Rifle barrel length vs muzzle velocity experiment.

Started by gitano, May 21, 2005, 10:59:17 AM

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gitano

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.
 
Paul
Be nicer than necessary.

Gmoney

That sounds very interesting.....can't wait to see some results.....
-Greg
 
Personal field testing trumps everything no matter what Field and Stream says, what your degree of perceived manhood is, or what your buddies think.

M1Garand

Great thread, I'm interested to see your results.  I usually estimate roughly 50 fps per inch with my rifles compared to reload data and their barrel lengths.  So far has been in the ball park.

drinksgin (deceased)

Paul;
You could use a pickup bed grade Mauser and keep the 18" result for a pickup bed rifle or use it as a doner action for something you really want[no, no not a .50-90].\
Don.  ;D
NRA life, TSRA life, SAF life, GOA, CCRKBA, DEF -CON

Alboy

I have a question abd suggestion on the Crowning. To possibly save considerable effort.

Since accuracy is not a factor in this experiment why crown at all unless the bullets will go astray enough to endanger your chrono? A dremmel tool and fine stone would remove burs from the hack saw.

Now crowning the last cut makes sense and that may very well make a suitable truck/knock around camp gun especially with some handloading fine tuning.
Alboy
BLACKPOWDER WATERFOWLER
KATY TEXAS PRAIRIE
 
THIS TOO SHALL PASS

gitano

Point well taken Don and Alboy. But I don't think too much of MNs to start with. They're not as ugly as Enfields, (threw that in just for BH, but it's too true. :D), but they can't hold a candle to a Mauser. I might end up using it for a tent stake. (Ol' John keeps mentioning this thing called a "tomato stake". I thought tomatos grew on trees. ;) )
 
I've since visited a MN site, and after a few of them were revived from the "vapors" after hearing that I was going to "destroy" a sacred milsurp, I think I've found a source of suckers, er... I mean "collectors", that will take the "abused" firearm off my hands for more than I originally paid for it. I hate it when that happens. :D I also found a souce in Texas that is selling bbl'd action for $8 apiece, but you gotta buy 10 of 'em. I'm gonna call 'em tomorow, and see if I can't wheedle a single out of 'em for $15.
 
Alboy, I don't think the crowning is a BIG issue, but I could see some pretty ragged edges from a hack saw. Without some dressing, that roughness might affect the numbers a bit. The Dremel is a good idea.
 
Keep those cards and letters coming in.
 
Paul
Be nicer than necessary.

drinksgin (deceased)

May Czar Nicholas curse you for desecrating his pride and joy, [ I hope you really stick it to some silly milsurp collector].
I just have this silly idea , if you buy a gun , you are supposed to take it out in the field , shoot it a bunch , and if it wears out where it cannot be repaired, just hang it up on the wall and tell big lies about what you did with it!
;D
Don
NRA life, TSRA life, SAF life, GOA, CCRKBA, DEF -CON

Jay Edward (deceased)

Well...reckon I should chime in.  The answer to the 'muzzle crown' problem is at hand.  The 'crown' does not need to be convex...it can be 'flat', 'level', 'even' or whatever one wants to call it.

So the procedure is this:

Cut off the barrel as  straight as you possible can.  File it flat as you possibly can.  Thennnnn...the couterbore.  a 'piloted' counterbore will allow you to finish the muzzle crown.

The pilot needs to be 'close' and you can spin it in a drill press with  fine emery to make it a 'slip fit'.  Chuck the thing into a hand drill and go slow with an even pressure.  And I mean 'even pressure'. Don't let the counterbore chatter!!!

Here is a picture of said counterbore with pilot.

drinksgin (deceased)

I would say, the Dremel tool with a ball ended tool, or the 1/4" round head brass stove bolt and Clover valve lapping compound would be much more economical than a machine tool reamer and a lot easier to locate.
Don
NRA life, TSRA life, SAF life, GOA, CCRKBA, DEF -CON

gitano

Glad you joined the fray Jay. Are we talkin' a real counterbore here? One in which the bore is reamed larger below the level of the muzzle? Or are we talking a 'crown'?
 
Paul
Be nicer than necessary.

Brithunter

Hi All,
 
For a quick easy clean countersunk bore with no or little clean up have you considered the Chamfering tool for the reloading bench. Sure you may need a new one after this experiment but it should do the job ;) Oh the RCBs one is certainly hard enough to cut the steel .............. and yes I have tried it :)
 
 
Now Paul may that Big Roe Buck never appear in your sights :frown ................. Huh? Enfields ugly Hurmmmmmph :mad:
Go Get them Floyd!

gitano

Another good suggestion BH.
 
Watch the cursing there Limey, this is a family site! :D
 
Paul
Be nicer than necessary.

Jay Edward (deceased)

Quote from: gitanoGlad you joined the fray Jay. Are we talkin' a real counterbore here? One in which the bore is reamed larger below the level of the muzzle? Or are we talking a 'crown'?[/size]

Paul
[/size]

What we are speaking of here is a 'target' type crown (actually no 'crown' at all).  We are just making it dead flat so that no unevenness can affect the the POI.  In the picture below you can see 'A' which is a normal crown, 'B' which is a slight flatter design and the ends of the rifling are more recessed for protection and 'C' where it is a two step level surface.

The two step is done for protection over a long period.  Dead level gives you an excellent surface but there is no protection.  In this case I don't think any protection is needed since you are going to shoot a few rounds and then cut it (the barrel) off again.  

There are specific barrel crowning tools but, as usual, they cost quite a bit more than just a plain counterbore.

Now...if you are really clever...you will get your hands on a counterbore that also can be used in conjunction with taper, plug and bottom taps to cut the breech end of a muzzle loading barrel to install said breech plug.  That way you justify the counterbore instead of just owning another unique tool.;)

gitano

As I said, glad you joined the fray. :)
 
On a side note, I love those pictures.
 
Paul
Be nicer than necessary.

CAfrica

Gitano,
 
Good experimental design there.  I like the statistical bit but then I would have expected that from you.
 
On an aside, whereas the data out of your experiment would be relatively transferrable to "similar" calibres, I would postulate that there would be major differences for the big bangers.  If you have a large case with a lot of powder, so much that by the time the bullet leaves the BBL on the short bbl, the powder has not even burnt completely, surely the short bbl would more markedly affect velocity?
 
In short (no pun intended), the data from something like a 51mm case (your favourite) would not be applicable to (say) a 300 H&H Magnum?
 
Regards.
 
C

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