PDA

View Full Version : Groove depth...



Marlin Junky
11-28-2008, 03:29 AM
How deep is too deep?

According to Krieger's website, they can provide .32 cal (8mm) barrels that measure .323" in the grooves and .311" across the lands. I'm not sure how many grooves there are but this would provide .006" high lands which is even taller than my old 336's in .35 Remington. Is this a good configuration for cast bullets? The rate of twist on these barrels is 1 in 10" which should provide stability for heavy .32 cal bullets.

MJ

Bass Ackward
11-28-2008, 08:51 AM
How deep is too deep?

According to Krieger's website, they can provide .32 cal (8mm) barrels that measure .323" in the grooves and .311" across the lands. I'm not sure how many grooves there are but this would provide .006" high lands which is even taller than my old 336's in .35 Remington. Is this a good configuration for cast bullets? The rate of twist on these barrels is 1 in 10" which should provide stability for heavy .32 cal bullets.

MJ


Too deep is defined by whether or not you want to shoot jacketed. Cause this WILL increase pressure. Big time.

But for cast, it is dependent on the angle of the rifling on the leade. If it is too steep, then lead is scraped instead of sized out of the way. This takes more time and causes more pressure which is not good for a base. It also means that you have to have space or depth in the grease groove to accommodate it, or it has to displace all the way back which takes even more time. So any advantage you sought from taller rifling is going to be lost in the wash from pressure damage.

Obviously, the taller the rifling, the longer the ramp will be at the same angle. My 45 has .006 and I love it and it loves cast. The top end is what 60,000 psi will produce. (458X2) My 35 Remington is @ .0065 and it does well soft too. Both shoot old technology jacketed which are full length jacketed. Solid copper can be launched, but powder burn rate must drop.

Is tall rifling any more accurate? This is what you have to answer based upon how you want to shoot. For accuracy, you only need rifling tall enough to hold your slug not to strip for the bullet hardness and velocity range you want to shoot so that you get a good launch. But putting tall rifling on a 7.62X39 is a waste cause pressure is going to kill you first. So cartridge choice (volume) dictates whether you need it or not too.

The faster your twist rate, the taller you need to hold it. The accuracy is going to come from the barrel itself and what you do in that same region of velocity. So if you want to shoot cast where current rifle construction establishes zones, then use standard height. If you want to push up, your are going to produce fouling so you need to slow your twist and increase your height to strengthen the slug for launch into the barrier.

As for BC, the wider the nose, the taller the rifling you will need or you will need a heavier bullet to have enough lead still in the bore until the nose cracks the barrier. Some people think this is matching twist rate when what they are doing with a heavier slug is ensuring the bullet is guided until air flow is established. That's why we hear meplat size is a negative to flight. It's not a negative to flight so much as it is a negative to launch. Many people think that GCs come off in flight, when they are loosened at this point in the launch. So tall rifling will help here as well.

Just depends on what you want to shoot or do. But you should be mindful of the next guy too. You shouldn't build a cast rifle with the intent that you will blow it out later to some unsuspecting soul. That's the trade off.

Larry Gibson
11-28-2008, 12:34 PM
Those might make a good choice for an 8mm barrel. With Lee sizers readily available you could tailor the nose of a cast bullet to be a perfect fit in the bore to ride the lands. I'd look more for a 12" twist myself as the heaviest 8mm bullet I'd look at would be 323471. With a barrel like that I'd get a custom reamer with a tight neck for .001" clearence over the outside diameter of the loaded round with a .324" bullet. I'd also have it with a 30-30 length neck. This means I'd be forming cases from '06 brass and outside neck turing for a perfect fit the chamber neck. Regular 8x57 cases and ammo could still be used in a pinch if necessary but the necks would just be shorter. I'd also have the throat on the reamer set up for the Lyman 323470 Lovern bullet with the GC at the bottom of the neck as it has the potential for best accuracy at higher velocity. Some thought would be given to the angle of the leade but I'd suspect a standard leade would do if softer hunting alloys were the norm (they will swage instead of stripp or chip). I would consider a shallower leade if hard alloy bullets were the norm. Just depends on the most intended use.

Just my thoughts and choice though.

Larry Gibson

Marlin Junky
11-28-2008, 03:12 PM
Thanks guys...

I'm considering either an 8mm-'06 or a .338-'06. I'd like to be successful with a custom mold in the 240 to 250 grain neighborhood but also want to be able to use all the "over-the -counter" mold offerings. Any additional input?

MJ

felix
11-28-2008, 04:57 PM
I'd save the 240+ boolits for the 35 guns at 14 twist. I'd use the 8mm with the Mauser case, chopping the 06 case to factory spec, but leaving a 30-30 neck length. I would stay with the 180 grainers or less at 16 twist, with zero freebore (why the long neck), for high pressure 2400 fps ammo. ... felix