|
|
楼主 |
发表于 2013-12-28 22:31:28
|
显示全部楼层
Here's some of the standards most all mfg's follow for common modern centerfire cartridges with the exception of custom, black powder and/or specialty bores.
Nominal Caliber - Bore diameter - Groove diameter - Groove depth
.22 - 0.218"/0.219" - 0.224" - 0.0025"/0.003"
6.5mm - 0.256" - 0.264" - 0.004"
.270 - 0.270" - 0.277" - 0.0035"
7mm - 0.277" - 0.284" - 0.0035"
.30 - 0.300" - 0.308" - 0.004"
8mm - 0.315" - 0.323" - 0.004"
.338 - 0.330" - 0.338" - 0.004"
.375 - 0.366"/0.367" - 0.375" - 0.004"/0.0045"
.416 - 0.408" - 0.416" - 0.004"
.50 - 0.500" - 0.510" - 0.005"
The above does NOT include micro-groove or custom bores, those are simply the common standards. To increase the rifling depth, the bore diameter is reduced - thus, a custom match "tight .30" will normally have a bore diameter of 0.298" and a goove diameter of 0.3065"-0.307" resulting in a groove depth of 0.00425" to 0.0045". That's all fine and dandy but when you choke down the groove diameter by 0.00025" from nominal, pressures start increasing rapidly with common diameter bullets.
In reference to Ed's post, the theory back in the day was that shallower grooves made for less trapped fouling and it does work to a point but you're again limited on horsepower to whatever shear-force the bullet will withstand and still maintain grip on the rifling. The shallower groove rifles were more often than not intended to be used with paper patched bullets. I don't have the history memorized as to what round was the first intended to be used with a naked lead bullet (perhaps it was the .405win?) but the the point remains that the old Ballard cartridges maintained deeper rifling grooves which is why they shined in accuracy pushing naked lead bullets. If one looks at the old .50cal BPCR rounds, it was not uncommon for the bore diameter to be 0.500" with a 0.515" groove making the groove depth 0.0075"
In reference to Ten2Six:
The "assumed" nominal barrel dimensions for .303 British rifles are .303" bore and .311" groove diameter with a groove depth of 0.004" - some wartime rifles have groove diameters as large as .316" or more so when you're dealing with ANY mil-surp rifle, second thing you do after checking the headspace is to slug the bore. SAAMI spec's for pressure test barrel are 0.303" bore diameter and 0.314" groove diameter putting the groove depth at 0.0055"
The main concern with rifling is that it imparts the right amount of spin onto the bullet. Some of the most accurate jacketed bullet barrels ever produced had an 8-groove buttress shaped cut just 0.002" deep but it didn't catch on as it was extremely difficult and time consuming to manufacture. Another one that proved very accurate was the 3-groove narrow lobed land style but this too went by the wayside because of the difficulty & time of manufacturing.
There are those who argue that button/broach/cut/hammered rifling is more accurate than button/broach/cut/hammered ... reality says that no matter what process is used, it's the end result that matters most. I've had button rifled bores that shot exceptionally well but quite frankly, it's my absolute last choice of rifling method. There are certain facts about each process that make the initial choice of method for most people.
Buttons swage the rifling into the bore, they displace barrel material using brute force which puts a lot of stress into the barrel. Button rifled barrel must be heat-treated post-rifling and 99% of them require straightening before they can be used - straightening is done by bending and 99.9% of the time, a straightened barrel will change the POI in relation to it's temperature at the time the shot is fired because the straightening process puts stress right back into the barrel again. Buttons and broaches are also common to chatter and if they hit a hard spot in the blank, they'll be forced away from it making a bore that may be uniform in diameter but not straight - thus the "air gauge" claim isn't really telling of how the bore is going to shoot. Cut rifling, if done correctly, does not put stress in the barrel. Hammer forging does stress the barrel but in most cases the stress is part of the work-hardening process where the barrel is not subject to a post-process heat treatment but again, the exact process all depends on the mfg.
Don't discount the amount of stress that can be induced during the chambering and profiling operations either. You get someone hogging out the profile and you'll have a barrel laden with stress no matter how it was produced up to that point. |
|
IP卡
狗仔卡
发表于 2013-12-25 12:41:01
提升卡
置顶卡
沉默卡
喧嚣卡
变色卡
显身卡