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> Freebore
Freebore
@ 02:31 am (GMT) |
Paul LevermanI sure hope this makes enough sense that someone can tell me where I went wrong in my thinking.I have cartridge based on the standard 300H&H belted case. The case has been modified (sized) to remove most of the taper, the shoulder angle has been increased (but not to the Ackley extreme, probably around 25*), the neck length has been set at one calibre plus 0.010". Neck tension and neck thickness is not a concern at the moment. With the Hornady 30 cal. 208gr. A-Max seated (where the bearing surface and the boattail meet) to the base of the neck, there is 0.290" of bearing surface forward of the case mouth. How much bullet length should be left in the case when the ogive contacts the leade? One-half? One-third? If I leave half the bullet in the case, then the freebore will be approx. 0.450". If I add 0.050" to that measurement for adjustment of jump, I end up with 0.109" of bullet left in the neck at contact. Is this enough, or should I go the other way and subtract the 0.050" to leave more bullet in the case? |
Replies
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@ 02:34 am (GMT) |
Paul LevermanRe: FreeboreJust so you know, COAL does not come into the equation, as it is a single shot. As it sits right now, with the dummy round, COAL is 3.644". |
@ 03:34 am (GMT) |
Nathan FosterRe: FreeboreHi Paul, you wrote:there is 0.290" of bearing surface forward of the case mouth. Guessing you meant freebore? If so, I would leave it at that and not seat forwards, treat it like a .308 Win or .300WBY. Should be able to use WBY start loads too. Wondering if 77gr H1000 would be a good start, then work up. Could start at 75 to play it safe. Obviously I cannot say for sure where safe is. STW brass might work also as a source of brass. Should be a really good cartridge, its an ideal case capacity for a .30 Magnum. |
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@ 05:00 am (GMT) |
Paul LevermanRe: FreeboreObviously, I am misunderstanding both what freebore is and what it is used for.If I was to ream out the barrel so that there was 0.290" of freebore ahead of the case mouth, the bullet would already be into the leade, or, 0.000" of jump. [/img] 
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@ 05:20 am (GMT) |
Paul LevermanRe: FreeboreAfter reading my original and Nathan's reply, maybe I'm asking the wrong question.Is it not possible to seat the bullet further out by X inches to increase powder capacity in the case? If so, how far could I move it out and still have suitable guidance? |
@ 05:24 am (GMT) |
Thomas KitchenRe: Freeborehi Paulwhen i was designing my 35/303 reamer i got a case filed open the neck and using a ogive gauge on my calipers i slowly seated projectiles until the start of the boat tail was sitting flush with the bottom of the case neck. i then got the reamer designed to that ogive coal with an extra .200 of freebore before it went into the leade/throat. now that amount of freebore is around the same as the 308win from my understand that gives a good mix of velocity vs accuracy. i would go shorter on your as you want accuracy as you have all the power you'll need my sounds of it. i have included copy of my reamer for my 308 norma mag as this was designed around the 208gr amax's by Nathan and Dave mason so may be some help. 
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@ 06:16 am (GMT) |
Paul LevermanRe: FreeboreOK....I need to do more reading or something. Maybe my meds are screwing with my head. None of this is making sense, even less than it did when I started.So thanks to Nathan and Thomas for at least trying to help me, but it just isn't working. Not a big deal, I have lots of time to figure this out. |
@ 01:10 pm (GMT) |
Bryan WebsterRe: FreeboreQuote: After reading my original and Nathan's reply, maybe I'm asking the wrong question.
