Figuring free bore
- Thread starter bigngreen
- Start date
You have to be more specific (well, maybe you don't.....it might be me just not understanding your question) but anyway, freebore is specifically the parallel section of barrel between the end of the chamber neck and the beginning of the rifling. "Measuring it" using bullets is pretty math heavy under the best of conditions but completely fruitless without you know intimately the leade angle and ogive configuration of your bullet. Plus your gapspace.
If you're trying to spec a reamer my suggestion is that you make a dummy round and send it to the reamer maker. Let heem do the math.
hth
al
Michael Huebner
Member
The only way I can think of measuring it is put the bullet (flat base) upside down in the case with medium neck tension and chamber
Measure that and see what you get
Never tried it but if I get a chance I will and let you know
Michael
Flipping a bullet around and seating it should give me a close idea of the free bore of an already chambered rifle which is part of what I'm wanting to know but I also want to take it a step farther and seat the given bullet in the position I want it and figure out what I need, I understand it may be a little math heavy but I want to know none the less. I partially want to know just so I can confirm if I got what I wanted or not, getting a reamer these days is kinda ridiculous so I want to have all the tools possible to get what I want!
Boyd Allen
Active member
One way to approach this is in relation to a chamber of known dimensions. It is common for benchrest gunsmiths to cut the front of a chamber into a short piece of barrel, that is included with new barrel jobs. If you have access to one of these in your caliber (bullet diameter) , you can find the point at which your preferred bullet(s) touches and then build another dummy round that has the bullet(s) seated where you would like. By comparing the lengths of these dummy rounds, you can come up with how much you need to shorten or lengthen freebore so that bullets are in your preferred relation to the rifling when seated to the length that you want. For instance, If someone wanted to design a 6mm reamer, we could use my PPC barrel stub, a couple of sized cases (PPC) and a couple of the bullets that he wanted to use, to make up a couple of dummy rounds, one with the bullet positioned as he wants, and the other with the same bullet seated to touch the rifling. If for instance he is building a rifle for a 6BR with a fast twist barrel, and wants to shoot a bullet that is in the 105-107 gr. range, we can use one of those bullets for both dummy rounds, even though the case dimensions are different, the difference in loaded lengths will tell us what we need to know, since I know that my reamer has a .060 free bore.
For a close reference, seat the bullet to your desire position in a case, lock a calipers at bore diameter and slide the seated bullet into the caliper jaws (it will stop on the ogive at bore diameter). Then with a second calipers measure from the case-head to the caliper jaws. Compute out the desired case-length of a reamer and subtract that from your measurement. This will give you a ballpark spec for your desired lead/throat.
My 2-cents
Donovan Moran
My 2-cents
Donovan Moran
Not to argue, but IME my bullets are marked by the ramp or top of the lands...... dunno as I've ever gotten a leade angle so shallow that I got a ring!
Dave Tooley
Active member
Not to argue, but IME my bullets are marked by the ramp or top of the lands...... dunno as I've ever gotten a leade angle so shallow that I got a ring!
What Donavan is talking about is how getting a starting point when designing a new reamer. I usually set my calipers .001" under bullet size. Scribe a ring at the junction of the ogive and body. Seat the bullet to the OAL length I want. Then measure from the base of the case to that ring. That has always gotten me within
.010" of the OAL length I wanted. If magazine length is a factor I always error on the short side about .015". Determining an exisiting barrel's freebore is about impossible as you know none of the other dimensions, being OAL case length and few others. But you can get come up with a number from the base of the case to ogive/body junction on the bullet.
Dave
If you really want to go the math route, you will need to know the leade angle and type and dimensions of the bullet ogive. This will get you started: http://mathscinotes.wordpress.com/2011/01/11/ballistics-ogives-and-bullet-shapes-part-1/
Cheers,
Keith
bigngreen, I'm gaining a sense that you may be mostly concerned with measuring existing chambers, "checking them" as it were.
??
If this is the case may I suggest you Cerrosafe the chambers, do a chamber cast and you can measure the freebore to within a thou or two using calipers.
hth
al
??
If this is the case may I suggest you Cerrosafe the chambers, do a chamber cast and you can measure the freebore to within a thou or two using calipers.
hth
al
The immediate issue is trying to get a reamer that would seat a bullet .100 longer than a current chambered 300win but I do not know the throat length of the reamer used, a chamber cast would do it. I want to take it farther to learn how to take a case and a bullet and set it up how I want and measure it then relate that info to a reamer maker or check a reamer to check the specs.
I've noticed that if I just measure the straight section of a chamber then seat a bullet it does not directly relate to the free bore spec, I assume this is because the lead angle and bullet ogive relationship. I think I've figured out how to get the lead numbers, using my machinist calculator I punch in the depth of the rifling and call it the opposite leg a then I punch in a lead angle of 1.5 degrees and get the adjacent leg and hypotenuse length, I think I should be able to figure at what length the bullet will contact the lead angle and add that to the throat and have a free bore length, at least that's what I think.
I've noticed that if I just measure the straight section of a chamber then seat a bullet it does not directly relate to the free bore spec, I assume this is because the lead angle and bullet ogive relationship. I think I've figured out how to get the lead numbers, using my machinist calculator I punch in the depth of the rifling and call it the opposite leg a then I punch in a lead angle of 1.5 degrees and get the adjacent leg and hypotenuse length, I think I should be able to figure at what length the bullet will contact the lead angle and add that to the throat and have a free bore length, at least that's what I think.
Dave Tooley
Active member
You've made one mistake in your initial input. The lead angle starts at the end of the freebore and the freebore has a diameter larger than the nominal groove diameter. That moves the ogive-land contact point further down the barrel.
Dave
R.G. Robinett
"That'll never work."
It's pretty easy to get confused measuring from the end of the case
to the marks on a bullet where it touches the rifling and think that
measurment is freebore.
You also have the safety factor "distance from the end of the neck to
the transition. Which can be .010 or .020 .
Then you have the transition from neck diameter to freebore,
at 45* it will add .015 to .020 depending on caliber.
You have all this before freebore starts. Kenny
to the marks on a bullet where it touches the rifling and think that
measurment is freebore.
You also have the safety factor "distance from the end of the neck to
the transition. Which can be .010 or .020 .
Then you have the transition from neck diameter to freebore,
at 45* it will add .015 to .020 depending on caliber.
You have all this before freebore starts. Kenny
david kiff
New member
Give me a call I can tell you how to figure out a free bore that will be with in tenths. I will teach anyone that will call . I learned the formula from Red Elliot and Kieth Francis... I now use a 1ooX comparator and a Lazar imaging machine but the old way made those men the master reamer makers for eternity.... Thanks Dave Kiff 541-826-5808