Bore rider geometry

[Digital Dan]

Question about proper dimensions for a bore rider design. Should the nose section of the bullet be bore, bore +?, or ?????

What portion of a bore rider bullet length is assignable to the nose with expectations it will work?

Looking at a bullet of about 1.1" length, .300" bore, .308 groove. Pressures in the range of 25-30 KPSI.

Thanks

[RickinTN]

I've found a bore ride section of .001" to .002" over bore to work pretty well for me. I'm sure there are several folks here with more experience than myself who will chime in with their experiences.
Rick

[guicksylver]

+1 On RickinTN. I have had excellent results with .002" over bore nose riders on several of my boolits for my 03A3.

The occasional 5/16"-3/8" groups @100yds no longer surprise me.

[44man]

Light engraving on the nose is good. But look at run out of your rounds. If engraved off center, you will suffer. The boolit must enter straight.

[GabbyM]

If you will be making hard bullets. Like water dropped. You'd be easier on yourself to keep the bore ride section same size as bore.
You'll have a hard time chambering a .002" over hard bullet. Unfortunately soft alloys usually drop smaller than the harder alloys.

[44man]

If you will be making hard bullets. Like water dropped. You'd be easier on yourself to keep the bore ride section same size as bore.
You'll have a hard time chambering a .002" over hard bullet. Unfortunately soft alloys usually drop smaller than the harder alloys.

True, best to stay at bore size. I have one boolit I made a mold for and have to size the nose.

[45 2.1]

Your question really depends on just where the bore ride part hits the lands. With a short nose, it would have to be fatter so it touches something in the throat. With a log nose, it would be smaller so it will engrave into the lands ahead of the throat. See the problem now?

[Digital Dan]

Well, I sort of imagined that, hence the question. I suppose one could address the short long of it by chamber/throat geometry, no?

[truckjohn]

Really accurate bore-riders can be tricky for exactly the reasons outlined above....

For example - if the bore-ride nose is larger than bore diameter - it's got to engrave when you close the bolt... which means that there is a risk of engraving off-center - and accuracy greatly suffers.... It ALSO means that you might stick a bullet in the rifling when you UNLOAD the rifle... and that sort of thing makes me VERY unhappy....

On the other hand - if they are undersize too much - the unsupported nose wobbles around and carries the whole projectile out of whack - which also ruins accuracy....

I think in a perfect world - I would shoot for a longer bore-ride nose that measures 0.0005" - 0.001" smaller than the actual measured bore/throat where in the actual area it's going to be chambered into the rifling... This way - it's a high precision slip fit, the length keeps it straight, AND you can get it back out if you need to unload the rifle.

In real life - we get what we can get.. If given the choice between too loose and a little too tight - I am going to go with a *Wee* bit too tight that ever so slightly engraves - but not enough to keep it from extracting an unfired round.... and that leads us into slightly tapered bore-riders and such like this....

Thanks

[W.R.Buchanan]

Have a look at Lyman 311299. That is a pretty good bore rider design.

The intention of the bore rider design is that the smaller portion of the boolit indexes the boolit into the bore concentric with the centerline of the bore and the rear part or driving bands engage the rifling to impart spin.

I don't think the percentage of front versus rear portions of the bullet/boolit actually have very much to do with it. The front portion must be long enough to go into the bore at least 1-2 diameters, and the rear portion must be long enough so that it doesn't strip on firing and transfers the twist of the rifling directly to the boolit.

For this reason I want my boolits as close to actual bore size as possible but on the low side. MY NOE 311299 drops at .2997 which is about as close as you can get in the hole without engraving the boolit some amount.

That way the boolit is pre-aligned with the bore before firing unlike other styles of bullets/boolits that must align themselves at firing and literally jump into the bore from a distance and may or may not actually go into the bore perfectly strait.

Randy

[Digital Dan]

Well, I kinda figured out where it's going...

[geargnasher]

Have a look at Lyman 311299. That is a pretty good bore rider design.

The intention of the bore rider design is that the smaller portion of the boolit indexes the boolit into the bore concentric with the centerline of the bore and the rear part or driving bands engage the rifling to impart spin.

