Rifling

[georgemu]

What is the best type of rifling for cast bullet shooting?

[versifier]

Many prefer deep cut rifling, but I have no trouble with button rifled barrels, and lots of folks have good results with microgroove and polygonal. The operative question is: Does your barrel shoot well with cast? - If it does, then it doesn't matter, and if it doesn't, get another rifle to play with and cast for. :-D

[Maven]

Versifier pretty much nailed it, but I don't think rifling type per se is the only or even main variable. Condition of rifling/bore; bullet fit in a particular bore; bullet design; how carefully they are cast and inspected; bullet hardness (BHN) of alloy; velocity & pressure to which an alloy/CB will be subjected (looking at shear strength here); bullet lube, both type (quality) and amount; cartridge brass and how carefully it's prepared; powder used, both relative burning rate and type/brand of; distance to target; etc. are at least as important as rifling type I think. If all of these can be controlled for, i.e., with perfect conditions over a given range (100yds? 200? 500?), then maybe one type of rifling will outshine the other to a significant degree. Hope this helps!

[grumpy one]

Whether correctly or not I tend to think in terms of a fairly simple model: how much area of rifling is bearing on the side of the grooves the rifling makes in the bullet? With my K98 I have a bore of 0.299 and a groove of 0.310, so the rifling depth is 0.011 divided by two, or 0.0055, which sounds like a lot, but I only have four grooves, so the bearing area is 4 x .0055 x the combined length of the driving bands. My microgroove 30-30 has a bore of 0.306 and a groove of 0.312 so the rifling depth is only 0.006 divided by two, or 0.003. This doesn't sound like much until you remember that there are twelve grooves, so the bearing area is 12 x .003 x the combined length of the driving bands. If I used the same type of bullet in both barrels, the 30-30 would have nearly 50% more bearing area on the sides of the grooves in the driving bands, despite its very shallow riflling.

In practice what dominates the picture with microgroove barrels is just how much rifling engagement you actually get. Sloppy bullet fit has vastly more effect with shallow rifling than with deep rifling; losing .001 of engagement from a total engagement of .0055 is less important than it would be from a total of .003. So, if a microgroove barrel is going to work, it will have to have a constant or slightly reducing diameter, and a smooth surface that doesn't tear up the surface of the driving bands, which would reduce the effective bearing area. Since cut rifling is typically a whole lot smoother than microgroove rifling, and may be less prone to diameter variations as well, the microgroove barrel's performance might not be as good in practice as it is in theory. However if you have a good microgroove barrel - smoothly tapered to a small diameter reduction from breech to muzzle, plus smooth bore and rifling - it might shoot better than quite a good deep-groove barrel.

Well, that's how I see it anyway, and it does seem to line up with the performance of my rifles so far.

Geoff

[Black Prince]

Grumpy

That is the best discussion of that I've ever heard and it makes a LOT of sense. Thanks.

THANKS.

[felix]

Grumpy, the only real disadvantage of the smaller land height is the buildup of gunk while shooting. If kept clean, the microgroove will outshoot the others for the reason you mentioned. It is just too easy to make the boolits strip, so speeds have to be kept down a little also to allow the gun to shoot more than 10 shots with good accuracy. ... felix

[Doc - J]

[QUOTE=versifier;110146]Many prefer deep cut rifling, but I have no trouble with button rifled barrels, and lots of folks have good results with microgroove and polygonal.

I thought that polygonal was bad for cast boolits? ie: Glock's and most of the Kahr's. ?

[Phil]

Evening all!

You really have to fit the bullet to the rifling form. I think this was covered in the NRA cast bullet book some years ago, can't remember right now.

For instance I have two Hoch tapered, very small meplat nose pour molds. They are identical in style and length (1.2"). The only difference is in diameter; one is in 30 caliber and has a gas check shank, the other is a plain base 8mm. I made a barrel (cut rifled) for the 8mm with seven grooves, the grooves are deeper and perhaps narrower than one would normally associate with a seven groove barrel, similar to Mauser or Enfield rifling form. I cut the chamber for the 8.15X46R cartridge and set the rifle up for breech seating. During a three day tournament (100 yards) I fired just under 90 five shot groups, the aggregate of these groups was .387 MOA. The smallest group was .162 MOA. This group was fired at the end of a shooting day, when I had already fired about 150 shots without cleaning the barrel. The smallest group I have ever fired with this bullet at 200 yards is 1.25" for ten shots . This bullet works well and exhibits no leading up to at least 1600 fps, which is as fast as I have ever tried to drive it. The lube I use is 50% petroleum jelly, 50% paraffin, and a teaspoon of RCBS case lube per pound.

