A design question [Archive]

Marlin Junky

09-10-2006, 04:35 AM

Here's one that's been bothering me for some time now:

What percentage of a boolit's overall length should be bearing surface in order for the boolit to maintain 1.5 MOA accuracy out to 300 yards? I'm referring to a boolit that is not supported on the nose by the rifling but supported only by its driving bands as it rides in the grooves. Let's assume a .35 caliber rifle boolit, leaving the muzzle at 2000 to 2500 fps.

Thanks for the input!
MJ

StarMetal

09-10-2006, 11:06 AM

MJ,

What you meant is a bullet that is not supported on the nose by the lands. Riflings are the grooves in a barrel. So with that said I'd say look at revolver bullets especially the SWC ones. I assume you're talking that the nose of the bullet touches no part of the interior of the barrel...that is no lands, no rifling grooves.

Joe

Marlin Junky

09-10-2006, 04:54 PM

I can see where a picture might help here

2107

This boolit would be for an old .35-336 with .005+" deep grooves. Just above the leading band measures about .350" +/- .001" in diameter while the driving bands measure .361+". The case mouth would land about half way up the leading band and the crimp would be applied by a LFC die. The very short throat would be filled by the leading ban forward the case mouth and the commencement of the ogive may or may not (depending on tolerances) touch the top of the lands. What I'm trying to avoid here is hard chambering and at the same time obtain adequate bearing surface.

My question is, would there be enough bearing surface for stability out to 300 yards. The boolit's overall length is .905" and the bearing surface is .529" long (about 58%).

MJ

44man

09-10-2006, 05:04 PM

You are treading on one of the greatest mysteries ever contemplated by man!

StarMetal

09-10-2006, 05:06 PM

MJ,

I reckon you mean is there enough bearing area, that is the area of the bullet that the rifling grooves grab to actually impart spin to the bullet. Looking at that bullet I would say yes providing the twist is correct for the caliber , length, and weight of the bullet. With that said you see by the bullets flat nose that it's not going to have a very high ballistic coefficient. That sir, I think will do it the most long distance harm, rather then its bearing surface.

Joe
P.S. There was much you could do about tubular magazine lever action bullet ballistics until Hornayd came out with those new pointed safe to use jacketed bullet, unless you wanted to load one single shot at a time, possibly just one in the magazine for a two shot.

Bass Ackward

09-10-2006, 05:37 PM

I always want at least 75% and usually push for 80%. The more you have, the faster you can go and or the softer you can go. It also cuts down on the bullet jump.

If you want to run rock hard stuff, you might get away with 60% but it will probably be finicky. 80% is what I always shoot for if designing a high velocity design.

But remember that it has to fit too.

montana_charlie

09-10-2006, 10:06 PM

I'm no bullet designer, but I take great interest in comparing bullets of similar style and weight to see what makes them different. It's obvious that some are long nosed, and others are rather stumpy up front. What makes one better than the other is a mystery to me, but it's fun to look.

I recently developed an interest in trying a grooveless bullet for BPCR shooting. That is why I happen to have the picture below.
It is not a paper patched bullet. It is a grooveless design that is meant to be 'dip lubed'.
This one is supposed to be a great 1000 yard bullet for the .45 rifles. I guess you would call that 'long range'.

Sure, most of the front end rides along on the lands, but look at the single 'driving band' on the rear.
If you want to refer to that as the 'bearing surface'...and you think that it takes a long one to make a bullet stable at long range...this shape might change your mind.

I don't think the length of the 'bearing surface' has a thing to do with how stable a bullet will be when it gets 'out there'. If the bullet was spun properly in the barrel, the 'bearing surface' could fall off...and the bullet would remain as stable as it is capable of being. After all...that's what happens with paper patched bullets, which were the first (and maybe still the best) 'long range' bullets.
CM

Marlin Junky

09-11-2006, 11:31 AM

I'm not sure I like this one better:

2111

The most noticeable mod. here is the .350" wide, .05" long bore ride section that may or may not prove beneficial due to machining tolerances.

MJ

45 2.1

09-11-2006, 11:43 AM

The most noticeable mod. here is the bore ride section that may or may not prove beneficial due to machining tolerances.MJ

Most molds aren't round, pick one that has a definite end for the front band, otherwise you'll have more bearing surface on one side than the other after you size it. Seat a jacketed bullet backwards in a fired case that you pinched the mouth on and chanmber it. Take it out and measure the distance from the base of the jacketed bullet to the base of the neck. That is how long you want the body of the boolit to be. A parallel sided nose that engraves the lands some is wanted also. Compare your design to other successfull ones around. If the boolit doesn't fit, your not going to hit much at 300 yards.

