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Rifle barrel length vs muzzle velocity experiment.
Old 05-21-2005, 01:59 PM
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Default Rifle barrel length vs muzzle velocity experiment.

I'm starting this thread to solicit comments. You are encouraged to do so.

Jay started a thread in which he posted some published data regarding muzzle velocity versus barrel length in a revolver. Of course this generated some discussion, and out of that discussion came a suggestion from drinksgin to use a cheap military surplus 91/30 and conduct a 'new' experiment to determine what the actual MV vs bbl length values were for a rifle today. Here are a list of initial thoughts and some preliminary experimental design.

The 91/30 was selected because:
1) Beater milsurp rifles can be had for very 'cheap' - about $60 including shipping.
2) The 91/30 has a bbl length of 28.75" (73mm), from bolt face to muzzle. Therefore, it provides plenty of bbl to work with.
3) The 7.62x54R cartridge (AKA the Russian '06), with its .312-ish bore is a very "middle-of-the-road" cartridge. Therfore, its performance should represent the performance of "standard" cartridges from say .270-ish to .338-ish.

A few realities:
1) The rifle's historical value, however small that may be, will be destroyed. The rifles I have still have cosmoline (or a Soviet equivalent) in the bores and actions. I am quite sure that cosmoline wasn't put in those bbls more recently than about 1950.
2) For all intents and purposes, the rifle will be destroyed. Who among you would chose a Mosin Nagant action upon which to build a custom rifle? Of course the action and stock could be parted out, but really... that's usually more trouble than it's worth.
3) I don't have access to the necessary hardware, nor do I have the skills of a gunsmith. Therefore, an acceptable method needs to be found for shortening the bbl and crowning the muzzle. I have read in reputable places, that bbls can be cut off with a hacksaw, and recrowned with a carriage bolt and valve grinding compound. Unless and until I hear otherwise, that is the method of choice at the moment.

"Experimental design"
1) I intend to cut the bbl off in 1" increments. I could do 2" increments, but why interpolate?
2) I intend to stop at an 18.75" bbl length for legal reasons. That will provide data from 11 bbl lengths.
3) The number of shots fired per bbl length will be determined statistically. (This is a MAJOR departure for ANY such experiment I have EVER seen.) Starting with the 28.75" original bbl length, I'll shoot until the 95% confidence interval about the mean is plus AND minus 2%. For example: Let's say that the average 28.75" MV is about 3000 f/s. Then the 95% confidence interval around the mean would need to be equal to or less than 45 f/s. Meaning that 95% of the shots fired from that 28.75" bbl should fall between 2940 fps and 3060 fps. Sample size will be increased until that level of precision can be achieved. (I would drop this to plus and minus 1% (2970-3030), but factors other than bbl length - such as uniformity of milsurp ammo - might prevent such precision from ever being reached.)
4) Subsequent sample sizes will be determined by;
a) a minimum of 10, or a maximum of 21 or
b) when the expected value (the average), is statistically significantly different (at an alpha of 0.05), than the previous bbl length. In other words, shots will be taken until it can be said that there is a 95% surety that the average muzzle velocities between adjacent bbl lengths are in fact statistically different.
5) Accuracy/precision of point of impact is a non-issue.
6) All shots will be taken using milsurp ammo. This provides a repeatable standard. Therefore, there will be NO attempts to 'optimize' the charge to the bbl length. What "charge tuning" might do to mitigate shortening a bbl is not the question, and it is vital to minimize the number of variables under consideration.
7) At this time I would install a pressure sensor on the receiver to determine the pressure of each shot. While interesting, pressure values don't go directly to determining the relationship between bbl length and muzzle velocity.
8) The bbl will only be cleaned between cut-off events, and the first shot out of the clean bbl will not be used for statistical purposes.

OK. That will do it until I hear from folks here at THL.


Last edited by gitano; 05-21-2005 at 02:17 PM..
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