I don't think the percentage of front versus rear portions of the bullet/boolit actually have very much to do with it. The front portion must be long enough to go into the bore at least 1-2 diameters, and the rear portion must be long enough so that it doesn't strip on firing and transfers the twist of the rifling directly to the boolit.

For this reason I want my boolits as close to actual bore size as possible but on the low side. MY NOE 311299 drops at .2997 which is about as close as you can get in the hole without engraving the boolit some amount.

That way the boolit is pre-aligned with the bore before firing unlike other styles of bullets/boolits that must align themselves at firing and literally jump into the bore from a distance and may or may not actually go into the bore perfectly strait.

Randy

At low pressures, that works. At high pressures, not so much. The bullet may be statically aligned by a close bore fit, but you have to look at the whole cartridge and ask yourself if the nose wouldn't be aligned properly if the bore wasn't there to guide it when chambered. Hopefully it would still be straight. Then consider what happens to that delicate guidance by the lands on the nose when the bullet is smacked in the pants, you may need more support in the throat than just the nose or the nose might dive into one of the lands harder than the other right when it starts to move, and remain crooked for the whole trip. If you "jump" the bullet, as is necessary with some rifles, all that has to be taken into consideration with the bullet design, so things come into alignment with good support before pressure is able to damage the bullet. Thinking just along the lines of bore and body diameter vs. throat entrance, bore, and groove diameter will leave unsupported areas in the leade which can be your nemesis to finding accuracy past a certain pressure.

Now, I see a nice, throat matching taper on Dan's drawing and almost no unsupported nose hanging out there..... but I have two questions: Where's the metal going to go when it engraves, and how big is the throat entrance compared to the .309" body?

Gear

[Digital Dan]

Well, that's a question. Theory I'm working with is a compatible throat cut that might just have a nose seated forward of the leade and throat of .308-309" in close proximity to the case mouth. Hope I got the terminology correct on that leade thing. The transition from throat to rifling on about a 1/2* angle, give or take. If you have contrary suggestions I'm all ears.

I'm guessing a bit here if that isn't obvious.

[geargnasher]

I'm going down a similar learning path, Dan, and am only about six tested bullet designs ahead of you. It appears that 45 2.1 was banned, which is too bad because he has a great deal of experience on the matter. Perhaps Tom Myers could give some input, and I know that he markets some design software that includes a lot of guidelines for successful designs, you might contact him through private message as he doesn't post here much. Another resource is NOE, and if you haven't visited the discussion board there yet, I think you could benefit. There already exists a design very similar to yours on both Accurate's catalog and on NOE's, but it's a much lighter bullet, at about 165 grains advertised. NOE's has a more pointed nose.

Gear

[Digital Dan]

Thanks for that, will head over that way.

[Forrest r]

A simple pound cast will tell you your throat size and leade angle. For every .001" of leade angle you should have around:
.038" for a 1 1/2*
.057" for a 1*
.077 for a 45 minute

So if you have a .310 throat with a 1* leade taper in a 308/300 bore, you'll have:
.310 - .300 ='s .010
10 x .057 ='s .570"

Typical 30cal throating reamers are .309/.310 with the most common being 1 1/2* total leade angle or your pound cast leade should measure around.
.380" for 1 1/2*
.57 for 1*
.77 for 45 minute

From there you can measure the length of your bullet from the .300 diameter to the .309/.310 diameter of your bullet. That measurement on your bullet should match the leade measurement on your pound cast.

[Digital Dan]

Thank for that, but I have a small problem in the short term. Barrel, bullet, mould, dies....none of them exist at present. The bullet will be first and the rest built around that, probably in the following order. Mould, Dies, Barrel. The case will be formed from .357 brass. No pound cast in my future for a spell. Shopping for a mould maker at present.

Conversation with Bullberry suggests they have a clue about throat/leade etc. and of course they will stick me with a bill for all of that, including the reamer. Guessing they speak "lead".

I do want to have a shooter out of the deal, but it's not destined to be a high pressure cartridge and I don't care if it ever shoots supersonic or a J-bullet. Really. Low pressure subsonic muffled is the exercise. If I can do MOA @ 100 yards I'll smile.