Marlin Junky

09-12-2006, 05:32 AM

Can a reamer be acquired to lengthen a .35-336 chamber's throat without touching any other part of the chamber or would such a tool need to be custom ground?

MJ

Bass Ackward

09-12-2006, 06:09 AM

Can a reamer be acquired to lengthen a .35-336 chamber's throat without touching any other part of the chamber or would such a tool need to be custom ground?

MJ

Yes. Cost about $70. Or you can rent one for about $20 if you have a CC to cover the deposit. Sometimes they will lend you the T-handle you will need to get in there.

But what bullet weight are you trying to achieve? I can tell you my experiencewith a Whelen. Remember that I use 14 BHN for all rifle mixes. All bullets were designed the same, just weight changed to meet my walking throat.

First bullet I designed weighed 210 grains. Load was 53 grains of RL15 at 2400 fps. Next bullet was 220gr. Load droped to 52 grains and 2300 fps. Then 250 grain and the load dropped to 47.5 grains at 2150. A 265 gr went to 44 grains RL15 at 1950 fps. See the pattern?

Just wanted you to know since you want a certain velocity. You can always harden a cast bullet slightly to radically penetrate farther if you need it. Most people "think" like jacketed guys that you need more bullet weight all the time. Ain't so really. If I had to pick a good weight and then match a throat to it, I would go with about 225 grains and use a 16 twist to stabilize it.

Remember that OAL is your limit. Bob laid out the bearing area for you. Just put a nose on that to make it aero dynamic enough and be under the OAL by a slight marin and see what it weighs. Remember that you can add another .050 to the bearing because a check is about .070.

Marlin Junky

09-12-2006, 03:13 PM

Regarding my .35-336, bearing length (among other variables) is limited by the short cartridge neck and the 1/16" chamber throat. I'll eventually have a bigger .35, but for now I want to go with a 220 grain boolit and drive it as fast as practical from my .35-336.

Where do I need to go in order to find out more about throat reamers?

MJ

Bass Ackward

09-12-2006, 08:06 PM

Marlin Junky

09-13-2006, 12:00 AM

Bass Ackward

09-13-2006, 06:27 AM

Thanks...

Does the barrel need to be removed from the action and placed in a lathe in order to use these throaters? I was hoping I could do the throating operation by hand without removing the barrel from the action. I couldn't find any spec's for the throaters on that website.

MJ

It depends. (doesn't it always?) If you can get the reamer on an extension to go through the back unobstructed, OK. So it also depends on how much metal you have to remove and how hard the steel / dull the reamer is. You are renting something that there is no control over how someone used it before. But usually they keep them sharp.

While the reamer will follow the hole that is there, it would be controlled more precisely and show you measurement in a lathe. Expecially if the reamer wants to start to chatter. (If you have ever seen the results of someone trying a hand crown cutter on a carbon steel Smith, it will make you cry.) But I have done one or two throats without any problems by hand using trial and error. (mostly light clean up on turn bolts)

Use an over abundance of oil. How much is an over abundance? If you aren't making a mess, you need more. Clean VERY well because you don't want chips giving a false read. And try a reversed jacketed dummy round several times. You will just have to picture the design in your mind when you make the dummy round.

The sequal to the Patriot is coming out soon. Everyone remembers where he told his boys to, "aim small, miss small". The sequal is called the Machinist. In it he tells them boys to, "cut small, cry small". :grin:

StarMetal

09-13-2006, 08:16 AM

It doesn't take very much turning for a throat reamer to remove alot of metal. What that means is you can screw up a barrel really fast if you don't know what you are doing.

If you've never done it and don't know how to do it I would suggest having a gunsmith do it, or if you want to learn, have a willing gunsmith teach and show you how to do it.

Joe

Marlin Junky

09-30-2006, 11:56 PM

Are there any books or videos that describe reaming throats? I want to push the throat of my .35-336 forward at least 1/32" and give it a gradual taper so I can load boolits with more weight in the nose.

MJ

Regarding 4D products, I don't see how they can rent throaters without providing spec's.