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Does a rifle bore taper?

11K views 41 replies 16 participants last post by  DK PHILLIPS In Memoriam 
#1 ·
I've got a MN 91/30 receiver & barrel that I want to cut to M44 length. My machinist friend has a lathe and the skills to cut and crown it.
He's not into guns whatsoever but thinks he heard somewhere that the bore of a rifle, by design, is tapered and becomes smaller towards the front. His concern is that the muzzle will be too large in diameter.
I've never heard of this and think it's untrue but I've agreed to find out before he cuts it.
What say you folks?
Thank you.
 
#2 ·
it depends. im not a barrel smith or machinist by any standards but i know a little something. there is such things as barrel contours. contour 1 is very skinny towards the front and thick in the back. countour 2 is thick in the back but not soo skinny in the front. countour 3 is thick in the back kinda thick in the front. contour 4 is like a bull barrel period. and military barrels was "made" for the design of the gun. now mosins have a billion different dementions. 762x54r net will tell you every diff on model
 
#4 ·
The bore's groove diameter should be consistent throughout its length, but the lands diameter will vary in a well used rifle, with the most wear, i.e. greater land to land diameter, near the breech.

Military rifles frequently have irregularly worn lands near the muzzle, resulting from careless use of steel cleaning rods. If the rifle in question has cleaning rod blight at the muzzle, cutting the barrel may well rejuvenate the rifle.
 
#9 ·
Not necessarily right but not pertinent to your rifle.

Tapering the bore by .001 or less increases accuracy, as discovered by the bench rest rifle makers over 100 years ago. It's hard to get that taper with conventional machinery so was pretty much ignored for mass production until hammer forging became popular. So today some quality hammer forged barrels have a bore taper.

But not your 91/30. And I doubt that you'll ever get one hole accuracy, so I'd have cut the barrel with a hacksaw, filed the end flat and square with the bore, then "crowned" it by chamfered it a little with a brass round head screw dipped in valve grinding compound chucked in an electric drill.
 
#11 ·
The older mini-30's had a tapered bore.
 
#12 ·
AFAIR only Herr Gehrlich made internally-tapered barrels. His so-called 'squeeze-bore' barrels were designed to shoot high-velocity armour-piercing projectiles, and , of necessity, did not last that long. The projectile had a set of soft 'squeezable' fins or flanges round it that deformed into the taper of the barrel as it neared the muzzle.

NOT a hand-held device, though.

tac
Supporter of the Cape meares Lighthouse Restoration Fund
 
#13 · (Edited)
I know of no shoulder fired rifle that has a tapered bore... the Germans had a taper bore AT gun. I have heard of progressive depth rifling in muzzleloaders... and gain twist rifling (Carcano)... a taper bore shoulder fired rifle would play havoc with operating pressure... you would need a robust cartridge case, and locking system. Gas escape provisions would be a real concern too.

There was a dolt that rechambered a Type 38 Arisaka to .30-06, but didn't have the barrel rebored from 6.5 to .30... I think it was written about in a 1959 American Rifleman article. I dont think I would have has the cojones to swedge a .308 projectile down a .264 bore. As fools are wont to do, he was uninjured, but complained that the recoil was bad.

P.S. The article is in the May 1959 American Rifleman, page 52.
 
#16 · (Edited)
jjk 308 is correct in saying at times guns have been developed to taper at end of barrel travel, squeeze bores i have hear them called.
but not your modle.
few made, custom, experimental black powder and smokeless- none--???? some hammer forged are given tapers made today that i have read recently.
 
#17 · (Edited)
In the context of 91/30s I have never heard of a tapered bore by design. Some bores in used rifles are uneven and show wear in different places.

Some military rifles have contoured or stepped barrels on the OUTSIDE.

However. We are talking a $80 M/N here. Some in VG to exc. are being advertised locally for less than that.

Why in the world would anyone consider having several hundred dollars worth of work done on a 91/30 when you can buy one with a new or like new bore for under $100 ?

If you want to just tinker, then cut the barrel with a hack saw and crown as posted.

Unless your machinist friend is doing it for free. Better to use a lathe. A good crowing tool, cost almost as much as the rifle. So using a another method is advisable.

If by his logic the bore would be too big after cutting. Nothing would happen except it would shoot like a shot gun. But in your case, just the opposite should happen. It should shoot better, since the lands half way down or so, should be in better shape than at the muzzle.

