rifling cross section [Archive]

Artful

10-10-2016, 02:53 PM

Any of these what your looking for?

From what I understand, conventional rifling bites into the bullet a little more and keeps it at the twist rate of the barrel better. In a polygonal-rifled barrel, the rifling may not bite as much and if the bullet twists less than the rifling, the rifling acts like an oblique scraping tool on the bullet (rather than a parallel track), and that scraped-off lead gets left behind in the barrel and a wave of hot propellant gasses pass over it (as well as subsequent bullets) and there's your leading.

http://upload.wikimedia.org/wikipedia/en/f/f8/Polygonal_vs_normal_rifling.gif

I have heard of people shooting lead in their factory Glock barrels, but for what you save casting your own, you can just buy a conventionally-rifled barrel and not have to worry about it. Plus, a black Glock (or any other black gun) with a nice SS barrel looks really nice.

S&W MP15 Sporter? - the 1:8? or the 1:9?

from boot's obermeyer web site
What does it mean when a barrel is said to be a "5R"?

5R is the form of rifling I developed for use in most target barrels and in many sporting barrels.
These barrels have 5 grooves, and the lands have angular sides. I have observed that bullet jackets will deform such that they remain closer to the R-form lands than they will to the sharp-edged lands present in conventional-style rifling. This reduces powder fouling at the corner of the grooves. The angled form of the lands also helps to reduce jacket failures in quick-twist barrels.

http://obermeyerbarrels.com/faq.html

https://precisionrifle.files.wordpress.com/2014/10/5r-barrel.jpg

Gain-twist rifling begins with very little change in the projectile's angular momentum (http://en.wikipedia.org/wiki/Angular_momentum) during the first few inches of bullet travel after ignition during the transition from chamber (http://en.wikipedia.org/wiki/Chamber_%28firearms%29) to throat. This enables the bullet to remain essentially undisturbed and trued to the case mouth. After engaging the rifling the bullet is progressively subjected to accelerated (http://en.wikipedia.org/wiki/Accelerate) angular momentum (http://en.wikipedia.org/wiki/Angular_momentum) as burning powder propels it down the barrel. By only gradually increasing the spin rate, torque is spread along a much longer section of barrel, rather than only at the throat where rifling is eroded through repeated rifling engagement.

http://lutzmoeller.net/English/Rifle-barrels.php
http://lutzmoeller.net/Waffen/Bilder/Progressivlauf.jpg
Gain twist rifling was used as early as the American Civil War (http://en.wikipedia.org/wiki/American_Civil_War) (1861-65). Colt Army and Navy revolvers both employed gain twist rifling. Gain twist rifling, however, is more difficult to produce than uniform rifling, and therefore is more expensive.

All this was extracted from "Cast Bullets", page 25, by E. H. Harrison, 5th Printing, Feb 1990, NRA. ISBN 0-935998-49-7 [ed.]

2-groove rifling- To speed production, most M 1903A3 rifle barrels were rifled with only 2 grooves. The rifling form and dimensions are the same as in the usual Springfield 4-groove rifling described below, but one pair of grooves is omitted. The lands cover 5/8 of the bore. Barrels so rifled are not popularly esteemed, but in fact give very reliable results with Service ammunition. Cast bullets with short groove-diameter body, the rest of the bullet bore-size, are guided positively by the abnormally broad lands, and shoot excellently in these barrels. Ideal No. 311334 is such a bullet, and is best even for light loads.

4-groove rifling- M 1903 and M1 rifle barrels have 4 broad grooves, and lands only 1/3 as wide as the grooves. The lands therefore occupy ¼ the circumference. They give this rifling a performance characteristic opposite to that of 2-groove - rifling, with cast bullets. The lands are too narrow to guide firmly anything riding on them, and the bullet must have a groove-size body of good length. Ideal bullets Nos. 311467, 311284, and 311291 are among the best for 4-groove rifling, because of their long bodies.

5-groove rifling- Of British origin, this is seen in M1917 “Enfield” rifles and British-made .30-‘06 sporting rifles, as well as in .303 Lee-Enfield rifles. Essential characteristic is not the number of grooves, but the equal width of lands and grooves. Lands therefore occupy half the bore. Cast bullets should be chosen as for use in barrels with 2-groove rifling.

6-groove rifling- Most sporting and target rifle barrels have been rifled with 6 grooves, with usual .300”-.308” diameters. As in the 4-groove Springfield, lands cover only ¼ the bore. These 6-groove barrels require the same cast-bullet forms as 4-groove barrels, and appear to handle cast bullets somewhat better.

8-groove rifling- Much shooting was done in one heavy target barrel so rifled, diameters .300”-.308”, lands covering ¼ the bore. It required bullets of same form as for 4- and 6-groove barrels. Performance was unusually good.

Multi-groove rifling- Many lands and grooves serve to spread the engraving and torque stresses favorably over the bullet. The Marlin Firearms Co. has adopted their Micro-Groove rifling for all their rifled arms. Cast-bullet test firing was done in an experimental heavy barrel with Micro-Groove rifling, in this case having 16 grooves, lands half as wide as the grooves, bore .3067”, grooves .3082” diameter.

Groove depth therefore was only ¾ of .001”. Light and medium cast-bullet loads fired from this barrel, using bullets with long groove-size bodies, gave very fine accuracy. Heavy cast-bullet loads gave wild shooting, and it was obvious the bullets did not receive sufficient rotation. This is the only such case with .30 cal. cast bullets that I have ever experienced or reliably heard of.

A Watts 24-groove barrel gave excellent results with cast-bullet loads. Obviously any bullet used in shallow rifling must depend on a long groove-size body, since no current cast bullet has a forepart large enough to fill the bore diameter.

:coffeecom