The same style bullet with a gas check shank in thirty caliber is the most accurate bullet I have in Enfield style rifling and M03A3 two groove rifling. That is rifling with rather broad lands and deep grooves. It works very well in those two groove barrels, Finnish M39 barrels, Ross M10 barrels, and Lee Enfields (if you can find one with a suitable groove diameter). It would work wonderfully in the 7.65 Mauser barrels also. But this bullet is a complete failure in the M1903-M70-M700, and commercial barrels, cut rifled, button, or hammer forged, with narrow lands. This with loads that develop 1500+ fps. There just isn't enough land area to support the tapered nose of this bullet. Bullets like the 311334 and 311 (314) 299 are great in this style rifling. For all my 30 caliber and pistol shooting I use the Alox/Beeswax lube.

With the narrow land style rifling you need (generally speaking) a bullet with at least half, and more is better, the shank diameter at least groove diameter so you aren't depending on the nose (bore) diameter portion of the bullet to keep the bullet straight in the barrel. Bullets like the 311291, 31141, and 311284 are much better in these barrels. My M700, M70, and Douglas barrels shoot much better with the 31141 bullet.

I have never owned a barrel with Microgroove rifling but if I were to try to shoot cast bullets out of one it would certainly be with the latter bullet styles. There just isn't enough support for the bullet nose in Microgroove rifling so you need a long groove riding section for support. I have a couple of friends with this style rifling and they have pretty fair success with the 31141 and 311291 bullet.

The polygonal rifling (Metford) was used in the Japanese T99 rifles and we used to shoot the 311467 bullet in those to good effect. I don't know why it wouldn't work in the above mentioned pistol barrels. You would just want to make sure you had sufficient diameter on the bullet shank.

Lots of good dope here guys, I am really enjoying these forums!

Cheers,

Phil

[versifier]

I thought that polygonal was bad for cast boolits? ie: Glock's and most of the Kahr's. ?

There are a lot of those spostabe's kicking around. More than a few have crashed and burned when members of this forum have really looked at them. No one wants to warranty their products for handloads with all the potential Bubbas their lawyers envision. Just because "everyone" knows something, it doesn't mean they have it right.
Glock's warranty is voided by any handloaded ammunition, including cast. Kabooms seem to only be a problem in high pressure .40cal rounds, due to the incompletely supported chamber and generous dimensions to insure them feeding anything put through them. With new cases and cast boolits, they perform well and with decent accuracy. I have not tested them extensively in my G23 - it pisses me off to only get one loading out of a case, but range brass fired in other makes and reloaded works fine. I still feel like I'm wasting perfectly good brass when I shoot it, though.

[Pop Gun]

Cast Boolit shooting how? If you want to shoot cast bullets like cast bullets with fast burning powders that leave little bore fouling, where you can shoot as hard of lead as you need, then you have your choice. Anything works. You simply need to match a bullet design to what you decide on.

Me, I shoot soft lead at jacketed bullet velocities. I can only do that with narrow rifling that has been cut tall. Notice I said "cut" tall. Cut rifling has the tool marks running parralel with the bore. Button rifling produces marks that run across bore at a 90 degree angle to the direction of the bullet. Even when you smooth it up to get rid of the file action, high pressure will still obturate soft lead into these places and pull off lead. Then it starts to build.

I need rifling height because to shoot cast like jacketed, you need slow powders. They tend to burn dirty. Softer lead means even slower powders. This carbon combines with lube to size down bullets in a bore and you must maintain enough surface area contact from the rifling to avoid stripping and thus the loss of your seal.

So I would add that in addition to your though process for rifling, that you think more about twist rates and what you want to do with this specifically. And there are bore diameters that offer cast advantages for twist rates to bullet weight ratios. But think about what you want to do with this, what caliber you are going to mate this to, then someone may be able to provide more than generalizations.