Lots of well used M/Ns have shot out muzzles, worn lands near the muzzle. Not neccesarily from cleaning. I think the muzzle wear from cleaning is internet myth.

Even the peasants that used M/Ns were smart enough to remove the bolt and clean from the receiver end. Give them some credit.

Due to crown and land wear in a lot of older rifle had the bores counter bored, drilled down to good lands, when they were refurbished.

We have a number of M/Ns in our gun club that have way over 10,000 rounds through them. Some of those rifles have throat and crown,(muzzle) erosion. Counterboring does help in the accuracy dept. None were ever cleaned from the muzzle end. Doesn't make sense to clean at the muzzle end. Just remove the bolt and clean from the other end. Muzzle land erosion comes with the territory of a well shot M/N, just from use, not from a cleaning rod.

Doing a expert job on a 91/30 barrel is not cost productive. The rifle with a cut down barrel will only shoot good enough for plinking, if that.

I have seen cut off sporter 91/30s that shot decent, but not stellar. Hacksaw and screw crowned will work if done right.

I cannot count the times I have seen people cut up M/Ns put synthetic stocks, scope, bi pod. etc etc. on them. The first question I ask is "How does it shoot?"
All to often I get a blank look "shoot??" "Well, I dont know" " I haven't shot it yet"

I would think shooting a rifle first to see if it warrants all that money and work, would be prudent.



If you want a an exc. shooting 91/30 find one with Tikka barrel in unissued or buy a M39 unissued the are exc. in accuracy up to 1,000 yds. as is.


On a side note. If your rounds keyhole after the work. It is usually a sign of a bad crown job. ( keyhole, is going into a paper target sideways) meaning the bullet was tumbling.
Its happened to me several times, when I first learned to re crown.

Bottom line IMO.

If you are into tinkering, it is a good project. But to what end ?
If things don't go right, like keyhole and other problems that crop up during a project like this. If nothing else it will be a learning experience. Not a good idea IMO to spend any amount of money on. Considering the end results of a project like this.
 
#21 ·
I'm not a gunsmith nor machinist, but have cut/crowned a few barrels by hand.

After cutting, have him check the bore center vs. the barrel OD center before crowning. Sometimes the bore is not perfectly in the center of the barrel. For proper crown, the bore, not the OD should be centered in the chuck before crowning.

Daniel39
 
#22 · (Edited)
Here's a link to an article in Precision Shooting Magazine that discusses the production of tapered bore rifle barrels by hammer forging. As a secondary benefit, aside from improving accuracy, the slight internal taper makes it easier to remove the mandrel after forging. All HK, Sako, Tikka, Sauer, Steyr and Ruger barrels are now made this way.

http://technology.calumet.purdue.ed...gazine - November- 2005 (Vol_ 53 - No_ 7).htm

One problem that has delayed the acceptance of hammer forging by custom barrel makers who still stick to the bored, cut rifling methods is that if the (expensive) mandrel is used for too long the rifling becomes irregular. Also the hammer forging equipment is expensive. But this is correctable with the application of some $$$.
 
#25 ·
#26 ·
Slightly tapered bores...

in rifle barrels are not uncommon, and the effect on accuracy is usually beneficial, if the taper is not excessive and the tighter end is at the muzzle. However, no standard military rifle barrel I am aware of has been so made by intent - the effect when present is usually an artefact of the methods of drilling and reaming.
I've made about a thousand cut-rifled barrels with traditional equipment and methods, and measured every one of them thoroughly (as well as many others made elsewhere). The methods we use produce a bore that is typically .0001" to .0003" tighter at the muzzle than at the breech, and other barrels I've inspected show a similar condition, which was common in carefully made barrels such as the National Match and other target-grade barrels made at Springfield Armory (and with the same equipment I use).
On the other hand, military barrels, particularly those made in wartime under increased production and relaxed inspection standards are often quite a bit less uniform: some I've air-gauged produced a graph of bore and groove diameters that looked like a corn snake that swallowed a dozen eggs - even one memorable BSA Martini MkIII heavy target barrel might have been a good one, had the maker kept the diameters to within, say, .001" from place-to-place internally (and no, it wasn't bulged, it was just badly made).
One other condition which is not uncommon is a bore which is not concentric with the outside of the barrel throughout its length (even sporting barrels), so that, when shortened any significant amount, the bore at the new muzzle is visibly off-center.
Having said all that, in most cases, shortening the barrel of the typical MILSURP rifle will not, of itself, result in decreased accuracy, so long as the work is properly done.
FWIW
PRD1 - mhb - Mike
 