Is it not possible to seat the bullet further out by X inches to increase powder capacity in the case? If so, how far could I move it out and still have suitable guidance? You can up to the point that the overall cartridge's length including the bullet will no longer fit in your magazine, or feed properly. You could try seating it out to just touching the lands, then back off -.002 inches, do up 3 or 5 loads like that, then seat it in small increments deeper until you get to where you want. Either way the rifle groups or magazine capacity will tell you somethings that you can use to decide on seating depths. I would be very careful to start with lower pressure loads while doing this. The above being said however, I often just choose to use a load that is accurate and short enough to feed reliably, and I hate magazine fed rifles anyway, so have a tiny bit more leeway sometimes. |
@ 08:26 pm (GMT) |
Mike DavisRe: FreeborePaul,Im just a simple truck driver but talk plain simple KiwispeakI think what you are asking is how much freebore/bullet jump/un rifled barrel/space walk should I make or how far should projectile travel BEFORE encountering start of rifling????? is that correct???? if so I believe question has been answered for you,either use freebore as per Thomas's diagram or get gunsmith to make it like a .308winchester...eg seat her deep enough in case so it gets lined up before space walk out to rifling. hope that helps clear it up. Im sure others will chime in when you answer . |
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@ 09:01 pm (GMT) |
Nathan FosterRe: FreeboreOk, in the 7mm Practical (and short throat .300 WBY Mag), I have the bullet body / ogive junction flush with the bottom of the case neck, the bullet is then 10 thou off the lands (I should have the COAL for this in the Cartridges book).I understand what you are saying now. In this context, you are using the term freebore with regards to the actual chamber design (as in the pic above). This is different to how the term is used and understood by a hand loader where they sit the bullet in the case and then call the jump ahead of this freebore (but is not the actual freebore which starts at the case mouth). Yes, you can ream forwards to increase powder capacity. If you ream way forwards like a Weatherby or RUM, you increase both powder capacity and also gas expansion area ahead of the case, but risk a space walk for the bullet and loss in control / concentricity the further forwards you ream. I like to have the bullet engage the lands pretty much straight away- unless the capacity of the case is a bit on the small side. For example, I would never want a short throated 6.5x55 or .308 Win. Ideal accuracy comes into question too. For example, there is no need to shorten a .35 Whelen throat right up for extreme accuracy as the BC's and general bullet designs just aren't ideal for true long range work. So its no loss if the bullet does not engage the lands till its halfway out of the case neck or even a bit further with some bullet designs. This way, we get the raw stopping power thanks to max velocities. With good reloading practices, can still get the rifle shooting reasonably well. But if the shooter must have sub half minute accuracy, they may want things a bit different. |
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@ 10:50 pm (GMT) |
Thomas KitchenRe: Freeborehi Paul.how are you thinking about doing your reamer? are you designing a reamer from scratch or are you using a reamer to do the body then a neck and throat reamer or use a separate throat reamer? you will want to design a throat around the match type reamers as they are designed for the sleek bullets you will be using. i would set bullet in the neck as described then allow maybe 1 thous jump that way you can seat slightly deeper if you need more jump to find an accurate load. you could add more jump to your reamer but if you need to seat projectile further out of the neck you'll end up with the doughnuts on your brass which isn't the end of the world but just need to consider everything from all angles. if your designing reamer form scratch i have had great dealings with Dave at Manson reamers so he might be worth contacting |
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@ 06:31 am (GMT) |
Paul LevermanRe: FreeboreThanks, Nathan. After weighing the pros and cons, what probably is going to happen is the chamber will be cut to suit the 208gr as in the picture above, plus 0.020". Using 0.020" will give me room to step out to find the sweet spots, and if I need more, then the bullet gets seated deeper. With a neck length of approximately 0.010" over calibre (0.308" + 0.010"), if I have to reach out the full 0.020" to the lands, I don't think that there will be any loss of guidance.To have any appreciable gains in powder capacity, the bullet would need to sit at the least another 0.100" further out, that's quite a ways for gaining 4 - 5 grains of powder. Somehow the potential loss of accuracy just to gain five or six percent more powder doesn't appeal to me. Bryan, Mike and Thomas - thanks for the input. All valuable info for magazine fed rifles. But as I said in the second post, this is a single shot, single load rifle. The difference being, in the case of Thomas' chamber, almost one quarter of an inch in freebore. This in itself enlarges the powder capacity considerably. Thomas - the reamer will be a SAAMI reamer followed by a throating reamer. Not sure (yet) of the leade angle, but I'm hoping it will be similar to yours. Have you done any loading for yours? I was kind of curious what your powder charges are going to be like. |
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@ 04:30 pm (GMT) |
Paul LevermanRe: FreeboreSome quick calculations and measurements show that bullets below 190gr. will have too much of a stroll before engaging the lands. Looks like heavy and long is the way to go, suits me.As a side note to this, while doing all this length-to-lands measuring, I had a look at the 6mm. The 58gr. V-Max may not work. Using the book figures, the bullet sits 0.066" away from the lands. That's a long ways for a short bullet. I will have to see if our local store has the 75gr V-Max, as it has a 0.040" longer COL. On the upside, the 95gr. and 105gr. fit just fine. |
@ 09:45 pm (GMT) |
Bob MavinRe: FreeboreHi PaulI have a 6mm Remington, when loading 100gn Serra Game Kings, If I load to fit magazine length, the projectile is 4mm off the lands but it sill shoots 1/4" groups. |
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@ 04:24 am (GMT) |
Paul LevermanRe: FreeboreHi Bob - Thanks for the heads-up on the SGKs. I'll have a look in the store, never hurts to have a back up plan.Doesn't look good, weatherwise. Two low fronts, back to back, heading right for us. But, you know the weather service. Fifty percent chance of rain means it will or it won't. |
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