ballistics for Excel Forum

## BfX => Ballistics => Topic started by: ThunderDownUnder on June 18, 2014, 11:31:03 PM

Title: Recoil, Torque & Barrel Vibration
Post by: ThunderDownUnder on June 18, 2014, 11:31:03 PM
I started shooting 230gr Berger hybrid bullets in my 300WM for use at long range in F-Open competition. While not driving these bullets hard at 2850 fps the 10kg rifle still generates considerable recoil. This set my accuracy back initially until I learned some new skills to cope with the increased recoil.

One of the things I have set out to do is understand the forces involved. I am hoping this might lead to understanding stock design and the best place to add weight to the rifle to maximise the allowed limit of 10kg. So far I have put together a spreadsheet for free recoil calculation, (force and velocity) and also for "torque". The spreadsheet can be downloaded from here: http://fclassdu.com/origin/wp-content/uploads/2014/06/Rifle-Torque-Recoil-Calculator.xls

To state the obvious, recoil comes into consideration for accuracy only for the time the bullet is in the barrel. That usually means for the first few milliseconds and the first few millimeters of rear recoil movement. The rotation of the barrel due to the torque effect for my loads is approx 0.06 degrees as the bullet leaves the muzzle.

I am also very interested in barrel vibration and wonder if anyone using this forum has looked at calculating the effects of barrel profile, weight and load effects on barrel oscillations. I found this link really interesting. http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/barrel_vibrations.htm

Ian
Title: Re: Recoil, Torque & Barrel Vibration
Post by: mman on June 22, 2014, 10:11:20 AM
You are digging into fundamental questions about rifle internal ballistics. I been interested in this for years and when I started using finite element method (FEM) for guns I had a dream of 'perfect' gun design. That design would minimize vibrations and ultimately leave only intermediate and external ballistic effects to harm accuracy.

First thing you have to accept is that you can't use analytical methods to accurately estimate recoil nor vibrations. Only sophistic numerical methods like Finite element method are capable of taking into account everything from time depended excitation to flexible materials and supports. Analytical methods like your excel spreadsheet can be pretty good in estimating what are total forces, moments and torques when whole recoil phenomenon is over. However you should be interested in what happens while bullet is still in the barrel. That's far more complicated since material flexibility plays major role in that. Usually only minor part of the recoil is transferred to your shoulder during barrel time. Rest of it comes after and that has only psychological effect (which can be important as well). See picture below. It is calculated recoil force felt on your shoulder for gun and ballistics very similar to your's. This is also very sensitive to hold, recoil pad and even your clothing and body composition. The point still remains; usually most part of the recoil comes after.

(http://i.imgur.com/7UkaLcc.jpg)

From the Internet you can find lot's of general rules what kind of designs can minimize or optimize vibrations. However first thing you learn doing FEM is that gun vibrations are actually really sensitive to many things. For example change of load, stock or rifle scope can significantly alter vibration behavior. This leads to conclusion that general rules that always have a positive effect to accuracy are very hard to come by.

Okay, how did I do in my way to ultimate gun design? As I'm still here writing and not enriched by my inventions so pretty poorly I guess. I have still found many useful trends that work most of the time. Beyond that you have to calculate everything case by case. Usually that requires calibration of calculation model with measurements. However in the past I have got great results even without calibration. I still believe luck has played a some role in those situations. Couple of the most successful cases are combination gun projects where I was able to calculate how two free floating barrels should be assembled in relation to each other so that zero would be the same. According to calculation relative vibration amplitude was about 6 MRAD and same zero was achieved withing 0,5 mrad. The other barrel was shotgun barrel so accuracy was good enough. At the moment I have couple of gun projects going on where I have optimized barrel profile for bending vibration. Load testing will give me feedback how it did work.

About "BARREL VIBRATIONS SIMULATOR" you posted; it's first time I see it. It still obvious that it can't be useful in optimizing barrel profile. Reason is simple; too few input parameters. Only barrel profile is taken into account and excitation can't be altered to mention couple of things that would alone ruin the accuracy. I still agree many points mentioned in the text below calculator.