#28 · (Edited)
You do it by lagging it out, polishing the bore and grooves while progressively increasing the diameter toward the breach end by working it more. Obviously this is real labor intensive and only the old bench rest maniacs of the turn of the 19th century did it back when labor was cheap. It would be insanely expensive today. And that's why tapered bores weren't really practical until hammer forging was developed.
 
#30 ·
Hawkins, et. al. ...

If you read carefully, you will see that I said 'about a thousand', which is a conservative number based on the nine years or so I've worked making barrels.
Traditional rifle barrels were/are not made by broaching, though many modern barrels are - the process having been first introduced on a mass scale about the beginning of WW2. Traditionally, the bore was drilled, reamed to size, and then rifled on a rifling bench or rifling machine which pulled a single cutter back and forth through the bore at the desired rifling pitch to cut one groove at a time until the desired groove depth was reached. The ancient rifling bench was usually used to cut a single groove at a time from start to finish, inserting a thin shim to raise the cutter to increase groove depth, the process being repeated for each groove, separately, and the work done by hand. The 'modern' rifling machines are designed to automatically index through the desired number of grooves, making a single pass at each and then raising the height of the cutter the preset amount for the next series of passes, again, the process being continued until the desired groove depth is reached - the machines being power-driven, of course.
The broaching process uses one or more 'broaches', which are very long cutting tools containing a successively larger- in -diameter series of cutting teeth, ending in the desired groove diameter, and machined on the rifling pitch desired in the barrel. This tool is driven through the drilled and reamed bore, and must also be mechanically rotated at the same pitch as the broach and finished rifling, since to drive it straight through the bore would not allow the broach to rotate, resulting in a larger bore, but no useable rifling.
The advantages of the traditional rifling machine are that it will accommodate any rifling design by making the appropriate cutter, and any rifling pitch within the range of adjustment of the machine - in the case of the Pratt and Whitney #1 sine-bar machine we use, any pitch between 0 and approximately one turn in five inches, infinitely adjustable (without fixed pitch stops, though with engraved scales as a guide), left or right-hand. The tooling is much less expensive and much longer-lasting (the typical broach costs thousands of dollars and has a finite service life due to the need to re-grind it to slightly smaller diameter each time it is sharpened).
While I did not say so originally (it being outside the scope of the original question), every barrel I made is hand-lapped: not to alter internal dimensions, but solely to improve interior surface finish by smoothing the remaining tool marks from reaming and rifling. My opinion is, that with modern barrel steels, you would lap yourself to death trying to achieve a taper by that method. The taper in the bore I spoke of is, as I said, an artefact of the drilling and reaming processes.
PRD1 - mhb - Mike
 
#32 ·
Hawkins:

In either case, the rifling tools do not establish the bore diameter or taper, if any: rather, the drilling and reaming do, prior to the rifling operation (though it would be possible to make a broach which also cut the original bore surface - which remains as the 'lands' of the rifling, it would be a poor practice to do so, since the original bore establishes the guiding surface for the rifling tools, and thereby controls the passage of the cutting instrument, serving as the basis of a finished barrel which is straight and in which the dimensions are uniform and accurate throughout its length).
It is the final reaming operation on the bore which establishes its diameter and surface finish (disregarding lapping after rifling, which, as I said, is done to refine surface finish, and not to alter bore or groove dimensions) - the factors which most affect the diameter, surface finish and taper, if any, in the finish-reamed bore are the speed and feed of the reaming machine, the geometry and diameter of the reamer itself, its condition, and the flow (pressure and volume) of the coolant/cutting lubricant delivered to the reamer in its passage through the bore: all of these factors are under the direct control of the barrel maker, and the last named (coolant flow) can be varied during the reaming pass to effect a taper in the finished bore.
Any well-trained, old-school machinist knows several tricks for 'influencing' the behavior of reamers, and anyone who has made good barrels by traditional methods has taken a post-grad course. It's not rocket science, but it is a highly skilled trade.
PRD1 - mhb - Mike
 
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